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Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.

7 votes
1 answer
789 views

Extensions of torsion modules

Given a regular local ring $R$ and an $R$-algebras $S$, which is torsion free and finitely generated (even free if needed) as an $R$-module. Assume we have a nontrivial surjective map $f: M \rightarr …
TonyS's user avatar
  • 1,391
5 votes
1 answer
2k views

Length of a module over different rings

Given a regular local ring $(R,m)$ and a finitely generated $R$-algebra $S$, which is free as an $R$-module. Let $M$ be a left $S$-module of finite length, $\ell_S(M)=r<\infty$. Under what conditions …
TonyS's user avatar
  • 1,391
4 votes
1 answer
549 views

Base change and relative Ext over noncommutative rings

Given two smooth projective schemes $X$ and $Y$ over some algebraically closed field $k$, we have $X\times Y$ with the projections $p$ to $X$ and $q$ to $Y$. Furthermore we have a "nice" sheaf of alge …
TonyS's user avatar
  • 1,391
3 votes
2 answers
580 views

Comparing homomorphisms over different base rings

I am trying to compare some homomorphism groups over different base rings, so given a commutative local ring $(A,\mathfrak{m})$ and a finite dimensional Azumaya algebra $R$ over $A$. If $M$ and $N$ …
TonyS's user avatar
  • 1,391
3 votes
2 answers
462 views

Chern character of Hom-sheaves

I'm reading the book about moduli spaces by Huybrechts and Lehn, and i'm stuck understanding a proof, it is Theorem 6.1.8.: Given a K3-surface $X$ and a 2-dimensional space $M$, coherent and torsion …
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  • 1,391
3 votes
1 answer
572 views

When does the forgetful functor S-Mod -> R-Mod induce injective maps on Ext-groups?

Assume we have a complete regular local ring $R$ and an $R$-algebra $S$. Is there a class of such algebras $S$ with the following property: Given two $S$-modules $M,N$, then the maps induced by the …
TonyS's user avatar
  • 1,391
2 votes
2 answers
967 views

Torsion in tensor products over noncommutative rings

I know that the problem of torsion in tensor products, even of torsion free modules, is a very delicate thing. Unfortunately i don't have a deeper insight into this subject, so i don't know how things …
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  • 1,391
2 votes
0 answers
245 views

Is simplicity preserved under completion of the base ring?

Let $(A,\mathfrak{m})$ be a noetherian local ring and $R$ be an $A$-algebra, which is finitely generated generated as an $A$-module (module finite $A$-algebra). Let $\widehat{A}$ be the $\mathfrak{m}$ …
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  • 1,391
2 votes
1 answer
790 views

How does torsion behave under the direct image functor?

Assume we have a finite morphism $f: X\rightarrow Y$ of smooth projective varieties of degree $d$ over $k=\mathbb{C}$. Then $f_{*}$ induces an equivalence between the categoy of coherent $O_X$-modules …
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  • 1,391
2 votes
1 answer
1k views

Direct image sheaf and tensor product (is the projection formula an isomorphism?)

Assume we have two "nice" schemes $X$ and $Y$ over $k=\mathbb{C}$, a finite flat map $f:X\rightarrow Y$ and a k-algbera $A$. Then we get an induced finite flat map $f_A:X\times_k A \rightarrow Y\times …
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  • 1,391
2 votes
1 answer
686 views

When is the restriction map on global sections an embedding

Given a scheme $X$ with generic point p and a quasi-coherent sheaf $F$ on $X$. Viewing $X$ as a scheme over $Spec(\mathbb{Z})$, let us assume $f: X \rightarrow Spec(\mathbb{Z})$ is a proper map. Wha …
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  • 1,391
2 votes
3 answers
645 views

Connection: locally free - locally projective

Given a smooth projective variety $X$ over some algebraically closed field $k$ and a locally free sheaf $R$ of $O_X$-algebras, e.g. central simple algebras or orders. If $M$ is a left $R$-module whic …
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  • 1,391
1 vote
1 answer
273 views

Q-Divisor and Determinant Map on a Maximal Order

Given a smooth projective surface $X$, let $A$ be a sheaf of maximal orders in a division ring. Let us for simplicity assume $A$ ramifies in one curve $C$ with ramification index $e$. Let $A^*$ be the …
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1 vote
1 answer
166 views

Why is multiplication with a scalar no global morphism?

Given a smooth projective surface $S$ over an algebraically closed field, a sheaf rings or algebras $R$ on $S$ and a simple left $R$-module $M$, i.e. $Hom_R(M,M)=k$.Then we have $Hom_R(M,M(-i))=H^{0}( …
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1 vote

Chern character of Hom-sheaves

This was to long for a comment, so i post this as an answer: Using Sasha's answer i tried my best, and here are my computations. Feel free to report any mistakes. Take a locally free resolution $G_{ …
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