Faster polynomial multiplicationover finite fields

 David Harvey

 School of Mathematics and Statistics University of New South Wales Sydney NSW 2052 Australia

 Email: d.harvey@unsw.edu.au

 Joris van der Hoevena, Grégoire Lecerfb

 Laboratoire d'informatique UMR 7161 CNRS École polytechnique 91128 Palaiseau Cedex France

 a. Email: vdhoeven@lix.polytechnique.fr

 b. Email: lecerf@lix.polytechnique.fr

 November 3, 2023
 Let be a prime, and let denote the bit complexity of multiplying two polynomials in of degree less than . For large compared to , we establish the bound , where is the iterated logarithm. This is the first known Fürer-type complexity bound for , and improves on the previously best known bound . Keywords: Polynomial multiplication, finite field, algorithm, complexity bound, FFT A.C.M. subject classification: G.1.0 Computer-arithmetic, F.2.1 Computations in finite fields A.M.S. subject classification: 68W30, 68Q17, 68W40

## 1.Introduction

Given a ring , a basic problem in complexity theory is to find an upper bound for the cost of multiplying two polynomials in of degree less than . Several complexity models may be considered. In algebraic complexity models, such as the straight-line program model [9, Chapter 4], we count the number of ring operations in , denoted . In this model, the best currently known bound was obtained by Cantor and Kaltofen [10]. More precisely, their algorithm performs additions and subtractions and multiplications in . Their bound may be viewed as an algebraic analogue of the Schönhage–Strassen result for integer multiplication [32] and generalises previous work by Schönhage [31]. These algorithms all rely on suitable incarnations of the fast Fourier transform (FFT) [12]. For details we refer the reader to [17, Chapter 8].

In this paper we are mainly interested in the case that is the finite field for some prime . The standard bit complexity model based on deterministic multitape Turing machines [26] is more realistic in this setting, as it takes into account the dependence on . We write for the bit complexity of multiplying two polynomials in of degree less than . Two basic approaches are known for obtaining good asymptotic bounds for .

If is not too large compared to , then one may use Kronecker substitution, which converts the problem to integer multiplication by packing the coefficients into large integers. Let denote the bit complexity of -bit integer multiplication. Throughout the paper, we make the customary assumption that is increasing, and for convenience we define for , . According to our recent sharpening [18] of Fürer's bound [15], we may take

 (1.1)

where denotes the iterated logarithm for , i.e.,

 (1.2)

If , then Kronecker substitution yields (see section 2.6), so we have

However, Kronecker substitution leads to inferior bounds when is large compared to , due to coefficient growth in the lifted product in . In this situation the best existing method is the algebraic Schönhage–Strassen algorithm (i.e., the Cantor–Kaltofen algorithm). In the Turing model this yields the bound

The first term dominates for large enough , say for , and we get simply

 (1.3)

Since Fürer's breakthrough result, there has been a gap between what is known for integer multiplication and for multiplication in . Namely, it has remained an open question whether the factor appearing in (1.3) can be improved to for some . It seems that Fürer's construction does not work in .

### 1.1.Our contributions

The main result of this paper is a new algorithm that completely closes the gap mentioned above.

Theorem 1.1. The bound

holds uniformly for all and all primes .

The bound follows immediately via Kronecker substitution if . The bulk of the paper concerns the reverse case where is large compared to . For example, if , the bound becomes simply

As promised, this replaces the factor in (1.3) by .

The basic idea of the new algorithm is as follows. We first construct a special extension of , where is exponentially smaller than , and for which has many small divisors. These divisors are themselves exponentially smaller than , yet they are so numerous that their product is comparable to . We now convert the given multiplication problem in to multiplication in , by cutting up the polynomials into small pieces, and we then multiply in by using FFTs over . Applying the Cooley–Tukey method to the small divisors of , we decompose the FFTs into transforms of exponentially shorter lengths (not necessarily powers of two). We use Bluestein's method to convert each short transform to a convolution over , and then use Kronecker substitution (from bivariate to univariate polynomials) to convert this to multiplication in . These latter multiplications are then handled recursively. We continue the recursion until is comparable to , at which point we switch to using Kronecker substitution (from polynomials to integers).

In many respects this approach is very similar to the algorithm for integer multiplication introduced in [18], but there are various additional technical issues to address. We recommend [18] as a gentle introduction to the main ideas.

We also prove the following conditional result.

Theorem 1.2. Assume Conjectures 8.1, 8.3 and 8.5. Then the bound

holds uniformly for all and all primes .

Of these conjectures, Conjecture 8.3 seems to be in reach for specialists in analytic number theory; Conjecture 8.5 is at least as hard as Artin's conjecture on primitive roots, but may be tractable under the generalised Riemann hypothesis (GRH); and Conjecture 8.1 is almost as strong as the Lenstra–Pomerance–Wagstaff conjecture on the distribution of Mersenne primes, and seems to be inaccessible at present.

### 1.2.Outline of the paper

The paper is structured as follows. In section 2, we start with a survey of relevant basic techniques: discrete Fourier transforms (DFTs) and FFTs, Bluestein's chirp transform, and Kronecker substitution. Most of this material is repeated from [18, Section 2] for the convenience of the reader; however, sections 2.5 and 2.6 differ substantially.

In section 3, we recall basic complexity results for arithmetic in finite fields. In particular, we consider the construction of irreducible polynomials in , algorithms for finding roots of unity in , and the cost of arithmetic in and .

In section 4, we show how to construct special extensions of whose multiplicative group has a large subgroup of highly smooth order, i.e., is divisible by many small integers. The main tool is [2, Theorem 3].

Section 5 gives complexity bounds for functions that satisfy recurrence inequalities involving postcompositions with “logarithmically slow” functions. The prototype of such an inequality is , where and are constants. The definitions and theorems are duplicated from [18, Section 5]; for the proofs, see [18].

