mpenciak / twist

Definition of Serre's twisting sheaf (work in progress)

Serre's Twisting Sheaf

This repo is aimed to formalise the concept of Serre's Twisting Sheaf. But before this, we need to build some missing pieces.

Localized Module

We need localized module, because we want the definition of quasicoherent sheaves which can be defined as locally $\tilde{M_i}$ over $\mathrm{Spec} R_i$.

So for a commutative ring $R$, a multiplicative subset $S\subseteq R$ and $R$-module $M$. We can defined the localized module $M_S$ (an $R_S$-module) to be $$ \left{[m, s]|[m, s]\sim[n, t]\iff\exists u\in S,u\cdot t\cdot m = u\cdot s\cdot n\right} $$

See src/module_localisation/bsaic.lean

Integer grading from natural number grading

Sometimes it is convinient to trade a natrual number graded algebra as an integer graded algebra by taking negative grade to be ${0}$. This is implemented at src/grading/nat_to_int.lean. We want this because we want to shift a grading by an arbitrary $m\in\mathbb Z$.

Graded modules and twisting

Given a graded ring $A\cong\bigoplus_i A_i$, an $A$-module $M\cong\bigoplus_j M_j$ is said to be graded if $a_i\cdot m_j\in M_{i+j}$ for all $a_i\in A_i$ and $m_j\in M_j$. Given a graded $A$-module $M$, we can twist $M$ by $j$ to give another graded $A$-module where the $i$-th grade component $M'j$ is defined as $M{i+j}$ . To verify this is a graded module with respect to this new grading, one needs to verify that $M\cong\bigoplus_i M'_i$ and this grading respects scalar multiplication. These are defined in src/grading/graded_module.lean.

$\tilde{M}$

So given a commutative ring $R$ and $R$-module $M$, we can define a sheaf of abelian group $\tilde{M}$ on $\mathrm{Spec} R$ by asking $$ U\mapsto \prod_{p\in U}M_p. $$

Then this is a $\mathrm{Spec} R$-module. This is defined in src/MSpec/basic.lean. I called this MSpec for module on spec.