## Section: Overall Objectives

### Research axes

The implementation of certified symbolic computations on special functions in the Coq proof assistant requires both investigating new formalization techniques and renewing the traditional computer-algebra viewpoint on these standard objects. Large mathematical objects typical of computer algebra occur during formalization, which also requires us to improve the efficiency and ergonomics of Coq. In order to feed this interdisciplinary activity with new motivating problems, we additionally pursue a research activity oriented towards experimental mathematics in application domains that involve special functions. We expect these applications to pose new algorithmic challenges to computer algebra, which in turn will deserve a formal-certification effort. Finally, DDMF is the motivation and the showcase of our progress on the certification of these computations. While striving to provide a formal guarantee of the correctness of the information it displays, we remain keen on enriching its mathematical content by developing new computer-algebra algorithms.

#### Computer algebra certified by the Coq system

Our formalization effort consists in organizing a cooperation between a computer-algebra system and a proof assistant. The computer-algebra system is used to produce efficiently algebraic data, which are later processed by the proof assistant. The success of this cooperation relies on the design of appropriate libraries of formalized mathematics, including certified implementations of certain computer-algebra algorithms. On the other side, we expect that scrutinizing the implementation and the output of computer-algebra algorithms will shed a new light on their semantics and on their correctness proofs, and help clarifying their documentation.

##### Libraries of formalized mathematics

The appropriate framework for the study of efficient algorithms for
special functions is *algebraic*.
Representing algebraic theories as Coq formal libraries
takes benefit from the methodology emerging from the success of
ambitious projects like the formal proof of a major classification
result in finite-group theory (the Odd Order
Theorem) [40].

Yet, a number of the objects we need to formalize in the present context has never been investigated using any interactive proof assistant, despite being considered as commonplaces in computer algebra. For instance there is up to our knowledge no available formalization of the theory of non-commutative rings, of the algorithmic theory of special-functions closures, or of the asymptotic study of special functions. We expect our future formal libraries to prove broadly reusable in later formalizations of seemingly unrelated theories.

##### Manipulation of large algebraic data in a proof assistant

Another peculiarity of the mathematical objects we are going to manipulate
with the Coq system is their size. In order to provide a formal guarantee
on the data displayed by DDMF, two related axes of research have to be
pursued.
First, efficient algorithms dealing with these large objects have
to be programmed and run in Coq.
Recent evolutions of the Coq system to improve the efficiency of
its internal computations [21], [24] make this objective
reachable. Still, how to combine the aforementioned formalization
methodology with these cutting-edge evolutions of Coq remains
one of the prospective aspects of our project.
A second need is to help users *interactively*
manipulate large expressions occurring in their conjectures, an objective
for which little has been done so far. To address this need,
we work on improving the ergonomics of the system
in two ways: first, ameliorating the reactivity of Coq in its interaction
with the user; second, designing and implementing extensions of its
interface to ease our formalization activity. We expect the outcome of
these lines of research to be useful to a wider audience, interested in
manipulating large formulas on topics possibly unrelated to special functions.

##### Formal-proof-producing normalization algorithms

Our algorithm certifications inside Coq intend to simulate
well-identified components of our Maple packages, possibly by
reproducing them in Coq. It would however not have been judicious to
re-implement them inside Coq in a systematic way. Indeed for a number of its
components, the output of the algorithm is more easily checked than
found, like for instance the solving of a linear system.
Rather, we delegate the discovery of the solutions to an
external, untrusted oracle like Maple. Trusted computations inside
Coq then formally validate the correctness of the a priori
untrusted output. More often than not, this validation consists in
implementing and executing normalization procedures *inside*
Coq. A challenge of this automation is to make sure they go to scale
while remaining efficient, which requires a Coq version of
non-trivial computer-algebra algorithms. A first, archetypal example we expect to
work on is a non-commutative generalization of the normalization
procedure for elements of rings [46].

#### Better symbolic computations with special functions

Generally speaking, we design algorithms for manipulating special functions symbolically, whether univariate or with parameters, and for extracting algorithmically any kind of algebraic and analytic information from them, notably asymptotic properties. Beyond this, the heart of our research is concerned with parametrised definite summations and integrations. These very expressive operations have far-ranging applications, for instance, to the computation of integral transforms (Laplace, Fourier) or to the solution of combinatorial problems expressed via integrals (coefficient extractions, diagonals). The algorithms that we design for them need to really operate on the level of linear functional systems, differential and of recurrence. In all cases, we strive to design our algorithms with the constant goal of good theoretical complexity, and we observe that our algorithms are also fast in practice.

##### Special-function integration and summation

Our long-term goal is to design fast algorithms for a general method
for special-function integration (*creative telescoping*), and
make them applicable to general special-function inputs. Still, our
strategy is to proceed with simpler, more specific classes first
(rational functions, then algebraic functions, hyperexponential
functions, D-finite functions, non-D-finite functions; two variables,
then many variables); as well, we isolate analytic questions by
first considering types of integration with a more purely algebraic
flavor (constant terms, algebraic residues, diagonals of
combinatorics). In particular, we expect to extend our recent
approach [29] to more general classes
(algebraic with nested radicals, for example): the idea is to speed up
calculations by making use of an analogue of Hermite reduction that avoids
considering certificates.
Homologous problems for summation will be addressed as well.

##### Applications to experimental mathematics

As a consequence of our complexity-driven approach to algorithms design, the algorithms mentioned in the previous paragraph are of good complexity. Therefore, they naturally help us deal with applications that involve equations of high orders and large sizes.

With regard to combinatorics, we expect to advance the algorithmic classification of combinatorial classes like walks and urns. Here, the goal is to determine if enumerative generating functions are rational, algebraic, or D-finite, for example. Physical problems whose modelling involves special-function integrals comprise the study of models of statistical mechanics, like the Ising model for ferro-magnetism, or questions related to Hamiltonian systems.

Number theory is another promising domain of applications. Here, we attempt an experimental approach to the automated certification of integrality of the coefficients of mirror maps for Calabi–Yau manifolds. This could also involve the discovery of new Calabi–Yau operators and the certification of the existing ones. We also plan to algorithmically discover and certify new recurrences yielding good approximants needed in irrationality proofs.

It is to be noted that in all of these application domains, we would so far use general algorithms, as was done in earlier works of ours [28], [32], [31]. To push the scale of applications further, we plan to consider in each case the specifics of the application domain to tailor our algorithms.

#### Interactive and certified mathematical web sites

In continuation of our past project of an encyclopedia at http://ddmf.msr-inria.inria.fr/1.9.1/ddmf, we ambition to both enrich and certify the formulas about the special functions that we provide online. For each function, our website shows its essential properties and the mathematical objects attached to it, which are often infinite in nature (numerical evaluations, asymptotic expansions). An interactive presentation has the advantage of allowing for adaption to the user's needs. More advanced content will broaden the encyclopedia: