An n × n matrix factorization of a polynomial f is a pair of n × n matrices (P, Q) such that PQ = f In, where In is the n × n identity matrix. In this dissertation, we study matrix factorizations of an arbitrary element in a given unital ring. This study is motivated on the one hand by the construction of the unit object in the bicategory LGK of Landau-Ginzburg models (of great utility in quantum physics) whose 1−cells are matrix factorizations of polynomials over a commutative ring K, and on the other hand by the existing tensor product of matrix factorizations b⊗. We observe that the pair of n × n matrices that appear in the matrix factorization of an element in a unital ring is not unique. Next, we propose a new operation on matrix factorizations denoted e⊗ which is such that if X is a matrix factorization of an element f in a unital ring (e.g. the power series ring K[[x1, ..., xr]] f) and Y is a matrix factorization of an element g in a unital ring (e.g. g ∈ K[[y1, ..., ys]]), then Xe⊗Y is a matrix factorization of f g in a certain unital ring (e.g. in case f ∈ K[[x1, ..., xr]] and g ∈ K[[y1, ..., ys]], then f g ∈ K[[x1, ..., xr , y1, ..., ys]]). e⊗ is called the multiplicative tensor product of X and Y. After proving that this product is bifunctorial, many of its properties are also stated and proved. Furthermore, if MF(1) denotes the category of matrix factorizations of the constant power series 1, we define the concept of one-step connected category and prove that there is a one-step connected subcategory of (MF(1),e⊗) which is semi-unital semi-monoidal. We also define the concept of right pseudo-monoidal category which generalizes the notion of monoidal category and we prove that (MF(1),e⊗) is an example of this concept. Furthermore, we define a summand-reducible polynomial to be one that can be written in the form f = t1 + · · · + ts + g11 · · · g1m1 + · · · + gl1 · · · glml under some specified conditions where each tk is a monomial and each gji is a sum of monomials. We then use b⊗ and e⊗ to improve the standard method for matrix factorization of polynomials on this class and we prove that if pji is the number of monomials in gji, then there is an improved version of the standard method for factoring f which produces factorizations of size 2 Qm1 i=1 p1i+···+ Qml i=1 pli−( Pm1 i=1 p1i+···+ Pml i=1 pli) times smaller than the size one would normally obtain with the standard method. Moreover, details are given to elucidate the intricate construction of the unit object of LGK. Thereafter, a proof of the naturality of the right and left unit maps of LGK with respect to 2−morphisms is presented. We also prove that there is no direct inverse for these (right and left) unit maps, thereby justifying the fact that their inverses are found only up to homotopy. Finally, some properties of matrix factorizations are exploited to state and prove a necessary condition to obtain a Morita context in LGK.

La combinatoire a permis de résoudre certains problèmes en Mathématiques, en Physique et en Informatique, en retour celles-ci inspirent des questions nouvelles à la combinatoire. Ce mémoire de thèse intitulé "Combinatoire et algorithme des factorisations tangentes à l'identité" regroupe plusieurs travaux sur la combinatoire des déformations du produit de Shuffle. L'objectif de cette thèse est d'écrire des factorisations dont le terme principal est l'identité à travers l'utilisation d'outils portant principalement sur la combinatoire des mots (ordres, graduation etc.). Dans le cas classique, soit F une algèbre libre. En raison du fait que F est une algèbre enveloppante, on a une factorisation exacte de l'identité de End(F) = F*⨶F comme un produit infini d'exponentielles (End(F) étant muni du produit de Shuffle sur la gauche et de la concaténation sur la droite, une représentation fidèle du produit de convolution). La procédure est la suivante : premièrement on commence avec une base de Poincaré-Birkhoff-Witt, deuxièmement on calcule la famille des formes coordonnées et alors les propriétés (combinatoires) non triviales de ces familles en dualité donne la factorisation. Si on part de l'autre côté, l'écriture pour le même produit ne donne exactement l'identité que sous des conditions très restrictives que nous précisons ici. Dans de nombreux autres cas (déformés), la construction explicite des paires de bases en dualité nécessite une étude combinatoire et algorithmique que nous fournissons dans ce mémoire. / Combinatorics has solved many problems in Mathematics, Physics and Computer Science, in return these domains inspire new questions to combinatorics. This memoir entitled "Combinatorics and algorithmics of factorization tangent to indentity includes several works on the combinatorial deformations of the shuffle product. The aim of this thesis is to write factorizations wich principal term is the identity through the use of tools relating mainly to combinatorics on the words (orderings, grading etc). In the classical case, let F be the free algebra. Due to the fact that F is an enveloping algebra, one has an exact factorization of the identity of End(F) = F⨶F as an infinite product of exponentials (End(F) being endowed with the shuffle product on the left and the concatenation on the right, a faithful representation of the convolution product) as follows : first on begins with a PBW basis, second one computes the family of coordinate forms and then non-trivial (combinatorial) properties of theses families in duality gives the factorization. Starting from the other side and writing the same product does give exactly identity only under very restrictive conditions that we clarify here. In many other (deformed) cases, the explicit construction of pairs of bases in duality requires combinatorial and algorithmic studies that we provide in this memoir.