Contents

Summary (random tour through the examples)

Let $k$ be some base field. We start with the constant group scheme $E_k$ defined by some classical group $E$ which gives in every component just the group $E$. Next we visit the notion of étale group scheme. This is not itself constant but becomes so by scalar extension to the separable closure $k_sep$ of $k$. The importance of étale affine is that the category of them is equivalent to that of Galois modules by $E\mapsto E \otimes_k k_sep=\cup_{K/k \,sep\,fin} E(K)$

So far these examples ‘’do nothing’‘ with the (multiplicative and additive) structure of the ring in which we evaluate our group scheme. But the next instances do this: We define the additive- and the multiplicative group scheme by $\alpha_k: R\mapsto R^+$ and $\mu_k:R\mapsto R^\times$ sending a $k$-ring to to its underlying additive- and multiplicative group, respectively. These have the ‘’function rings’‘ $O_k(\alpha_k)=k[T]$ and $O_(\mu_k)=K[T,T^{-1}]$ and since $(O_k\dashv Spec_k):k.Ring\stackrel{Spec_k}{\to} k.Aff$ we note that our basic building blocks $\alpha_k$ and $\mu_k$ are in fact representable $k$-functors aka. affine group schemes. We observe that we have $k.Gr(\mu_k,\alpha_k)=0$ and call in generalization of this property any group scheme $G$ satisfying $k.Gr(G,\alpha_k)=0$ multiplicative group scheme. (We could have also the idea to call $G$ satisfying $k.Gr(\mu_k,G)=0$ ‘’additive’‘ but I didn’t see this.) By some computation of the hom spaces $k.Gr(G,\mu_k)$ involving co- and birings we see that these are again always values of a representable $k$-functor $D(G)(-):=(-).Gr(G\otimes_k (-),\mu_{(-)})$; this functor we call the Cartier dual of $G$. If for example $G$ is a finite group scheme $D(G)$ also is, and moreover $D$ is a contravariant autoequivalence (’‘duality’’) of $k.fin.comm.Grp$; in general it is also a duality in some specific sense. By taking the Cartier dual $D(E_k)$ of a constant group scheme we obtain the notion of a diagonlizable group scheme. To justify this naming we compute some value $D(E_k)(R)=hom_{Grp}(E_k\otimes_k R, \mu_R)\simeq hom_{Grp}(E_k,R^\times)\simeq hom_Alg(k[E_k],R)$ where $k[E_k]$ denotes the group algebra of $E_k$ and the last isomorphism is due to the universal property of group rings; we observe that the last equality tells us that $G=Spec\,k[E_k]$ and recall that a $\zeta\in E_k\subset k[E_k]$ is called a character of $G$ (and one calls a group generated by these ‘’diagonalizable’’). Revisiting the condition $k.Gr(H,\alpha_k)=0$ by which we defined multiplicative group schemes and considering a group scheme $G$ satisfying this condition for all sub group-schemes $H$ of $G$ we arrive at the notion of unipotent group scheme. By the structure theorem of decomposition of affine groups we can proof that $G$ is unipotent iff the completion of group schemes (which gives us-by the usual technic of completion- a formal (group) scheme $\hat X$ if $X$ is a group scheme) of the Cartier dual of $G$, i.e. $\hat D(G)$ is a connected formal group scheme also called local group scheme since a local group scheme $Q=Spec_k A$ is defined to be the spectrum of a local ring; this requirement in turn is equivalent to $Q(K)=hom(A,K)=\{0\}$ hence the first name ‘’connected’’. There is also a connection between connected and étale schemes: For any formal group scheme there is an essentially unique exact sequence $0\to G^\circ\to G\to \pi_0(G)\to 0$ where $G^\circ$ is connected and $\pi_0(G)$ is étale. Such decomposition in exact sequences we obtain in further cases: $0\to G^{ex}\to G\to G_{ex}\to 0$ where

$k$-group	$G^{ex}$	$G_{ex}$
formal	connected	étale		p.34
finite	infinitesimal	étale	splits if $k$ is perfect	p.35
affine	multiplicative	smooth?	$G/G_{red}$ is infinitesimal	p.43

where a smooth (group) scheme is defined to be the spectrum of a finite dimensional (over k) power series algebra, a (group) scheme is called finite (group) scheme if we restrict in all necessary definitions to $k$-ring which are finite dimensional $k$-vector spaces, and a (group) scheme is called infinitesimal (group) scheme if it is finite and local. If moreover $k$ is a perfect field any finite affine $k$-group $G$ is in a unique way the product of four subgroups $G=a\times b\times c\times d$ where $a\in Fem_k$ is a formal étale multiplicative $k$ group, $b\in Feu_k$ is a formal étale unipotent $k$ group, $c\in Fim_k$ is a formal infinitesimal multiplicative $k$ group, and $d\in Fem_k$ is a infinitesimal unipotent $k$ group.

If we now shift our focus to colimits- or more generally to codirected systems of finite group schemes, in particular the notion of p-divisible group is an extensively studied case because the $p$-divisible group $G(p)$ of a group scheme encodes information on the p-torsion of the group scheme $G$. To appreciate the definition of $G(p)$ we first recall that for any group scheme $G$ we have the relative Frobenius morphism $F_G:G\to G^{(p)}$ to distinguish it from the absolute Frobenius morphism $F^{abs}_G:G\to G$ which is induced by the Frobenius morphism of the underlying ring $k$. The passage to the relative Frobenius is necessary since in general it is not true that the absolute Frobenius respects the base scheme. Now we define $G[p^n]:=ker\; F^n_G$ where the kernel is taken of the Frobenius iterated $n$-times and the codirected system

is then called the $p$-divisible group of $G$.

constant (group) scheme

Recall that $Spec_k k=*$ is the terminal object of $k.Sch$.

$k.Sch$ is copowered (= tensored)? over $Set$. We define the constant $k$-scheme on a set $E$ by

For a scheme $X$ we compute $M_k(E_k,E)=Set(*,X)^E=X(k)^E=Set(E,X(k))$ and see that there is an adjunction

This is just the constant-sheaf-global-section adjunction.

étale (group) scheme

An étale $k$-scheme is defined to be a directed colimit of $k$-spectra $Spec_k k^\prime$ of finite separable field-extensions $k^\prime$ of $k$.

For an étal group scheme $X=colim_{k^\prime \in T} Spec_k k^\prime$ we have

affine (group) scheme

(see also coalgebras, corings and birings in the theory of group shemes)

An affine $k$-scheme $G:=Spec_k A$ is a representable object in $k.Fun$.

We obtain a group law $G\times G\to G$ induced by $A$ if $A$ satisfies the dual axioms of a group object. We denote the structure maps called comultiplication, counit, and converse by

$\Delta:A\to A\otimes A$

$\epsilon: A\to *$

$\sigma:A\to A$

Examples

The additive group $\alpha_k$

The multiplicative group $\mu_k$

The kernels of group homomorphisms. In particular the kernel $ker\, (-)^n:\mu_k\to \mu_k$.

Mapping spaces

formal (group) scheme

local (=connected) group scheme

multiplicative group scheme

diagonalizable group scheme

unipotent group scheme

smooth formal group scheme

$p$-divisible group scheme