
The class of reductionbased algorithms was introduced recently as a new approach towards creative telescoping. Starting with Hermite reduction of rational functions, various reductions have been introduced for increasingly large classes of holonomic functions. In this paper we show how to construct reductions for general holonomic functions, in the purely differential setting.
I.1.2 Algebraic algorithms 
Let be an effective field of characteristic zero and a nonzero polynomial. Consider the system of differential equations
where is an matrix with entries in and is a column vector of unknown functions. Notice that any system of differential equations with can be rewritten in this form by taking to be a multiple of all denominators.
Let be a formal solution to (1) and consider the module of linear combinations where is a row vector. Then has the natural structure of a Dmodule for the derivation defined by . A linear mapping is said to be a reduction if for all . Such a reduction is said to be confined if its image is a finite dimensional subspace of and normal if for all .
In this paper, we will show how to construct confined reductions. Such reductions are interesting for their application to creative telescoping [13, 7], as we briefly recall in section 2. The first reduction of this kind is Hermite reduction [2, 4], in which case . The existence of normal confined reductions has been shown in increasingly general cases [12, 6] and most noticeably for Fuchsian differential equations [5]. We refer to [4, 9] for more details and the application to creative telescoping.
Our construction of confined reductions proceeds in two stages. In section 4, we first focus on the submodule of of linear combinations with . We will construct a linear head reduction such that and is bounded from above for all . Here we understand that for all . The construction uses a variant of Gaussian elimination that will be described in section 3.
The head reduction may also be regarded as a way to reduce the valuation of in , at the point at infinity. In section 5 we turn to tail reductions, with the aim to reduce the valuation of at all other points in and its algebraic closure . This is essentially similar to head reduction via a change of variables, while allowing ourselves to work in algebraic extensions of .
In the last section 7, we show how to glue the head reduction and the tail reductions at each of the roots of together into a global confined reduction on . Using straightforward linear algebra and suitable valuation bounds, one can further turn this reduction into a normal one, as will be shown in detail in section 7.2.
The valuation bounds that are required in section 7.2 are proved in section 6. In this section we also prove degree and valuation bounds for so called head and tail choppers. The existence of head and tail choppers whose size is polynomial in the size of the original equation makes it possible to derive polynomial bounds for the complexity of creative telescoping: this follows from polynomial bounds for the dimension of and for the reduction of an elements in . We intend to work out the details in a forthcoming paper.
Let be an effective subfield of and let and denote the partial derivations with respect to and . Consider a system of differential equations
(2) 
where is nonzero and are such that
Setting , the first part of (2) then becomes of the form (1). Notice that any bivariate holonomic function is an entry of a solution to a system of the form (2).
Let be a complex analytic solution of the above system of equations and let be the module generated by the entries of . Notice that is stable under both and . For any with and any nonsingular contour in between two points , we may consider the integral
which defines a function in the single variable . It is natural to ask under which conditions is a holonomic function and how to compute a differential operator with .
The idea of creative telescoping is to compute a differential operator and a function with such that
Integrating over , we then obtain
If the contour has the property that for all (where the equality is allowed to hold at the limit if necessary), then yields the desired annihilator with . In general, we need to multiply on the left with an annihilator of .
Now assume that we have a computable confined reduction . Then the functions in the sequence can all be computed and they belong to a finite dimensional vector space . Using linear algebra, that means that we can compute a relation
(4) 
with . Taking
we thus obtain (3). If the relation (4) has minimal order and the reduction is normal, then it can be shown [9] that there exist no relations of the form (3) of order lower than .
Let be a matrix and denote the th row of by . Assuming that , its leading index is the smallest index with . We say that is in row swept form if there exists a such that and for all . Notice that has rank in this case.
An invertible matrix such that is in row swept form will be called a row sweaper for . We may compute such a matrix using the routine RowSweaper below, which is really a variant of Gaussian elimination. Whenever we apply this routine to a matrix such that the truncated matrix with rows is in row swept form, we notice that these first rows are left invariant by the row sweaping process. In other words, the returned row sweaper is of the form . If, in addition, the matrix has rank , then is of the form .
Algorithm RowSweaper 
, for from to do
if for all and then return Let be minimal such that for some Swap the th and th rows of and
for from to do , return 
Let and . We may regard as a Laurent polynomial with matrix coefficients :
If , then we denote and . For any , we also denote . Setting
the equation (1) implies
for any constant matrix . The matrix can also be regarded as a Laurent polynomial with matrix coefficients . We say that is a head chopper for (1) if is an invertible matrix.
