Frobenius endomorphism

An endomorphism $\phi : X \rightarrow X$ of a scheme $X$ over the finite field $\mathbf{F}_q$ of $q$ elements such that $\phi$ is the identity mapping on $X(\mathbf{F}_q)$, and the mapping of the structure sheaf $\phi^* : \mathcal{O}_X \rightarrow \mathcal{O}_X$ is that of raising to the $q$-th power. The Frobenius endomorphism is a purely-inseparable morphism and has zero differential. For an affine variety $X \subset \mathbf{A}^n$ defined over $\mathbf{F}_q$, the Frobenius endomorphism $\phi$ takes the point $(x_1,\ldots,x_n)$ to $(x_1^q,\ldots,x_n^q)$.

The number of geometric points of $X$ that are defined over $\mathbf{F}_q$ is the same as the number of fixed points of $\phi$, which enables one to use the Lefschetz formula to determine the number of such points.

References

 [1] R. Hartshorne, "Algebraic geometry" , Springer (1977) MR0463157 Zbl 0367.14001

Here, $X(\mathbf{F}_q)$ is the set of $\mathbf{F}_q$-points of $X$, i.e. the set of points of $X$ that are defined over $\mathbf{F}_q$.