Symplectic resolutions for nilpotent orbits (III)

We prove that two symplectic resolutions of a nilpotent orbit closures in a simple complex Lie algebra of classical type are related by Mukai flops in codimension 2. To cite this article: B. Fu, C. R. Acad. Sci. Paris, Ser. I 342 (2006).     

Symplectic resolutions for coverings of nilpotent orbits

Let $\text{O}$ be a nilpotent orbit in a semisimple complex Lie algebra $\text{g}$. Denote by G the simply connected Lie group with Lie algebra $\text{g}$. For a G-homogeneous covering $\text{M→}\text{O}$, let X be the normalization of $\text{O}$ in the function field of M. In this Note, we study the existence of symplectic resolutions for such coverings X. To cite this article: B. Fu, C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

When is a symplectic quotient an orbifold?

Let K be a compact Lie group of positive dimension. We show that for most unitary K-modules the corresponding symplectic quotient is not regularly symplectomorphic to a linear symplectic orbifold (the quotient of a unitary module of a finite group). When K is connected, we show that even a symplectomorphism to a linear symplectic orbifold does not exist. Our results yield conditions that preclude the symplectic quotient of a Hamiltonian K  -manifold from being locally isomorphic to an orbifold. As an application, we determine which unitary _SU2${\mathrm{SU}}_2$-modules yield symplectic quotients that are Z⁺${\mathbb{Z}}^{+}$-graded regularly symplectomorphic to a linear symplectic orbifold. We similarly determine which unitary circle representations yield symplectic quotients that admit a regular diffeomorphism to a linear symplectic orbifold.     

A remark on the transformation formula of theta functions associated to positive definite quadratic forms

Let L be a positive Z-lattice with level N = cd, (c, d) = 1. Then the Fourier expansion at cusp $\text{1}\text{d}$ of the theta function associated to L is a theta function associated to L₁, where a lattice L₁ is defined by Z_pL₁ = Z_pL for $\text{p?c}$, Z_pL₁ = the dual of Z_pL for p | c.     

Classes of orderings of measures and related correlation inequalities. I. Multivariate totally positive distributions

A function f(x) defined on $\text{X}$ = $\text{X}$₁ × $\text{X}$₂ × … × $\text{X}$_n where each $\text{X}$_i is totally ordered satisfying f(x ∨ y) f(x ∧ y) ≥ f(x) f(y), where the lattice operations ∨ and ∧ refer to the usual ordering on $\text{X}$, is said to be multivariate totally positive of order 2 (MTP₂). A random vector Z = (Z₁, Z₂,…, Z_n) of n-real components is MTP₂ if its density is MTP₂. Classes of examples include independent random variables, absolute value multinormal whose covariance matrix Σ satisfies ?DΣ^?1D with nonnegative off-diagonal elements for some diagonal matrix D, characteristic roots of random Wishart matrices, multivariate logistic, gamma and F distributions, and others. Composition and marginal operations preserve the MTP₂ properties. The MTP₂ property facilitate the characterization of bounds for confidence sets, the calculation of coverage probabilities, securing estimates of multivariate ranking, in establishing a hierarchy of correlation inequalities, and in studying monotone Markov processes. Extensions on the theory of MTP₂ kernels are presented and amplified by a wide variety of applications.     

A new linear quotient of C 4 admitting a symplectic resolution

We show that the quotient C ⁴/G admits a symplectic resolution for ${G = Q_8 \times_{{\bf Z}/2} D_8 < {\sf Sp}_4({\bf C})}$ . Here Q ₈ is the quaternionic group of order eight and D ₈ is the dihedral group of order eight, and G is the quotient of their direct product which identifies the nontrivial central elements ?Id of each. It is equipped with the tensor product representation ${{\bf C}^2 \boxtimes {\bf C}^2 \cong {\bf C}^4}$ . This group is also naturally a subgroup of the wreath product group ${Q_8^2 \rtimes S_2 < {\sf Sp}_4({\bf C})}$ . We compute the singular locus of the family of commutative spherical symplectic reflection algebras deforming C ⁴/G. We also discuss preliminary investigations on the more general question of classifying linear quotients V/G admitting symplectic resolutions.     