To minimise the constant in the bound (for large relative to ), we need one more tool: in section 6, we present a polynomial analogue of the Crandall–Fagin convolution algorithm [13]. This allows us to convert a cyclic convolution over of length , where is arbitrary, into a cyclic convolution over of somewhat smaller length , where is prescribed and where . (We can still obtain without using this Crandall–Fagin analogue, but we do not know how to reach without it.)

Section 7 is devoted to the proof of Theorem 1.1. Section 8 gives the proof of Theorem 1.2, and discusses the three number-theoretic hypotheses of that theorem.

The last section offers some final notes and suggested directions for generalisation. We first quickly dispense with the bit complexity of multiplication in for prime powers , and in for arbitrary integers . We then sketch some algebraic complexity bounds for polynomial multiplication over -algebras and -algebras, especially in the straight-line program model. Our techniques also give rise to new strategies for polynomial evaluation-interpolation over . This may for instance be applied to the efficient multiplication of polynomial matrices over . Although we have not implemented any of our algorithms yet, we conclude by a few remarks on their expected efficiency in practice.

Notations. We use Hardy's notations for , and for and . The “soft-Oh” notation means that (see [17, Chapter 25, Section 7] for details). The symbol denotes the set of non-negative real numbers, and denotes . For a ring and , we write .

We will write . In expressions like or , we tacitly assume that the function is adjusted so as to take positive values for small primes such as .

Acknowledgments. We would like to thank Karim Belabas for directing us to the paper [2], and Igor Shparlinski for answering some analytic number theory questions.

## 2.Survey of classical tools

This section recalls basic facts on Fourier transforms and related techniques used in subsequent sections. For more details and historical references we refer the reader to standard books on the subject such as [3, 9, 17, 30].

### 2.1.Arrays and sorting

In the Turing model, we have available a fixed number of linear tapes. An array of -bit elements is stored as a linear array of bits. We generally assume that the elements are ordered lexicographically by , though this is just an implementation detail.

What is significant from a complexity point of view is that occasionally we must switch representations, to access an array (say 2-dimensional) by “rows” or by “columns”. In the Turing model, we may transpose an matrix of -bit elements in time , using the algorithm of [7, Appendix]. Briefly, the idea is to split the matrix into two halves along the “short” dimension, and transpose each half recursively.

We will also require more complex rearrangements of data, for which we resort to sorting. Suppose that is a totally ordered set, whose elements are represented by bit strings of length , and suppose that we can compare elements of in time . Then an array of elements of may be sorted in time using merge sort [22], which can be implemented efficiently on a Turing machine.

### 2.2.Discrete Fourier transforms

Let be a commutative ring with identity and let . An element is said to be a principal -th root of unity if and

 (2.1)

for all . In this case, we define the discrete Fourier transform (or DFT) of an -tuple with respect to to be where

That is, is the evaluation of the polynomial at .

If is a principal -th root of unity, then so is its inverse , and we have

Indeed, writing , the relation (2.1) implies that

where if and otherwise.

Remark 2.1. In all of the new algorithms introduced in this paper, we actually work over a field, whose characteristic does not divide . In this setting, the concept of principal root of unity coincides with the more familiar primitive root of unity.

### 2.3.The Cooley–Tukey FFT

Let be a principal -th root of unity and let where . Then is a principal -th root of unity and is a principal -th root of unity. Moreover, for any and , we have

 (2.2)

If and are algorithms for computing DFTs of length and , we may use (2.2) to construct an algorithm for computing DFTs of length as follows.

For each , the sum inside the brackets corresponds to the -th coefficient of a DFT of the -tuple with respect to . Evaluating these inner DFTs requires calls to . Next, we multiply by the twiddle factors , at a cost of operations in . (Actually, fewer than multiplications are required, as some of the twiddle factors are equal to . This optimisation, while important in practice, has no asymptotic effect on the algorithms discussed in this paper.) Finally, for each , the outer sum corresponds to the -th coefficient of a DFT of an -tuple in with respect to . These outer DFTs require calls to .

Denoting by the number of ring operations needed to compute a DFT of length , and assuming that we have available a precomputed table of twiddle factors, we obtain

For a factorisation , this yields recursively

 (2.3)

The corresponding algorithm is denoted . The operation is neither commutative nor associative; the above expression will always be taken to mean .

The above discussion requires several modifications in the Turing model. Assume that elements of are represented by bits.

First, for , we must add a rearrangement cost of to efficiently access the rows and columns for the recursive subtransforms (see section 2.1). For the general case , the total rearrangement cost is bounded by .

Second, we will sometimes use non-algebraic algorithms to compute the subtransforms, so it may not make sense to express their cost in terms of . The relation (2.3) therefore becomes

 (2.4)

where is the (Turing) cost of a transform of length over , and where is the cost of a single multiplication in .

Finally, we point out that requires access to a table of twiddle factors , ordered lexicographically by , for , . Assuming that we are given as input a precomputed table of the form , we must show how to extract the required twiddle factor table in the correct order. We first construct a list of triples , ordered by , in time ; then sort by in time (see section 2.1); then merge with the given root table to obtain a table , ordered by , in time ; and finally sort by in time . The total cost of the extraction is thus .

The corresponding cost for is determined as follows. Assuming that the table is given as input, we first extract the subtables of ()-th roots of unity for in time . Extracting the twiddle factor table for the decomposition then costs ; the total over all is again .

Remark 2.2. An alternative approach is to compute the twiddle factors directly in the correct order. When working over , as in [18, Section 3], this requires a slight increase in the working precision. Similar comments apply to the root tables used in Bluestein's algorithm in section 2.5.

### 2.4.Fast Fourier multiplication

Let be a principal -th root of unity in and assume that is invertible in . Consider two polynomials and in . Let be the polynomial defined by

where the product of the DFTs is taken pointwise. By construction, we have , which means that for all . The product of and modulo also satisfies for all . Consequently, , , whence .

