Complex analytic variety

Denote the constant sheaf on a topological space with value ${\displaystyle \mathbb {C} }$ by ${\displaystyle {\underline {\mathbb {C} }}}$. A ${\displaystyle \mathbb {C} }$-space is a locally ringed space ${\displaystyle (X,{\mathcal {O}}_{X})}$, whose structure sheaf is an algebra over ${\displaystyle {\underline {\mathbb {C} }}}$.

Choose an open subset ${\displaystyle U}$ of some complex affine space ${\displaystyle \mathbb {C} ^{n}}$, and fix finitely many holomorphic functions ${\displaystyle f_{1},\dots ,f_{k}}$ in ${\displaystyle U}$. Let ${\displaystyle X=V(f_{1},\dots ,f_{k})}$ be the common vanishing locus of these holomorphic functions, that is, ${\displaystyle X=\{x\mid f_{1}(x)=\cdots =f_{k}(x)=0\}}$. Define a sheaf of rings on ${\displaystyle X}$ by letting ${\displaystyle {\mathcal {O}}_{X}}$ be the restriction to ${\displaystyle X}$ of ${\displaystyle {\mathcal {O}}_{U}/(f_{1},\ldots ,f_{k})}$, where ${\displaystyle {\mathcal {O}}_{U}}$ is the sheaf of holomorphic functions on ${\displaystyle U}$. Then the locally ringed ${\displaystyle \mathbb {C} }$-space ${\displaystyle (X,{\mathcal {O}}_{X})}$ is a local model space.

A complex analytic variety is a locally ringed ${\displaystyle \mathbb {C} }$-space ${\displaystyle (X,{\mathcal {O}}_{X})}$ which is locally isomorphic to a local model space.

Morphisms of complex analytic varieties are defined to be morphisms of the underlying locally ringed spaces, they are also called holomorphic maps.

The associated complex analytic space (variety) ${\displaystyle X_{h}}$ is says that;[1]

Let X is scheme finite type over ${\displaystyle \mathbb {C} }$, and cover X with open affine subset ${\displaystyle Y_{i}=\operatorname {Spec} A_{i}}$ (${\displaystyle X=\cup Y_{i}}$). Then each ${\displaystyle A_{i}}$ is an algebra of finite type over ${\displaystyle \mathbb {C} }$, and ${\displaystyle A_{i}\simeq \mathbb {C} [z_{1},\dots ,z_{n}]/(f_{1},\dots ,f_{m})}$. Where ${\displaystyle f_{1},\dots ,f_{m}}$ are polynomial in ${\displaystyle z_{1},\dots ,z_{n}}$, which can be regarded as a holomorphic function on ${\displaystyle \mathbb {C} }$. Therefore, their common zero of the set is the complex analytic subspace ${\displaystyle (Y_{i})_{h}\subseteq \mathbb {C} }$. Here, scheme X obtained by glueing the data of the set ${\displaystyle Y_{i}}$, and then the same data can be used to glueing the complex analytic space ${\displaystyle (Y_{i})_{h}}$ into an complex analytic space ${\displaystyle X_{h}}$, so we call ${\displaystyle X_{h}}$ a associated complex analytic space with X. The complex analytic space X is reduced if and only if the associated complex analytic space ${\displaystyle X_{h}}$ reduced.

• Algebraic variety - Roughly speaking, an (complex) analytic variety is a zero locus of a set of an (complex) analytic function, while an algebraic variety is a zero locus of a set of a polynomial function and allowing singular point, especially if the algebraic variety is algebraically closed field, then that is reduced.
• Analytic space
• Complex algebraic variety
• GAGA