Albanese and Picard 1-motives

We define, in a purely algebraic way, 1-motives Alb⁺(X), Alb⁻(X), Pic⁺(X), and Pic⁻(X) associated with any algebraic scheme X over an algebraically closed field of characteristic zero. For X over C of dimension n, the Hodge realizations are, respectively, H^{2n − 1} (X, Z(n))/(torsion), H₁ (X, Z)/(torsion), H¹ (X, Z(1)), and H_{2n − 1} (X, Z(1 n))/(torsion).     

Graded generalizations of Weyl- and Clifford algebras

Graded skew bilinear forms {,} on graded vector spaces V are defined such that their restrictions to the even resp. odd subspaces are skew resp. odd. Over such graded symplectic vector spaces a (universal) factor algebra of the tensor algebra of V is described which reduces to a Weyl- resp. Clifford algebra if only one even resp. odd subspace is nontrivial. Introducing the total graduation on this polynomial algebra and graded symmetrization it is shown that the elements up to second power are closed under graded commutation. If the graduation is of type Z₂ the elements of second power are a Lie-graded algebra and this is the only graduation for which this is true. The graded commutation relations of this algebra are calculated. It is isomorphic to the graded symplectic algebra of (V,{,}) which is contained in the graded derivation algebra of the graded Heisenberg algebra of elements up to first power.     

On Ochoa's special matrices in matrix classes

It is well known that the ideal classes of an order Z[μ], generated over Z by the integral algebraic number μ, are in a bijective correspondence with certain matrix classes, that is, classes of unimodularly equivalent matrices with rational integer coefficients. If the degree of μ is ?3, we construct explicitly a particularly simple ideal matrix for an ideal which is a product of different prime ideals of degree 1. We obtain the following special n×n matrix (c_ij) in the matrix class corresponding to the ideal class of our ideal: c_i+1,_i=1(i=1,…,n?2); c_ij=0(?i?n, 1?j?n? 2, and i≠j+1); c_nj=0(j)=2,…,n?1). The remaining coefficients are given as explicit polynomials in an integer z which depends on the ideal. It is shown that the matrix class of every regular ideal class of Z[μ] contains a special matrix of this kind.     

Reverse processes and some limit theorems of multitype Galton-Watson processes

We classify the reverse process {X_n} of a multitype Galton-Watson process {Z_n}. In the positive recurrent cases we give the stationary measure for {X_n} explicitly, and in the critical case, supposing that all the second moments of Z₁ are finite, we establish the convergence in law to a gamma distribution. Limit distributions of {Z_cn}, 0 < c < 1, conditioned on Z_n, are also given in the subcritical, supercritical and critical cases, respectively. These extend the previous one-type work of W. W. Esty.     

Irreducible 4-manifolds with abelian non-cyclic fundamental group of small rank

In this paper we construct several irreducible 4-manifolds, both small and arbitrarily large, with abelian non-cyclic fundamental group. The manufacturing procedure allows us to fill in numerous points in the geography plane of symplectic manifolds with π₁=Z⊕Z,Z⊕Zp and Zq⊕Zp (gcd(p,q)≠1). We then study the botany of these points for π₁=Zp⊕Zp.     

Normalisation d'un processus de branchement critique dans un environnement aléatoire

Let (Z_n) be a critical branching process in an independent and identically distributed (i.i.d.) random environment. For each fixed environment ω, let C_n=E^ω[Z_n∣Z_n>0] be the conditional expectation of Z_n given Z_n>0. We prove an analogue of Yaglom's law: as n→∞, the conditional law of Z_n/C_n, conditional on Z_n>0, converges to a non-degenerate law on [0,∞). We give also an analogue of Kolmogorov's law, as well as a local limit theorem for the semi-group of probability generating functions. To cite this article: Y. Guivarc'h et al., C. R. Acad. Sci. Paris, Ser. I 337 (2003).     

