arXiv subjects
Publications and source records attributed to Carlos Sancho.
We calculate the automorphism group of a complete toric variety $X$ with torus $T_M$. We prove that the radical unipotent of $Aut_k^0X$ is a semidirect product of additive groups, the reductive part is a quotient of a product of lineal groups and we give the action of linear groups on the additive groups. We also prove that $Aut_kX/Aut_k^0X$ is a quotient of the group of automorphisms of $M$ leaving invariant the fan.
Let $R$ be a commutative ring. Roughly speaking, we prove that an $R$-module $M$ is flat iff it is a direct limit of $R$-module affine algebraic varieties, and $M$ is a flat Mittag-Leffler module iff it is the union of its $R$-submodule affine algebraic varieties.
We give a functorial characterization of Mittag-Leffler modules and strict Mittag-Leffler modules.
We give some functorial characterizations of flat strict Mittag-Leffler modules. We characterize reflexive functors of modules with similar tools, definitions and theorems.
Let $G = Spec A$ be an affine $K$-group scheme and $\tilde{A} = \{w \in A*: dim_K A^* \cdot w \cdot A^* < \infty \}$. Let $< -,-> : A^* \times \tilde{A} \to K, (w,\tilde{w}) := tr(w \tilde{w})$, be the trace form. We prove that $G$ is linearly reductive if and only if the trace form is non-degenerate on $A^*$.
Let $G= {\rm Spec} A$ be an affine $R$-monoid scheme. We prove that the category of dual functors (over the category of commutative $R$-algebras) of $G$-modules is equivalent to the category of dual functors of ${\mathcal A}^*$-modules. We prove that $G$ is invariant exact if and only if $A^*= R \times B^*$ as $R$-algebras and the first projection $A^* \to R$ is the unit of $A$. If $\mathbb M$ is a dual functor of $G$-modules and $w_G := (1,0) \in R \times B^* = A^*$, we prove that $\mathbb M^G = w_G \cdot \mathbb M$ and $\mathbb F = w_G \cdot \mathbb M \oplus (1-w_G) \cdot \mathbb M$; hence, the Reynolds operator can defined on $\mathcal M$.
Let $G={\rm Spec} A$ be a linearly reductive group and let $w_G\in A^*$ be the invariant integral on $G$. We establish the harmonic analysis on $G$ and we compute $w_G$ when $G=Sl_n, Gl_n, O_n, Sp_{2n}$ by geometric arguments and by means of the Fourier transform.