Suppose that $X = \mathbb{P}^n\_k$ and $G$ is a coherent sheaf on $X$. 


**Question:** Is there a way to determine *some* integer $n\_0$ such that $H^1(X, G \otimes O\_X(n)) = 0$ for all $n \geq n\_0$? (obviously $n\_0$ depends on $G$)


Of course, the higher cohomologies would be very interesting as well, but perhaps the above is simpler. One can also ask this for the relative case as well $f : Y \to X$ a projective morphism.


What I'd be particularly interested in is something that can be implemented into a computer. 


In my particular case, I know I only have to compute some other things until Serre vanishing hits, at that point, I know I can stop computing. Unfortunately, I don't know how to tell when I've *arrived*. You can assume I have a presentation of my module $G$ if it helps.


**PS:** *Actually, what I really want is vanishing of* $$R^1 f\_\* (G(-nE))$$ *for a blow-up $f : Y \to X$ with $E$ the pullback of the blownup ideal.* 



Effective Serre Vanishing
Take $n\_0$ as (Castelnuovo–Mumford) regularity of $G$ minus $1$.
This generalizes to higher cohomology: for $H^i$ take the regularity minus $i$


Let $M$ be a module representing $G$.
Then, the (Castelnuovo–Mumford) regularity of $M$ is an upper bound of the regularity of $G$.
So for an implementation you can take the regularity of $M$, which can be computed using a free resolution of $M$.


If you need a better bound then you can compute a "better" module for $G$ by approximating its module of global sections.
However, this is rather expensive to my experience.


