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We extend the work of Pappas-Rapoport-Zhu on twisted affine Grassmannians to wildly ramified, quasi-split, and residually split groups, assuming the maximal torus is induced. This relies on the construction, inspired by Tits, of certain smooth, affine, and connected $\mathbb{Z}[t]$-groups of parahoric type, which should be regarded as $\mathbb{Z}$-families of parahoric group schemes, and naturally extends a similar construction in the above articles after inverting $e$. The resulting $\mathbb{F}_p(t)$-groups are pseudo-reductive possibly non-standard in the sense of Conrad--Gabber--Prasad, and their $\mathbb{F}_p[[t]]$-models are parahoric in our generalized sense. We study their affine Grassmannians, establishing normality of Schubert varieties and Zhu's coherence theorem., 82 pages, in French. The paper was almost completely rewritten and now treats non reduced root systems as well. Applications to the Scholze-Weinstein conjecture on local models were left out for future work

Let $F$ be a family of graphs. Then for every graph $G$ the maximum number of disjoint subgraphs of $G$, each isomorphic to a member of $F$, is at most the minimum size of a set of vertices that intersects every subgraph of $G$ isomorphic to a member of $F$. We say that $F$ packs if equality holds for every graph $G$. Only very few families pack. As the next best weakening we say that $F$ has the Erdős-Posa property if there exists a function $f$ such that for every graph $G$ and integer $k>0$ the graph $G$ has either $k$ disjoint subgraphs each isomorphic to a member of $F$ or a set of at most $f(k)$ vertices that intersects every subgraph of $G$ isomorphic to a member of $F$. The name is motivated by a [classical 1965 result of Erdős and Posa](https://en.wikipedia.org/wiki/Erd%C5%91s%E2%80%93P%C3%B3sa_theorem) stating that for every graph $G$ and integer $k>0$ the graph $G$ has either $k$ disjoint cycles or a set of $O(k\log k)$ vertices that intersects every cycle. Thus the family of all cycles has the Erdős-Posa property with $f(k)=O(k\log k)$. In contrast, the family of odd cycles fails to have the Erdős-Posa property. For every integer $\ell$, a sufficiently large Escher Wall is a non-bipartite graph that has an embedding in the projective plane such that every face is even and every homotopically non-trivial closed curve intersects the graph at least $\ell$ times. In particular, it contains no set of strictly less than $\ell$ vertices such that each odd cycle contains at least one them, yet it has no two disjoint odd cycles. By now there is a large body of literature proving that various families $F$ have the Erdős-Posa property. A very general theorem of Robertson and Seymour says that for every planar graph $H$ the family $F(H)$ of all graphs with a minor isomorphic to $H$ has the Erdős-Posa property. (When $H$ is non-planar, $F(H)$ does not have the Erdős-Posa property.) The present paper proves that for every planar graph $H$ the family $F(H)$ has the Erdős-Posa property with $f(k)=O(k\log k)$, which is asymptotically best possible for every graph $H$ with at least one cycle.

In the line of classical work by Hardy, Littlewood and Wilton, we study a class of functional equations involving the Gauss transformation from the theory of continued fractions. This allows us to reprove, among others, a convergence criterion for a diophantine series considered by Chowla, and to give additional information about the sum of this series., Comment: in French. Aequationes Mathematicae, Springer Verlag, A para\^itre

In this paper we consider the multidimensional binary vector assignment problem. An input of this problem is defined by m disjoint multisets $$V^1, V^2, \ldots , V^m$$ , each composed of n binary vectors of size p. An output is a set of n disjoint m-tuples of vectors, where each m-tuple is obtained by picking one vector from each multiset $$V^i$$ . To each m-tuple we associate a p dimensional vector by applying the bit-wise AND operation on the m vectors of the tuple. The objective is to minimize the total number of zeros in these n vectors. We denote this problem by , and the restriction of this problem where every vector has at most c zeros by . was only known to be -hard, even for . We show that, assuming the unique games conjecture, it is -hard to -approximate for any fixed and . This result is tight as any solution is a -approximation. We also prove without assuming UGC that is -hard even for . Finally, we show that is polynomial-time solvable for fixed (which cannot be extended to ).