A generalization of a result of Sylvester's

Let v₁,…,v_n be vectors in Zⁿ with D = det(v₁,…,v_n) > 0. Let v_{n + 1} be in the cone generated by v₁,…,v_n and such that v₁,…,v_v, v_{n + 1} generate Zⁿ as a Z-module. There exists a unique “largest“ χ not expressible as a nonnegative integer combination of v₁,…,v_n, v_{n + 1} and χ = Dv_{n + 1} ? (v₁ + … v_n + v_{n + 1}).     

Ternary forms as invariants of Markoff forms and other SL2 (Z)-bundles

Consider the free group Γ = {A,B} generated by matrices A, B in SL₂(Z). We can construct a ternary form Φ(x,y,z) whose GL₃(Z) equivalence class is invariant, as it depends on Γ and not the choice of generators. If Γ is the commutator of SL₂(Z), then the generating matrices have fixed points corresponding to different fields and inequivalent Markoff forms, but they are all biuniquely determined by Φ = -z²+ y(2x+y+z) to within equivalence. When referred to transformations A, B of the upper half plane, this phenomenon is interpreted in terms of inequivalent homotopy elements which are primitive for the perforated torus.     

Integral Kirillov model

We prove that an irreducible cuspidal Q̄_ℓ-representation of GL(n, Q_p) with a central character with values in Z̄_ℓ^* has a unique Z̄_ℓ-integral structure, given by the Kirillov Z̄_ℓ-representation.     

Partially hyperbolic geodesic flows are Anosov1Les flots géodésiques partiellement hyperboliques sont Anosov

We prove that if a $\text{Z}$ or $\text{R}$-action by symplectic linear maps on a symplectic vector bundle E has a weakly dominated invariant splitting E=S⊕U with dimU=dimS, then the action is hyperbolic. In particular, contact and geodesic flows with a dominated splitting with dimS=dimU are Anosov. To cite this article: G. Contreras, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 585–590.     

L'anneau de cohomologie d'une variété de Seifert

All manifolds M considered in this Note are orientable Seifert 3-manifolds with base surface S² and infinite fundamental group π₁ (M). Our goal is to compute the cohomology ring H^* (M; Z/2Z). The ring structure will enable us to determine whether M admits a degree 1 map into RP³ or not. We describe the equivariant chain complex for the universal cover M of M, and give a diagonal approximation. The cohomology ring H^* (M; Z/2Z) is computed.     

Some discontinuous translation-invariant linear forms

The author proved in [3] that every translation-invariant linear form on $\text{D}$(Rⁿ), as well as on other spaces of test functions and distributions, is necessarily continuous. The same result has also been proved for the Hilbert space L²(G) where G is a compact connected Abelian group. In contrast to this it is proved here that there do exist discontinuous translation-invariant linear forms on the Banach spaces l₁(Z) and L¹(R), and on the Hibert spaces L²(D) and L²(R). Here Z denotes the additive group of the integers, D denotes the totally disconnected compact Abelian Cantor discontinuum group, and R denotes the additive group of the real numbers. The proofs divide into two parts: A general criterion (Theorem 1) and proofs that the spaces l₁(Z), L²(D), L²(R), and L¹(R) satisfy this criterion (Theorems 2, 3, 4, and 5, respectively).     

Weierstrass points and Z2 homology

In a recent paper (1993), Lustig established a beautiful connection between the six Weierstrass points on a Riemann surface M₂ of genus 2 and intersection points of closed geodesics for the associated hyperbolic metric. As a consequence, he was able to construct an action of the mapping class group Out(π₁M₂) of M₂ on the set of Weierstrass points of M₂ and a virtual splitting of the natural homomorphism Aut(π₁M₂) → Out(π₁M₂). Our discussion in this paper begins with the observation that these two results of Lustig's are direct consequences of the work of Birman and Hilden (1973) on equivariant homotopies for surface homeomorphisms.It is well known that Γ₂ acts naturally on the Z₂ symplectic vector space of rank 4, H₁(M₂, Z₂). We identify this action with Lustig's action by constructing a natural correspondence between pairs of distinct Weierstrass points on M₂ and nonzero elements in H₁(M₂,Z₂). In this manner, the well-known exceptional isomorphism of finite group theory, S₆ ≅ Sp(4, Z₂), arises from a natural isomorphism of Γ₂ spaces.