For polynomials with and , we thus obtain an algorithm for the computation of modulo using at most operations in . Modular products of this type are also called cyclic convolutions. If , then we may recover the product from its reduction modulo . This multiplication method is called FFT multiplication.

If one of the arguments (say ) is fixed and we want to compute many products (or cyclic convolutions) for different , then we may precompute , after which each new product can be computed using only operations in .

### 2.5.Bluestein's chirp transform

We have shown above how to multiply polynomials using DFTs. Inversely, it is possible to reduce the computation of DFTs — of arbitrary length, not necessarily a power of two — to polynomial multiplication [5], as follows.

Let be a principal -th root of unity and consider an -tuple . First consider the case that is odd. Let

Note that and are integers, and that

Let , and modulo . Then

In other words, the computation of a DFT of odd length reduces to a cyclic convolution product of the same length , together with additional operations in . Notice that the polynomial is fixed and independent of in this product.

Now suppose that is even. In this case we require that be invertible in , and that . Let , and put

Then

and

As above, let , and modulo . Then for we have

and similarly

Again, the DFT reduces to a convolution of length , together with additional operations in .

The only complication in the Turing model is the cost of extracting the , and in the correct order. For example, consider the in the case that is odd. Given as input a precomputed table , we may extract the in time by applying the strategy from section 2.3 to the pairs for . The other sequences are handled similarly. For the in the case that is even, we also need to perform additions in ; assuming that additions in have cost , this is already covered by the above bound.

Remark 2.3. Bluestein's original formulation used the weights , . This has two drawbacks in our setting. First, it requires the presence of -th roots of unity, which may not exist in . Second, if is odd, it leads to negacyclic convolutions, rather than cyclic convolutions. The variants used here avoid both of these problems.

Remark 2.4. It is also possible to give variants of the new multiplication algorithms in which Bluestein's transform is replaced by a different method for converting DFTs to convolutions, such as Rader's algorithm [29].

### 2.6.Kronecker substitution

Multiplication in may be reduced to multiplication in using the classical technique of Kronecker substitution [17, Corollary 8.27]. More precisely, let and suppose that we are given two polynomials of degree less than . Let be lifts of these polynomials, with coefficients and satisfying . Then for the product we have , and the coefficients of may be read off the integer product where . We deduce the coefficients of by dividing each by . This shows that . Using the assumption that is increasing, we obtain

If we also assume that , i.e., that the degree is not too large, then this becomes simply

 (2.5)

A second type of Kronecker substitution reduces bivariate polynomial multiplication to the univariate case. Indeed, let and , and suppose that , where and . We may then recover from the univariate product in . Note that and have degree less than , so the cost of the bivariate product is .

The same method works for computing cyclic convolutions: to multiply , the same substitution leads to a product in . The cost is thus , where denotes the cost of a multiplication in .

## 3.Arithmetic in finite fields

Let be a prime and let . In this section we review basic results concerning arithmetic in and .

We assume throughout that is represented as for some irreducible monic polynomial of degree . Thus an element of is represented uniquely by its residue modulo , i.e., by a polynomial of degree less than .

Lemma 3.1. Let be a prime and let . We may compute a monic irreducible polynomial of degree in time .

Proof. See [33, Theorem 3.2].

The above complexity bound is very pessimistic in practice. Better complexity bounds are known if we allow randomised algorithms, or unproved hypotheses. For instance, assuming GRH, the bound reduces to [1].

We now consider the cost of arithmetic in , assuming that is given. Addition and subtraction in have cost . For multiplication we will always use the Schönhage–Strassen algorithm. Denote by the cost of multiplying polynomials in by this method, i.e., using the polynomial variant [31] for the polynomials themselves, followed by the integer version [32] to handle the coefficient multiplications. Then we have

Of course, we could use the new multiplication algorithm recursively for these products, but it turns out that Schönhage–Strassen is fast enough, and leads to a simpler complexity analysis in section 7. Let denote the cost of dividing a polynomial in by , returning the quotient and remainder. Using Newton's method [17, Chapter 9] we have . Thus elements of may be multiplied in time .

Let be a divisor of . To compute a DFT over of length , we must first have access to a primitive -th root of unity in . In general it is very difficult to find a primitive root of , as it requires knowledge of the factorisation of . However, to find a primitive -th root, Lemma 3.3 below shows that it is enough to know the factorisation of . The construction relies on the following existence result.

Lemma 3.2. Let be a positive integer such that , where is the number of distinct prime divisors of , and where is a certain absolute constant. Then there exists a monic irreducible polynomial of degree such that modulo is a primitive root of unity for .

Proof. This is [34, Theorem 1.1].

Lemma 3.3. Assume that divides and that the factorisation of is given. Then we may compute a primitive -th root of unity in in time .

Proof. Testing whether a given is a primitive -th root of unity reduces to checking that and for every prime divisor of . According to Lemma 3.2, it suffices to apply this test to for running over all monic polynomials of degree , where is as in the lemma. The number of candidates is at most , and each candidate can be tested in time using binary exponentiation.

Of course, we can do much better if randomised algorithms are allowed, since is reasonably likely to be a primitive -th root for randomly selected .

Finally, we consider polynomial multiplication over . Let , and let . Let be their natural lifts, i.e., of degree less than with respect to . The bivariate product may be computed using Kronecker substitution (section 2.6) in time . Writing , to recover we must divide each by . Denoting by the complexity of the original multiplication problem, we obtain

As in section 2.6, the same method may be used for cyclic convolutions. If denotes the cost of multiplication in , then we get

Remark 3.4. In the other direction, we can also reduce multiplication in to multiplication in by splitting the inputs into chunks of size . This leads to the bound for . A variant of this procedure is developed in detail in section 6.

## 4.Preparing for DFTs of large smooth orders

The aim of this section is to prove the following theorem, which allows us to construct “small” extensions of containing “many” roots of unity of “low” order. Recall that a positive integer is said to be -smooth if all of its prime divisors are less than or equal to .

