arXiv subjects
Publications and source records attributed to James B. Carrell.
We consider a Chinese remainder theorem for (labeled) graphs. For $X$ a GKM $T$-variety and $Y$ an invariant subvariety, we use this to give a condition for surjectivity of the restriction map $H^*(X) \to H^*(Y)$. In particular, this applies to certain invariant subvarieties in a smooth toric variety.
The purpose of this note is to give a refinement of the product formula proved in [1] for the Poincare polynomial of a smooth Schubert variety in the flag variety of an algebraic group G over C. This yields a factorization of the number of elements in a Bruhat interval [e,w] in the Weyl group W of G provided the Schubert variety associated to w is smooth. This gives an elementary necessary condition for a Schubert variety in the flag variety to be smooth.
Let G be a complex semi-simple linear algebraic group without G_2 factors, B a Borel subgroup of G and T a maximal torus in B. The flag variety G/B is a projective G-homogeneous variety whose tangent space at the identity coset is isomorphic, as a B-module, to Lie(G)/Lie(B). Recall that if w is an element of the Weyl group W of the pair (G,T), the Schubert variety X(w) in G/B is by definition the closure of the Bruhat cell BwB. In this note we prove that X(w) is non-singular iff the following two conditions hold: 1) its Poincaré polynomial is palindromic and 2) the tangent space TE(X(w)) to the set T-stable curves in X(w) through the identity is a $B$-submodule of Lie(G)/Lie(B). This gives two criteria in terms of the combinatorics of W which are necessary and sufficient for X(w) to be smooth: \sum_{x\le w} t^{\ell(x)} is palindromic, and every root of (G,T) in the convex hull of the set of negative roots whose reflection is less than w (in the Bruhat order on W) has the property that its T-weight space (in Lie(G)/Lie(B)) is contained in TE(X(w)). However, as we show by example, these conditions don't characterize the smooth Schubert varieties when G has type G_2.
By a $B$-regular variety, we mean a smooth projective variety over $C$ admitting an algebraic action of the upper triangular Borel subgroup $B \subset SL_2(C)$ such that the unipotent radical in $B$ has a unique fixed point. A result of M. Brion and the first author describes the equivariant cohomology algebra (over $C$) of a $B$-regular variety $X$ as the coordinate ring of a remarkable affine curve in $X \times P^1$. The main result of this paper uses this fact to classify the $B$-invariant subvarieties $Y$ of a $B$-regular variety $X$ for which the restriction map $i_Y:H^*(X) \to H^*(Y)$ is surjective.
Let G be a semi-simple algebraic group over the complex numbers, B a Borel subgroup of G, T a maximal torus in B and P a parabolic in G containing B. This paper deals with singularities of T-stable subvarieties of G/P. It turns out that under the restriction that G doesn't contain any G_2-factors, the key geometric invariant determining the singular T-fixed points of X is the linear span of the reduced tangent cone to X at a T-fixed point x provided the singularity is isolated. The goal of this paper is to describe this invariant at the maximal singular T-fixed points when X is a Schubert variety in G/P and G doesn't contain any G_2-factors. We first describe the span of the tangent cone solely in terms of Peterson translates, which were the main tool in a previous paper. Then, taking a further look at the Peterson translates (with the G_2-restriction), we are able to describe the span of the tangent cone at x in terms of its isotropy submodule and the Bruhat graph of X at x. This refinement gives a purely root theoretic description, which should be useful for computations. It also leads to an algorithm for the singular locus of X.
Let $B$ denote the upper triangular subgroup of $SL_2(C)$, $T$ its diagonal torus and $U$ its unipotent radical. A complex projective variety $Y$ endowed with an algebraic action of $B$ such that the fixed point set $Y^U$ is a single point, is called regular. Associated to any regular $B$-variety $Y$, there is a remarkable affine curve $Z_Y$ with a $T$-action which was studied by the second author. In this note, we show that the coordinate ring of $Z_Y$ is isomorphic with the equivariant cohomology ring $H_T^*(Y)$ with complex coefficients, when $Y$ is smooth or, more generally, is a $B$-stable subvariety of a regular smooth $B$-variety $X$ such that the restriction map from $H^*(X)$ to $H^*(Y)$ is surjective. This isomorphism is obtained as a refinement of the localization theorem in equivariant cohomology; it applies e.g. to Schubert varieties in flag varieties, and to the Peterson variety studied by Kostant. Another application of our isomorphism is a natural algebraic formula for the equivariant push forward.
Let G be a semi-simple algebraic group over ${\mathbb C}$, B a Borel subgroup of G and T a maximal torus in B. A beautiful unpublished result of Dale Peterson says that if G is simply laced, then every rationally smooth point of a Schubert variety X in G/B is nonsingular in X. The purpose of this paper is to generalize this result to arbitrary T-stable subvarieties of G/B, the only restriction being that G contains no $G_2$ factors. In particular, we show that a Schubert variety X in such a G/B is nonsingular if and only if all the reduced tangent cones of X are linear.