According to a theorem of Brieskorn and Slodowy, the intersection of the nilpotent cone of a simple Lie algebra with a transverse slice to the subregular nilpotent orbit is a simple surface singularity. At the opposite extremity of the poset of nilpotent orbits, the closure of the minimal nilpotent orbit is also an isolated symplectic singularity, called a minimal singularity. For classical Lie algebras, Kraft and Procesi showed that these two types of singularities suffice to describe all generic singularities of nilpotent orbit closures: specifically, any such singularity is either a simple surface singularity, a minimal singularity, or a union of two simple surface singularities of type A 2k−1 . In the present paper, we complete the picture by determining the generic singularities of all nilpotent orbit closures in exceptional Lie algebras (up to normalization in a few cases). We summarize the results in some graphs at the end of the paper.In most cases, we also obtain simple surface singularities or minimal singularities, though often with more complicated branching than occurs in the classical types. There are, however, six singularities that do not occur in the classical types. Three of these are unibranch non-normal singularities: an SL 2 (C)-variety whose normalization is A 2 , an Sp 4 (C)-variety whose normalization is A 4 , and a two-dimensional variety whose normalization is the simple surface singularity A 3 . In addition, there are three 4-dimensional isolated singularities each appearing once. We also study an intrinsic symmetry action on the singularities, extending Slodowy's work for the singularity of the nilpotent cone at a point in the subregular orbit.
Let G be a reductive group over a field k of characteristic = 2, let g = Lie(G), let θ be an involutive automorphism of G and let g = k⊕p be the associated symmetric space decomposition. For the case of a ground field of characteristic zero, the action of the isotropy group G θ on p is well understood, since the well-known paper of Kostant and Rallis [17]. Such a theory in positive characteristic has proved more difficult to develop. Here we use an approach based on some tools from geometric invariant theory to establish corresponding results in (good) positive characteristic.Among other results, we prove that the variety N of nilpotent elements of p has a dense open orbit, and that the same is true for every fibre of the quotient map p → p/ /G θ . However, we show that the corresponding statement for G, conjectured by Richardson, is not true. We provide a new, (mostly) calculation-free proof of the number of irreducible components of N , extending a result of Sekiguchi for k = C. Finally, we apply a theorem of Skryabin to describe the infinitesimal invariants k[p] k . morphism π : P → P/ /K is false; finally, we apply a theorem of Skryabin to describe the ring k[p] K i , where K i is the i-th Frobenius kernel of K.A torus A in G is θ-split if θ(a) = a −1 for all a ∈ A. It was proved by Vust that the set of maximal θ-split tori are K-conjugate. Let a be a toral algebra contained in p. If a is maximal such, then by abuse of terminology we say that a is a maximal torus of p.Lemma 0.1. Let a be a maximal torus of p. Then z g (a) ∩ p = a, and there exists a unique maximal θ-split torus A of G such that Lie(A) = a.
Esquisse d'une théorie de la multiplication des variables aléatoires Annales scientifiques de l'É.N.S. 3 e série, tome 76, n o 1 (1959), p. 59-82
Systèmes markoviens et stationnaires. Cas dénombrable Annales scientifiques de l'É.N.S. 3 e série, tome 68 (1951), p. 327-381
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