Proof. Setting , and , we have
is a head chopper for
is a head chopper for
Notice that the equations (5–7) and Proposition 1 generalize to the case when for some arbitrary . Notice also that , where . Given and , let
It is easy to see that both and are modules.
Now consider a matrix with rows ordered by increasing degree . Let , let be the matrix with rows , and let be maximal such that . We say that is a head annihilator for (1) if the following conditions are satisfied:
The rows of form a basis for the module ;
The matrix is invertible;
The first rows of are linearly independent.
The matrix is obviously a head annihilator. If , then we notice that HA3 implies that is a head chopper for (1). The following proposition is also easily checked:
Proof. We have for all and for all . In particular, we have and is maximal with the property that . Setting , we also observe that for all . Since and the last rows of vanish, the first rows of both and are linearly independent. In other words, HA3 is satisfied for . As to HA2, we observe that , whence is invertible.
Propositions 4 and 5 allow us to compute head annihilators for (1) with arbitrarily large . Assuming that we have in HA3 for sufficiently large , this yields the following algorithm for the computation of a head chopper for (1):
Algorithm HeadChopper 
repeat
if is invertible then return
,

Proof. Assume that
Let be maximal such that . Using the observation made at the end of the above proof, we have , so there exist an index and with for all . Furthermore,
and
Moreover, for , the row sweaper is even of the form
Let be a head chopper for (1). Replacing by if necessary, we may assume without loss of generality that and . Let . Writing with and , let to be the set of exceptional indices for which or . For any , let
If and , then the matrix is invertible. We define the linear mapping by
We indeed have , since . The mapping also induces a mapping that we will still denote by . Setting , the relation (7) yields
This shows that the mapping is a reduction. If and , then we have and the identity map is clearly a reduction as well.
Since compositions of reductions are again reductions, we also obtain a reduction for each . Now let be the unique mapping with for all and . Then is clearly a reduction as well and it has a finite dimensional image . For any , we call the head reduction of . The following straightforward algorithm allows us to compute head reductions:
Algorithm HeadReduce 
repeat
if for all then return Let be maximal with

Remark
Remark
Remark
Head reduction essentially allows us to reduce the valuation in of elements in via the subtraction of elements in . Tail reduction aims at reducing the valuation in in a similar way for any in the algebraic closure of . More precisely, let , and . We may regard as a Laurent polynomial in with matrix coefficients :
If , then we denote its valuation in by . Setting
the equation (1) implies
for any matrix . The matrix can also be regarded as a Laurent polynomial with matrix coefficients . We say that is a tail chopper at for (1) if is an invertible matrix. In fact, it suffices to consider tail choppers at the origin:
There is also a direct link between head choppers and tail choppers at via the change of variables .
Proof. Setting , we have
Finally, the matrix is a tail chopper at almost all points :
Now consider a monic squarefree polynomial and assume that we wish to compute a tail chopper for (1) at a root of in . First of all, we have to decide how to conduct computations in . If is irreducible, then we may simply work in the field instead of and take to be the residue class of , so that becomes a generic formal root of . In general, factoring over may be hard, so we cannot assume to be irreducible. Instead, we rely on the well known technique of dynamic evaluation [8].
For convenience of the reader, let us recall that dynamic evaluation amounts to performing all computations as if were irreducible and were a field with an algorithm for division. Whenever we wish to divide by a nonzero element (with ) that is not invertible, then provides us with a non trivial factor of . In that case, we launch an exception and redo all computations with or in the role of .
So let be a formal root of and define , , and . Let be a head chopper for the equation , as computed using the algorithm from section 4.3. Then is a tail chopper at by Lemmas 13 and 14.
Let be a tail chopper for (1) at . Let , , and be as above, so that , is a head chopper for the equation . In particular, rewriting linear combinations with as linear combinations with , we may head reduce as described in section 4.4. Let be such that . Then we may rewrite as an element of . We call the tail reduction of at and write .
Let be the finite set of exceptional indices for the above head reduction and . Setting and , it can be checked that the following algorithm computes the tail reduction at :
Algorithm TailReduce 
repeat
if for all then return Let be minimal with

In the particular case when
(11) 
for some operator of order , the system (1) is equivalent to
Given a general system (1), there always exists an element such that are linearly independent. Such an element is called a cyclic vector and, with respect to the basis of , the equation (1) transforms into an equation of the form (12). For efficient algorithms to compute cyclic vectors, we refer to [3].