Theorem 4.1. There exist computable absolute constants and with the following properties. Let be a prime and let . Then there exists an integer in the interval

and a -smooth integer , such that . Moreover, given and , we may compute and the prime factorisation of M in time .

The proof depends on a series of preparatory lemmas. For , define

For example, and . Note that is always squarefree and -smooth. We are interested in finding for which is unusually large; that is, for which has many divisors such that happens to be prime. Most of the heavy lifting is done by the following remarkable result.

Lemma 4.2. Define . There exist computable absolute constants such that for any integer we have

Proof. This is part of [2, Theorem 3]. (The threshold could of course be replaced by any fixed power of . It is stated this way in [2] because that paper is concerned with primality testing.)

The link between and is as follows.

Lemma 4.3. Let be a prime and let . Then there exists a -smooth integer such that .

Proof. We take if divides , and otherwise . In the former case we must have , so in both cases . To see that , consider any prime divisor of . Then , so by Fermat's little theorem.

Remark 4.4. The integer constructed in Lemma 4.3 only takes into account the structure of , and ignores itself. In practice, will often have small factors other than those in , possibly including repeated factors (which are never detected by ). For example, , but . In an implementation, one should always incorporate these highly valuable “accidental” factors into . We will ignore them in our theoretical discussion.

Next we give a simple sieving algorithm that tabulates approximations of for all up to a prescribed bound.

Lemma 4.5. Let . In time , we may compute integers with

 (4.1)

Proof. Initialise a table of with for . Since , it suffices to set aside bits for each .

For each integer , perform the following steps. First test whether is prime, discarding it if not. Using trial division, the cost is , so overall. Now assume that is found to be prime. Using a fast algorithm for computing logarithms [8], compute an integer such that , where , in time . Scan through the table, replacing by for those divisible by , i.e., every -th entry, in time . The total cost for each prime is , and there are primes, so the overall cost is . At the end, we divide each by , yielding the required approximations satisfying (4.1).

Proof of Theorem 4.1. We are given and as input. Applying Lemma 4.2 with , we find that for large enough there exists some with

and such that . Choose any positive and any . Since

we have

for large . Using Lemma 4.5, we may find one such in time , where .

Let be as in Lemma 4.3. Then divides , and

for large . We may compute the prime factorisation of by simply enumerating the primes , , and checking whether divides for each . This can be done in time .

The main multiplication algorithm depends on a reduction of a “long” DFT to many “short” DFTs. It is essential to have some control over the long and short transform lengths. The following result packages together the prime divisors of the abovementioned , to obtain a long transform length near a given target , and short transform lengths near a given target .

Theorem 4.6. Let , , , be as in Theorem 4.1. Let and be positive integers such that . Then there exist -smooth integers with the following properties:

1. divides (and hence divides ).

2. .

3. for all .

Given , , , and the prime factorisation of , we may compute such (and their factorisations) in time .

Proof. Let be the prime decomposition of . Taking minimal with , we ensure that (a) and (b) are satisfied. At this stage we have for all . As long as the tuple contains an entry with , we pick the two smallest entries and and replace them by a single entry . Clearly, this does not alter the product of all entries, so (a) and (b) continue to hold. Furthermore, as long as there exist two entries and with and , new entries will always be smaller than . Only at the very last step of the loop, the second smallest entry might be larger than . In that case, the product of the entry with with is still bounded by . This shows that condition (c) is satisfied at the end of the loop.

Determining requires at most multiplications of integers less than . There are at most iterations of the replacement loop. Each iteration must scan through at most integers of bit size and perform one multiplication on such integers.

## 5.Logarithmically slow recurrence inequalities

Let be a smooth increasing function, for some . We say that is an iterator of if is increasing and if

 (5.1)

for all sufficiently large .

For instance, the standard iterated logarithm defined in (1.2) is an iterator of . An analogous iterator may be defined for any smooth increasing function for which there exists some such that for all . Indeed, in that case,

is well-defined and satisfies (5.1) for all . It will sometimes be convenient to increase so that is satisfied on the whole domain of .

We say that is logarithmically slow if there exists an such that

 (5.2)

for . For example, the functions , , and are logarithmically slow, with respectively.

Lemma 5.1. Let be a logarithmically slow function. Then there exists such that for all . Consequently all logarithmically slow functions admit iterators.

In this paper, the main role played by logarithmically slow functions is to measure size reduction in multiplication algorithms. In other words, multiplication of objects of size will be reduced to multiplication of objects of size , where for some logarithmically slow function . The following result asserts that, from the point of view of iterators, such functions are more or less interchangeable with .

Lemma 5.2. For any iterator of a logarithmically slow function , we have

The next result is our main tool for converting recurrence inequalities into actual asymptotic bounds for solutions.

Proposition 5.3. Let , and . Let , and let be a logarithmically slow function such that for all . Then there exists a positive constant (depending on , , , and ) with the following property.

Let and . Let , and let be any function satisfying the following recurrence. First, for all , . Second, for all , , there exist with , and weights with , such that

Then we have for all , .

## 6.The Crandall–Fagin algorithm

Consider the problem of computing the product of two -bit integers modulo . The naive approach is to compute their ordinary -bit product and then reduce modulo . The reduction cost is negligible because of the special form of . If is divisible by a high power of two, one can save a factor of two by using the fact that FFTs naturally compute cyclic convolutions.

An ingenious algorithm of Crandall and Fagin [13] allows for the gain of this precious factor of two for arbitrary . Their algorithm is routinely used in the extreme case where is prime, in the large-scale GIMPS search for Mersenne primes (see http://www.mersenne.org/).

A variant of the Crandall–Fagin algorithm was a key ingredient of our algorithm for integer multiplication that conjecturally achieves the bound [18, Section 9]. In this section we present yet another variant, for computing products in .

### 6.1.The Crandall–Fagin reduction for polynomials

Let , and be positive integers with and . Our aim is to reduce multiplication in to multiplication in . In the applications in subsequent sections, will be a divisor of with many small factors, so that we can multiply efficiently in using FFTs over .

