Ramblings on quasi-projective varieties

by ayushkhaitan3437

This blog post is mean to be an exposition on quasi-projective varieties, something that I am having problems understanding. A quasi-projective variety is a locally closed projective variety. What does that mean? It means that it is the intesection of a Zariski open set and a Zariski closed set in some projective space. Does this align with what a locally closed set means? This article  would confirm that this is indeed the case.

The wikipedia article on quasi-projective varieties states that an affine space is an open set in a projective space. How is this? I can surely understand that an affine space can be embedded in projective space. Then? Oh c’mon. Say the affine space A^n is embedded as U_i in \Bbb{P}^n. The variety that lies in its complement is z_i=0. Hence, being the complement of a closed set, the affine subspace is open.

As any affine variety in \Bbb{A}^n can be embedded in \Bbb{P}^n as the intersection of its topological closure in \Bbb{P}^n and the affine chart U_i, we see that any affine variety is a quasi-projective variety.

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