In the remainder of this section, we focus on systems (1) that are equivalent to (12), with , and as in (11).
Let us start with a quick review of some well known results about the asymptotic behaviour of solutions to (12) when . We define
to be the set of polynomials in whose coefficients are Puiseux series in , together with the corresponding set of monomials. The set is asymptotically ordered by
and elements of can be written as series with . We call the support of . If , then the maximal element of the support is called the dominant monomial of .
We may also regard elements of as series in with coefficients in . If , then we denote by the corresponding valuation of in . Notice that we have for some .
Let . We write for the set of finite linear combinations of elements of . The sets and are defined likewise. Consider the formal exponential monomial group
It is well known [1, 11] that the equation (12) admits a basis of formal solutions of the form
with , and such that the monomials are pairwise distinct. We will write
Notice that this result generalizes to the case when via a straightforward change of variables with .
Let us now consider a linear differential operator . Such an operator can also be expanded with respect to ; we denote by the valuation of in and by the coefficient of in this expansion.
From the valuative point of view it is more convenient to work with linear differential operators , where is the Euler derivation. Such operators can be expanded with respect to in a similar way and and are defined as above.
For , let be the corresponding operator in . If has order , then
For , let be the corresponding operator in . If has order , then
Given , we notice that its logarithmic Euler derivative belongs to . Let be the operator obtained from by substituting for . Then
for all . If has order , then
We call the twist of by .
Now consider an operator and . If , then it can be shown that . Otherwise, , whence . In other words,
More generally, if and , then
Let
If , then it can be shown using the Newton polygon method that .
Let and notice that for all .
Proof. Let be a transcendental constant over with and let . Then satisfies , , and
We may rewrite for the linear differential operator . Notice that . Similarly, we may rewrite for some operator with .
Let be the operators given by and , so that and . By construction,
Since has order at most , there exists a monomial with and . Now the solutions of are spanned by the solutions to and any particular solution to the inhomogeneous equation . In [11, Chapter 7] it is shown that the latter equation admits a so called distinguished solution with and . Since is transcendental over , it follows that . Let be a solution to with . Then satisfies
whereas
(The last equality follows from (13) and the fact that .) Now implies
whence
We notice that our definition of coincides with the one from section 4.2, since the degree in coincides with the opposite of the valuation in . Theorem 16 immediately yields a bound for the number of iterations that are necessary in HeadChopper in order to obtain a head chopper. More precisely:
Let , , and let be the smallest number for which there exists a head chopper with
(14) 
We will call the defect of (1) at infinity. Collarary 17 shows that . In a similar way, given , let and let be the smallest number for which there exists a tail chopper with
(15) 
We call the defect of (1) at . Defining in a similar way as , but at the point , one has .
Let and , so that . Let . The proof technique from Theorem 16 can also be used for studying as a function of :
Proof. Let and rewrite and with . Then we have
We may rewrite and for some with and . Let be the operators given by and , so that and . In a similar way as in the proof of Theorem 16, we deduce that
Since has order at most , there exists a monomial with and . The distinguised solution to the equation with has valuation , so that . Since , it follows that , whence . In a similar way as in the proof of Theorem 16, we obtain
whence
Proof. Let be a head chopper for (1) such that satisfies . Let be sufficiently small such that and is defined and invertible for all . We take .
Let us now study how head and tail reductions can be glued together into a global confined reduction on . More generally, we consider the case when , where is a monic squarefree polynomial such that divides for some .
We assume that we have computed a head chopper for (1) and tail choppers for (1) at each the roots of in . In particular, we may compute the corresponding head and tail reductions. Given an element of the Galois group of over , we may also assume without loss of generality that the tail choppers were chosen such that for all .
Partial fraction decomposition yields linear mappings
and
with
for all . This allows us to define a global reduction of by
By our assumption that the tail choppers were chosen in a way that is compatible with the action of the Galois group, we have whenever . Furthermore, the restriction of the reduction on to is still a reduction.
Remark
Given the confined reduction from section 7.1, let us show how to construct a normal confined reduction . For each , assume that we have computed constants and with the property that for all with and , we have .
Consider the finite dimensional vector space and let for all . Let be the subvector space of of all with for all . This space is finite dimensional and our assumptions imply that we cannot have for . In other words, for any with , we have .
Now let and let be a supplement of in so that . We may compute bases of and using straightforward linear algebra. The canonical linear projections and with are also computable. We claim that we may take for every .
Acknowledgments. We would like to thank Pierre Lairez for a helpful remark on an earlier version of this paper.
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