Suppose that is represented as , for some irreducible of degree . The reduction relies on the existence of an element such that . (This is the analogue of the real -th root of appearing in the usual Crandall–Fagin algorithm, which was originally formulated over .) It is easy to see that such may not always exist; for example, there is no cube root of in . Nevertheless, the next result shows that if is large enough, then we can always find some modulus for which a suitable exists.

Proposition 6.1. Suppose that . Then we may compute an irreducible polynomial of degree , and an element such that , in time .

Proof. We first observe that if is a primitive root, then cannot lie in a proper subfield of . (This property is independent of .) Indeed, if for some proper divisor of , then every -th power in lies in . This contradicts the fact that the number of -th powers in is at least .

Now we give an algorithm for computing and . We start by using Lemma 3.1 to compute an irreducible of degree in time , and we temporarily represent as .

Using Lemma 3.2, we may construct a list of candidates for primitive roots of (see the proof of Lemma 3.3). For each candidate , we compute and its powers . If these are linearly dependent over then belongs to a proper subfield (and so, as shown above, is not actually a primitive root). Thus we must eventually encounter some for which they are linearly independent. We take to be the minimal polynomial of this , which may be computed by finding a linear relation among . Let be the isomorphism that sends to . The matrix of with respect to the standard bases and is given by the coefficients of . The inverse of this matrix yields the matrix of . We then set , so that .

For each candidate , the cost of computing the necessary powers of is , and the various linear algebra steps require time using classical matrix arithmetic.

In the remainder of this section, we fix some and as in Proposition 6.1, and assume that is represented as . Suppose that we wish to compute the product of . The presentation here closely follows that of [18, Section 9.2]. We decompose and as

 (6.1)

where

Notice that takes only two possible values: or .

For , let

 (6.2)

so that . For any , define as follows. Choose so that lies in the interval , and put

From (6.2), we have

Since the left hand side lies in the interval , this shows that . Now, since , we have

where

Since and similarly for , we have . Note that we may recover from in time , by a standard overlap-add procedure (provided that ).

Now, regarding and as elements of by sending to , define polynomials by and for , and let be their (cyclic) product. Then

coincides with the reinterpretation of as an element of . Moreover, we may recover unambiguously from , as and . Altogether, this shows how to reduce multiplication in to multiplication in .

Remark 6.2. The pair can be computed from in bit operations, so we may compute the sequences and in time . Moreover, since takes on only two possible values, we may compute the sequence using multiplications in .

## 7.The main algorithm

Consider the problem of computing products with , i.e., products with one fixed operand. Denote the cost of this operation by . Our algorithm for this problem will perform forward DFTs and inverse DFTs, so it is convenient to introduce the normalisation

This is well-defined since clearly . Roughly speaking, may be thought of as the notional cost of a single DFT.

The problem of multiplying two polynomials in of degree less than may be reduced to the above problem by using zero-padding, i.e., taking and . Since , we obtain . Thus it suffices to obtain a good bound for .

The next theorem gives the core of the new algorithm for the case that is large relative to .

Theorem 7.1. There exist and a logarithmically slow function with the following property. For all integers , there exist integers , and weights with , such that

 (7.1)

uniformly for .

Proof. We wish to compute products with , for some sufficiently large . We assume throughout that .

Choose parameters. Using Theorem 4.1, we obtain integers and with

and so that , , and is -smooth. We choose long and short target transform lengths and . For large enough we then have , so we may apply Theorem 4.6. This yields -smooth integers , with known factorisations, such that divides and lies in the range , and such that for all . Finally we set , so that also divides , and we put . All of these parameters may be computed in time .

For large , we observe that the following estimates hold. First, we have

Since , it follows that

and also that . We have , so

To estimate , note that and , so

Since we have

 (7.2)

and as , we similarly obtain

 (7.3)

Crandall–Fagin reduction. Let us check that the hypotheses of section 6.1 are satisfied, to enable the reduction to multiplication in . We certainly have and . For Proposition 6.1, observe that , so for large . Thus we obtain an irreducible of degree , and an element such that , in time . Each multiplication in has cost (see section 3).

Computing the sequences and costs , and computing the sequence costs . The initial splitting and final overlap-add phases require time , and the multiplications by and cost .

Long transforms. The factorisation of is known, and divides , so by Lemma 3.3 we may compute a primitive -th root of unity in time .

We will multiply in by using DFTs with respect to . The table of roots may be computed in time . In a moment, we will describe an algorithm for computing a “short” DFT of length with respect to ; we then use the algorithm for the main transform of length (see section 2.3). The corresponding twiddle factor tables may be extracted in time .

Let denote the complexity of , and for let denote the cost of performing independent DFTs of length using . Then by (2.4) we have

The last term is simply .

Bluestein conversion. We now begin constructing , assuming that independent transforms are sought. We first use Bluestein's algorithm (section 2.5) to convert each DFT of length to a multiplication in . We must check that is invertible in if is even; indeed, if is even, then so is , so . The Bluestein conversion contributes to the cost of .

We must also compute a suitable table of roots, once at the top level. We first extract the table from the top level table in time , and then sort them into the correct order (and perform any necessary additions) in time . Over all the cost is .

Kronecker substitution. We finally convert each multiplication in to a multiplication in using Kronecker substitution (see section 2.6). The latter multiplications have cost , since one argument is fixed. After the multiplications, we must also perform divisions by to recover the results in , at cost . Consolidating the estimates for the Bluestein conversion and Kronecker substitution, we have

For , this becomes

Conclusion. Summing over yields

Since

and

this becomes

Let , so that . Since

and , we get

To compute the desired products, we must execute forward transforms, and inverse transforms. Each product also requires pointwise multiplications in and multiplications by . These have cost , which is absorbed by the term. Thus we obtain

Dividing by , taking suprema over , and using (7.2) and (7.3), yields the bound (7.1).

