> **1 Preliminaries.**

> **1.1 Functoriality of Presheaves.**

Let $\phi\colon R\longrightarrow S$ be a morphism of rings. This induces a functor
$$f\colon\mathbf{Alg}_S\longrightarrow\mathbf{Alg}_R$$
taking an $S$-algebra $S\rightarrow A$ to the $R$-algebra $R\rightarrow S\rightarrow A$.

As remarked in [this nLab page](https://ncatlab.org/nlab/show/functoriality+of+categories+of+presheaves) (and developed in detail in SGA IV, Exposé I, Section 5), we get an adjoint triple of functors between the corresponding presheaf categories:

[adjunction][1]

where $f^*\colon\mathrm{PSh}(\mathbf{Alg}_R)\longrightarrow\mathrm{PSh}(\mathbf{Alg}_S)$ is given by precomposition with $f$.

> **1.2 Relation to Schemes.**

We may restrict $f^*$ to the full subcategory $\mathbf{Aff}_R$ of $\mathrm{PSh}(\mathbf{Alg}_R)$ spanned by the representable presheaves on $\mathbf{Alg}_R$, i.e. by affine $R$-schemes.

This procedure gives a functor
$$f^*|_{\mathbf{Aff}_R}\colon\mathbf{Aff}_R\longrightarrow\mathbf{Aff}_S$$
sending an affine $R$-scheme $\mathrm{Spec}(A)\rightarrow\mathrm{Spec}(R)$ to the affine $S$-scheme $\mathrm{Spec}(A)\rightarrow\mathrm{Spec}(R)\rightarrow\mathrm{Spec}(S)$.

This leads us to the following questions:

> **2 Questions.**

 1. Is the image of the restriction of the functors $f_!$ and $f_*$ to $\mathbf{Aff}_R$ contained in the category $\mathbf{Aff}_S$? That is, do $f_!$ and $f_*$ send $R$-schemes to $S$-schemes?
 2. If yes, do they admit a nice description in terms of standard scheme-theoretic constructions? (Maybe base change or something like that)
 3. Do we also get a corresponding adjoint triple of functors between $\mathbf{Aff}_R$ and $\mathbf{Aff}_S$ from the triple $(f_!\dashv f^*\dashv f_*)$?

  [1]: https://i.sstatic.net/405KE.png