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For a $k$-functor $X\in coPsh(M_k)$ and $E\subseteq O(X)=M_k(X,O_k)$ a set of functions on $X$, Definition in k-ring?, we define
and
(The ‘’unit ideal of $R$’‘ is just $R$ itself.) For a transformation $u:Y\to X$ of $k$-functors and $Z\subseteq X$ a subfunctor we define
A subfunctor $Y\subseteq X$ is called open subfunctor resp. closed subfunctor if for every transformation $u:T\to X$ we have $u^{-1}(Y)$ is of the form $V(E)$ resp. $D(E)$.
A $k$-functor $X$ is called a $k$-scheme if the following two conditions hold:
($X$ is a sheaf for the Zarisky Grothendieck topology on $M_k^{op}$) For all $k$-rings and all families $(f_i)_i$ such that $R=\coprod_i R f_i$ we have: if for all $x_i\in R[f_i^{-1}]$ such that the images of $x_i$ and $x_j$ coincide in $X(R[f_i^{-1} f_j^{-1}])$ there is a unique $x\in X(R)$ mapping to the $x_i$.
($X$ has a cover of Zarisky open immersions of affine $k$-schemes) There exists a family $(U_i)_i$ of open affine subfunctors of $X$ such that for all fields $K\in M_k$ we have that $X(K)=\coprod_i U_i(K)$.
The category of $k$-schemes is closed under limits, forming open- and closed subfunctors and skalar extension.