Finally, since

we have and hence for some constant and all large . We may then take .

Now we may prove the main theorem announced in the introduction.

Proof of Theorem 1.1. For define

It suffices to prove that , uniformly in and .

Let and be as in Theorem 7.1. Increasing if necessary, we may assume that for , and that .

Let for each . We will consider the regions and separately. First consider the case . There are only finitely many primes , so we may assume that and that . In this region we use Kronecker substitution: by (2.5) and (1.1) we have

and since

we get uniformly for . In fact, this even shows that uniformly for .

Now consider the case . Here we invoke Theorem 7.1 to obtain absolute constants such that for every , there exist and such that

Set . Applying Proposition 5.3 with , , , we find that uniformly for . Since and , we conclude that uniformly for .

## 8.Conjecturally faster multiplication

Recall that in [18] we established the bound with for the complexity of integer multiplication. We also proved that this can be improved to , assuming the following slight weakening of the Lenstra–Pomerance–Wagstaff conjecture on the distribution of Mersenne primes [28, 35] (see [18, Section 9] for further discussion).

Conjecture 8.1. Let be the number of Mersenne primes less than . Then there exist constants such that for all ,

The source of the (conjectural) speedup is as follows. The algorithm of [18, Section 6] computes DFTs over , and so we encounter the “short product obstruction”. Namely, to compute the product of two real numbers with significant bits using FFT algorithms, we are forced to compute a full product of two -bit integers, and then truncate the result to bits. To achieve , we replaced the coefficient ring by the finite field , where is a Mersenne prime. Multiplication modulo is a “cyclic product” rather than a short product, and this saves a factor of two at each recursion level.

The aim of this section is to outline a credible strategy for achieving the same improvement, from to , in the context of multiplication in , at least under certain plausible number-theoretic hypotheses.

Consider the problem of multiplying polynomials in of degree less than . In the Kronecker substitution region, i.e., for , we can of course achieve if we assume Conjecture 8.1.

Now consider the region where is much larger than . In the algorithm of section 7, we reduced the multiplication problem to computing products in , where for some monic irreducible of degree , and where is a “short” transform length. These multiplications were in turn converted to multiplications in via Kronecker substitution. It is exactly this factor of two in zero-padding that we wish to avoid.

Taking our cue from the integer case, we observe that if the modulus is of a particularly special form, then this factor of two can be eliminated. Indeed, suppose that

Then is isomorphic to , which is a quotient of . Assuming further that , the latter ring is isomorphic to . In other words, we can reduce a cyclic convolution over of length to a cyclic convolution over of length , rather than length . This saves a factor of two at this recursion level; if we can manage something similar at every recursion level, we reduce from to .

The essential question is therefore how to choose so that is irreducible over . The following lemma gives a simple number-theoretic characterisation of such .

Lemma 8.2. Let . The polynomial is irreducible over if and only if is prime and is a primitive root modulo .

Proof. If is a nontrivial factor of , then is divisible by , so has the nontrivial factor . On the other hand, suppose that is prime. If , then is not irreducible; otherwise, the number of irreducible factors of over is exactly , where is the order of in [36, Theorem 2.13 and Proposition 2.14] (this last statement is sometimes called the “cyclotomic reciprocity law”).

A first attempt to reach might run as follows. First choose as in the proof of Theorem 7.1, so that has many roots of unity of smooth order. Then use the same multiplication algorithm as before, but now working over , where is the smallest positive multiple of such that is prime and such that is a primitive root modulo .

Unfortunately this plan does not quite work, for reasons that we now explain.

First, if , there may be a trivial obstruction to the existence of suitable . For example, if and , then for any we have by quadratic reciprocity, so cannot be a primitive root modulo . The only way to avoid this seems to be to insist that not be divisible by in the first place. Therefore we propose the following strengthening of Lemma 4.2.

Conjecture 8.3. Define

There exist computable absolute constants such that for any integer and any prime we have

It seems likely that this conjecture should be accessible to specialists in analytic number theory. Experimentally, the “missing” factor does not seem to have much effect on the propensity of to have many divisors such that is prime. We could not find a quick way to prove the conjecture directly from Lemma 4.2.

The second problem is more serious. A special case of Artin's conjecture on primitive roots (see [25] for a survey) asserts that for any prime , there are infinitely many primes for which is a primitive root modulo . Unfortunately, there is not even a single prime for which Artin's conjecture is known to hold! Therefore, in general we cannot prove existence of a suitable .

However, we do have the following result of Lenstra, which states that the only obstruction to the existence of is the trivial one noted above, provided we are willing to accept GRH.

Lemma 8.4. Assume GRH. Assume that if is odd, or that if . Then there exist infinitely many primes such that is a primitive root modulo .

Proof. This is a special case of [23, Theorem 8.3]. (For a description of exactly which number fields are supposed to satisfy GRH for this result to hold, see [23, Theorem 3.1].)

The third problem is that existence of is not enough; we also need a bound on its size, as a function of and . The literature provides little guidance on this question. Under the hypotheses of Lemma 8.4 (including GRH), it is known that the number of primes such that and for which is a primitive root modulo grows as

 (8.1)

for some positive (see for example [24, Theorem 2]). Both and the implied big- constant depend on and . While the value of is reasonably well understood, we could not find in the literature a similarly precise description of the implied constant, so we have been unable to derive a bound for .

In the interests of getting our conjectural algorithm off the ground, we make the following guess. Define to be the smallest prime such that and such that is a primitive root modulo (assuming that such exists).

Conjecture 8.5. There exists a constant and a logarithmically slow function with the following property. Suppose that if is odd, or that if , and suppose that . Then .

We have deliberately given an absurdly weak formulation of this conjecture, so as to provide the largest possible target for analytic number theorists. For example, it would be enough to prove that for all . We suspect that the conjecture is accessible under GRH; presumably a proof would require analysing the implied constant in (8.1).

