arXiv subjects

Letterio Gatto

Publications and source records attributed to Letterio Gatto.

25 records · Page 2Linked to original sources

arXiv2007

Equivariant Schubert Calculus

Let $T$ be a torus acting on $\CC^n$ in such a way that, for all $1\leq k\leq n$, the induced action on the grassmannian $G(k,n)$ has only isolated fixed points. This paper proposes a natural, elementary, explicit description of the corresponding $T$-equivariant Schubert calculus. In a suitable natural basis of the $T$-equivariant cohomology, seen as a module over the $T$-equivariant cohomology of a point, it is formally the same as the ordinary cohomology of a grassmann bundle. The main result, useful for computational purposes, is that the $T$-equivariant cohomology of $G(k,n)$ can be realized as the quotient of a ring generated by derivations on the exterior algebra of a free module of rank $n$ over the $T$-equivariant cohomology of a point.

math.AG↗

arXiv2007

Schubert Calculus on a Grassmann Algebra

The ({\em classical}, {\em small quantum}, {\em equivariant}) cohomology ring of the grassmannian $G(k,n)$ is generated by certain derivations operating on an exterior algebra of a free module of rank $n$ ({\em Schubert Calculus on a Grassmann Algebra)}. Our main result gives, in a unified way, a presentation of all such cohomology rings in terms of generators and relations. It also provides, by results of Laksov and Thorup, a presentation of the universal splitting algebra of a monic polynomial of degree $n$ into the product of two monic polynomials, one of degree $k$.

math.AG↗

arXiv2007

Special ramification loci on the double product of a general curve

Let C be a general connected, smooth, projective curve of positive genus g. For each nonnegative integer i we give formulas for the number of pairs (P,Q) em C x C off the diagonal such that (g+i-1)Q-(i+1)P is linearly equivalent to an effective divisor, and the number of pairs (P,Q) em C x C off the diagonal such that (g+i+1)Q-(i+1)P is linearly equivalent to a moving effective divisor.

math.AG↗

arXiv2007

Limits of special Weierstrass points

Let C be the union of two general connected, smooth, nonrational curves X and Y intersecting transversally at a point P. Assume that P is a general point of X or of Y. Our main result, in a simplified way, says: Let Q be a point of X. Then Q is the limit of special Weierstrass points on a family of smooth curves degenerating to C if and only if Q is not P and either of the following conditions hold: Q is a special ramification point of the linear system |K_X+(g_Y+1)P|, or Q is a ramification point of the linear system |K_X+(g_Y+1+j)P| for j=-1 or j=1 and P is a Weierstrass point of Y. Above, g_Y stands for the genus of Y and K_X for a canonical divisor of X. As an application, we recover in a unified and conceptually simpler way computations made by Diaz and Cukierman of certain divisor classes in the moduli space of stable curves. In our method there is no need to worry about multiplicities, an usual nuisance of the method of test curves.

math.AG↗

arXiv2005

Schubert Calculus via Hasse-Schmidt Derivations

A natural Hasse-Schmidt derivation on the exterior algebra of a free module realizes the (small quantum) cohomology ring of the grassmannian $G_k(\CC^n)$ as a ring of operators on the exterior algebra of a free module of rank $n$. Classical Pieri's formula can be interpreted as Leibniz's rule enjoyed by special Schubert cycles with respect to the wedge product.

math.AG↗

arXiv2004

The Algebra of Schubert Calculus

A flexible unified framework for both classical and quantum Schubert calculus is proposed. It is based on a natural combinatorial approach relying on the Hasse-Schmidt extension of a certain family of pairwise commuting endomorphisms of an infinite free Z-module M to its exterior algebra.

math.AG↗

arXiv2000

A geometric interpretation and a new proof of a relation by Cornalba and Harris

In the 80's M. Cornalba and J. Harris discovered a relation among the Hodge class and the boundary classes in the Picard group with rational coefficients of the moduli space of stable, hyperelliptic curves. They proved the relation by computing degrees of the classes involved for suitable one-parameter families. In the present article we show that their relation can be obtained as the class of an appropriate, geometrically meaningful empty set, thus conforming with C. Faber's general philosophy to finding relations among tautological classes in the Chow ring of the moduli space of curves. The empty set we consider is the closure of the locus of smooth, hyperelliptic curves having a special ramification point.

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