We prove the quasiinvariance of Gaussian measures (supported by functions of increasing Sobolev regularity) under the flow of one-dimensional Hamiltonian partial differential equations such as the regularized long wave, also known as the Benjamin–Bona–Mahony (BBM) equation.

We define a compactification of symmetric spaces of noncompact type, seen as spaces of isometry classes of marked lattices, analogous to the Thurston compactification of the Teichm\"uller space, and we show that it is equivariantly isomorphic to a Satake compactification. We then use it to define a new compactification of the Torelli space of a hyperbolic surface with marked points, and we show that it is equivariantly isomorphic to the Satake compactification of the image of the period mapping. Finally, we describe the natural stratification of a subset of the boundary., Comment: In french, 35 pages

International audience; We define a formal framework for the study of algebras of type Max-plus, Min-Plus, tropical algebras, and more generally algebras over a commutative idempotent semi-field. This work is motivated by the increasingly diversified use of these algebras which occur also in control theory, automata theory as well as in algebraic geometry, and in more specific ways in other parts of mathematics such as the theory of monoids.In this first article, we expecially re-examine linear algebra over idempotent semi-fields: the most delicate, but undoubtedly the most interesting point is the notion of a singular point seen as a generalization of the notion of zero. We thus rediscover many notions of regularity already introduced for matrices, and this allows us to define further notions, new in this context, such as that of the kernel of a linear form, and to apply duality to obtain a good notion of tropical dimension of a submodule.

Rate-independent systems allow for solutions with jumps that need additional modeling. Here we suggest a formulation that arises as limit of viscous regularization of the solutions in the extended state space. Hence, our parametrized metric solutions of a rate-independent system are absolutely continuous mappings from a parameter interval into the extended state space. Jumps appear as generalized gradient flows during which the time is constant. The closely related notion of BV solutions is developed afterwards. Our approach is based on the abstract theory of generalized gradient flows in metric spaces, and comparison with other notions of solutions is given.

International audience; We prove that the incidence coloring number of every k-degenerated graph G is at most \Delta(G)+2k-1. For K_4-minor free graphs (k=2), we decrease this bound to \Delta(G)+2, which is tight. For planar graphs (k=5), we decrease this bound to \Delta(G)+7.

In 1991, Dhar \cite{Dhar},\cite{DharRuelle} proves that the recurrent configurations of the sandpile automaton form an abelian group for the addition operator $\oplus$. In this article we study the identity element of this group for the sandpile automaton on rectangular grids of size $p \times q$. We prove that for $q \geq p(2+3 \sqrt 2)/2$, this identity is made of $3$ parts ($x < \frac{p(2+3\sqrt{2})}{4}$,$\frac{p(2+3\sqrt{2})}{4} < x < p - \frac{p(2+3\sqrt{2})}{4}$, $x > p - \frac{p(2+3\sqrt{2})}{4}$.) Extremal parts are symmetric whereas the central one has $2$ grains of sand on every vertex. We give a new method to compute the identity element of the group. This method is twice as fast experimentally as the other known methods.

In the French tradition of Bourbaki, the theory of vector spaces is usually presented in a very formal setting, which causes severe difficulties to many students. The aim of this paper is to analyze the underlying reasons of these difficulties and to suggest some ways to make the first teaching of the theory of vector spaces less ineffective for many students. We do not reject the necessity for formalism. On the contrary, on the basis of a historical analysis we can explain the specific meaning it has in the theory. From this mathematical analysis with a historical perspective, we analyze the teaching and the apprehension of vector space theory in a new approach. For instance, we will show that mistakes made by many students can be interpreted as a result of a lack of connection between the new formal concepts and their conceptions previously acquired in more restricted, but more intuitively based areas. Our conclusions will not plead for avoiding formalism but for a better positioning of the formal concepts with regard to previous knowledge of the students as well as special care to be given in making the role and the meaning of formalism in linear algebra explicit to the students.

International audience