Remark 8.6. The reason we need to be large compared to is that it is easy to construct, using the Chinese remainder theorem, a prime which fails to be a primitive root modulo for all primes up to a prescribed bound.

Proof of Theorem 1.2. The proof is similar to that of Theorem 1.1. We will only give a sketch, highlighting the main differences.

In the region , we use Kronecker substitution together with [18, Theorem 2], which states that under Conjecture 8.1.

Now assume that . We will pursue a strategy similar to Theorem 7.1, recursing from down to an exponentially smaller . Assume that we wish to compute products in with one fixed operand.

Using an appropriate modification of Theorem 4.1, in which the use of Lemma 4.2 is replaced by Conjecture 8.3, we find integers and such that , (or if ), , , and is -smooth.

Now we wish to apply Conjecture 8.5 to construct suitable . However, that conjecture requires that . If we have instead , we simply use the algorithm of Theorem 7.1. This yields the expansion factor , but only for recursion levels (depending on ), since . So it is permissible to assume that , losing only a constant factor in the main complexity bound for the last few recursion levels.

Applying Conjecture 8.5 and Lemma 8.2, we obtain a positive multiple of , such that is prime, , and is irreducible in . We will take as our model for ; note that .

Notice that for some logarithmically slow function Indeed, there exists and such that and for large . Increasing if necessary, we get , the latter being a logarithmically slow function of .

In the proof of Theorem 7.1, we selected the transform length first, and then chose to fine-tune the total bit size. Here we have less control over , so we must use a different strategy. Choose target long and short transform lengths and . Applying Theorem 4.6, we obtain -smooth integers such that divides and for each , and such that for large . Let , and let be the smallest prime greater than and different to . By [4] we have , so .

We now apply the Crandall–Fagin algorithm to reduce multiplication in to multiplication in . The prerequisite is satisfied (since is even). Instead of using Proposition 6.1 to construct satisfying , we may simply take . The required modular inverse exists, as and are distinct primes, and since is -smooth.

We now take advantage of the isomorphism , where . We multiply in by using DFTs over : one DFT of length over reduces to DFTs of length over . The latter are handled by decomposing into short transforms of length , which are in turn converted to multiplications in via Bluestein's algorithm. Finally –- and this is where all the hard work pays off –- each such multiplication reduces to a multiplication in , since . The multiplications in itself are handled using the algebraic Schönhage–Strassen algorithm.

We omit the rest of the complexity argument, which is essentially the same as that of Theorem 7.1 and Theorem 1.1. We mention only that was chosen sufficiently large that the multiplications in and in make a negligible contribution overall.

## 9.Notes and generalisations

In this section we outline some directions along which the results in this paper can be extended. We also provide some hints concerning the practical usefulness of the new ideas. Our treatment is more sketchy and we plan to provide more details in a forthcoming paper.

### 9.1.Multiplication of polynomials over

Recall that denotes the cost of multiplying polynomials in of degree less than , where we assume that some model for has been fixed in advance.

Theorem 9.1. We have

uniformly for all and all primes . Assuming Conjectures 8.1, 8.3 and 8.5, we may replace by .

Indeed, we saw in section 3 that

where denotes the cost of dividing a polynomial of degree less than by . Having established the bound , it is now permissible to assume that is increasing, so the usual argument for Newton iteration shows that . Using again that is increasing, we obtain .

### 9.2.Multiplication of polynomials over

For any prime and any integer , denote by the bit complexity of multiplying polynomials in .

Theorem 9.2. We have

uniformly for all and all primes . Assuming Conjectures 8.1, 8.3 and 8.5, we may replace by .

For we may simply use Kronecker substitution. For we must modify the algorithm of section 7. Recall that in that algorithm we reduced multiplication in to multiplication , where is a transform length dividing . Our task is to define a ring , analogous to , so that multiplication in can be reduced to multiplication in .

Let be the natural projection , and write also for the corresponding map . Let be any monic irreducible polynomial of degree , and let be an arbitrary lift via , also monic of degree . Then is irreducible, and we will take .

Any primitive -th root of unity has a unique lift to a principal -th root of unity . (This can be seen, for example, by observing that must be the image in of a Teichmüller lift of in the -adic field whose residue field is .) Given , we may compute efficiently using fast Newton lifting [11, Section 12.3].

Moreover, if we choose and as in Proposition 6.1, so that , then fast Newton lifting can also be used to obtain such that in .

### 9.3.Multiplication of polynomials over

For any , denote by the bit complexity of multiplying polynomials in .

Theorem 9.3. We have

uniformly for all . Assuming Conjectures 8.1, 8.3 and 8.5, we may replace by .

We use the isomorphism , where is the prime decomposition of . The cost of converting between the and representations is [17, Section 10.3]. By Theorem 9.2 we get

The first term dominates if . If we may simply use Kronecker substitution.

### 9.4.Complexity bounds for straight-line programs

Until now we have considered only complexity bounds in the Turing model. The new techniques also lead to improved bounds in algebraic complexity models. In what follows, is always a commutative ring with identity.

For the simplest case, first assume that is an -algebra for some prime . In the straight-line program model [9, Chapter 4], we count the number of additions and subtractions in , the number of scalar multiplications (multiplications by elements of ), and the number of nonscalar multiplications (multiplications in ). As pointed out in the introduction, the best known bound for the total complexity was previously , which was achieved by the Cantor–Kaltofen algorithm.

Theorem 9.4. Let be an -algebra. We may multiply two polynomials in of degree less than using additions, subtractions, and scalar multiplications, and nonscalar multiplications. These bounds are uniform over all primes and all -algebras .

The idea of the proof is to use the same algorithm as in section 7, but instead of switching to Kronecker substitution when we reach , we simply recurse all the way down to . The role of the extension is played by , where is monic and irreducible of degree (here we have used the fact that may be viewed as a subring of , since contains an identity element and hence a copy of ). Thus we reduce multiplication in to multiplication in . The latter multiplication may be handled by DFTs over , since any primitive -th root of unity in corresponds naturally to a principal -th root of unity in .

The bound covers the cost of the DFTs, which are accomplished entirely using additions, subtractions and scalar multiplications. Nonscalar multiplications are needed only for the pointwise multiplication step. To explain the bound, we observe that at each recursion level the total “data size” grows by a factor of four: one factor of two arises from the Crandall–Fagin splitting, and another factor of two from the Kronecker substitution.

Note that in the straight-line program model, we give a different algorithm for each . Thus all precomputed objects, such as the defining polynomial and the required roots of unity, are obtained free of cost (they are built directly into the structure of the algorithm). The uniformity in follows from the fact that the bounds for and in Theorem 4.1 are independent of , although of course the algorithm will be different for each . For each fixed , we may use the same straight-line program for all -algebras .

Assuming Conjectures 8.3 and 8.5, the bounds may be improved to respectively and . However, in this case we lose uniformity in , due to the issue raised in Remark 8.6.

Theorem 9.4 can be generalised to -algebras, along the same lines as section 9.2. It is also possible to handle -algebras for any integer , i.e., rings of finite characteristic, but we cannot proceed exactly as in section 9.3, because the straight-line model has no provision for a “reduction modulo operation. Instead, we use a device introduced in [10]. Suppose that and that is now a -algebra. For each we may construct a straight-line program that takes as input polynomials of degree less than , where is any -algebra, and computes . By replacing every constant in by a compatible constant in , we obtain a straight-line program that takes as input of degree less than , and computes such that . By the Chinese remainder theorem we may choose such that and for each . The linear combination is then equal to in . We conclude that we may multiply polynomials in using additions, subtractions and scalar multiplications (by elements of ), and nonscalar multiplications (i.e., multiplications in ). These bounds are not uniform in , but for each they are uniform over all -algebras.

### 9.5.Other algebraic models

If we wish to take into account the cost of precomputations, and give a single algorithm that works uniformly for all and , we may use a more refined complexity model such as the BSS model [6]. In this model, a machine over is, roughly speaking, a Turing machine in which the tape cells hold elements of . Actually, we need a multi-tape version of the model described in [6]. The machine can perform arithmetic operations on elements of in unit time, but can also manipulate data such as index variables in the same way as a Turing machine, and must deal with data layout in the same way as a Turing machine. In this model we obtain similar bounds to Theorem 9.4, but without uniformity in . Alternatively, we could obtain bounds uniform in if we added extra terms to account for the precomputations.

Another point of view in Theorem 9.4 is that we have described a new evaluation-interpolation strategy for polynomials over an -algebra . We refer the reader to [21, Sections 2.1–2.4] for classical examples of evaluation-interpolation schemes, and [16] for algorithms specific to finite fields. Such schemes are characterised by two quantities: the evaluation/interpolation complexity and the number of evaluation points. The new algorithms yield the bounds and , and these can be used to prove complexity bounds for problems more general than polynomial multiplication. For example, we can multiply matrices with entries in using ring operations, where is an exponent of matrix multiplication. One of the main advantages of our algorithms is that is almost linear, contrary to synthetic FFT methods [10, 31] derived from Schönhage–Strassen multiplication [32], which achieve only .

In the setting of bilinear complexity [9, Chapter 14], the new algorithms do not improve asymptotically on the best known bounds. For example, it is known [27] that for any there exist -linear maps and polynomials for , with , such that for all . The new method yields the asymptotically inferior bound . Nevertheless, these bounds are asymptotic and do not take into account that our new algorithm would rather work over (say) instead of . In practice, the new method might therefore outperform the bilinear algorithms from [27].

### 9.6.Implementations

The new complexity bounds have a quite theoretic flavour: in practice, slowly growing functions such as and really behave as constants. Furthermore, for concrete applications, it would be more relevant to redefine as

With this definition, we observe that for all feasible values of in the foreseeable future. Nevertheless, we think that variants of the ideas in this paper could still be useful in practice.

Let us briefly sketch how an efficient multiplication algorithm over could be implemented. Various modern processors offer a special instruction for the multiplication of two polynomials in . The practical translation of the results from section 4 is to work over the field where . This has two advantages. First, if we represent elements of by their residues modulo , i.e., using a redundant representation, then each multiplication in requires a single hardware multiply instruction, followed by simple shift and XOR instructions. Second, there exist primitive roots of unity of high smooth order .

Transforms of small prime lengths over can be implemented using specialised codelets, similar to those implemented in FFTW3 [14]. Roughly speaking, the constant factor of a radix- DFT is . In our case, , so specialised codelets should remain reasonably efficient for . Consequently, through the mere use of such small radices, we obtain efficient algorithms for computing DFTs of lengths dividing . To handle larger primes, we may use any algorithm for converting DFTs to convolutions. Rather than use Bluestein's algorithm as in the theoretical portion of the paper, it is more economical in this case to use Rader's algorithm, which reduces a DFT of length to a cyclic convolution of length , together with additions in (see also Remark 2.4).

More precisely, consider the remaining transform lengths , , , , and . We have , , , , and , so in each case, the DFT reduces to a two-dimensional convolution of size , where is a divisor of and . Such a convolution in turn reduces to DFTs of length together with convolutions of length . The latter cannot be handled using DFTs over , since there are no roots of unity of order , or , but in any case they are all very short convolutions and could be performed directly, or by using Karatsuba's algorithm.

Transforms of other lengths dividing may be reduced to these cases via the Cooley–Tukey algorithm. Finally, to handle arbitrary input lengths efficiently, we may use a mixed-radix generalisation of the truncated Fourier transform [19, 20].

We also reemphasise the fact that DFTs of this kind can be used as an evaluation-interpolation technique. This approach is for instance attractive for the multiplication of polynomial matrices over .

We have not yet implemented any of these algorithms. We expect that much fine-tuning will be necessary to make them most effective in practice. We intend to report on this issue in a future work.

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