p-torsion

- group, ∞-group
- group object, group object in an (∞,1)-category
- abelian group, spectrum
- group action, ∞-action
- representation, ∞-representation
- progroup
- homogeneous space

Since any abelian group $G$ is a $\mathbb{Z}$-module we can form for any $z\in \mathbb{N}$ the torsion subgroup

$G[z]:=\{g|g\in G, z g = 0\}$

Of particular interest in this article are those cases where $z=p^n$ for a prime number $p$ and a natural number $n$.

There are two important constructions to perform with these $G[p^n]$ namely taking limits and colimits:

$S_p(G):=colim_n G[p^n]$

$T_p(G):=lim G[p^n]$

Here $S_p(G)$ sometimes is called *$p$-torsion subgroup*; if $G$ is finite $S_p(G)$ is also called *Sylow p-subgroup of $G$*.

$T_p(G)$ is called p-adic Tate module of $G$.

Note that sometimes by “the Tate module” is meant a specific example of a Tate module. This example is mentioned below.

$G[p]$ is obviously the kernel of the Frobenius endomorphism of $G$:

$G[p]=(ker\, (g\mapsto g^p))$

In this form we can extend the Frobenius and hence this notion of $p$-torsion from abelian groups to fields if we require our field to be of characteristic $p$ such that we have $(a+b)^p=a^p+b^p$.

In fact the definition of $p$-torsion via the Frobenius has the advantage that we get additionally an adjoint notion to $p$-torsion which is sometimes called *Verschiebung*; this is explained at Demazure, lectures on p-divisible groups, I.9, the Frobenius morphism.

If $X$ denotes some scheme over a $k$-ring for $k$ being a field of characteristic $p$, we define its $p$-torsion component-wise by $X^{(p)}(R):=X(s_* R)$.

Let $G$ is a commutative group scheme over a scheme $S$. Define the multiplication by $p$ map as follows - $[p]: G \xrightarrow{\Delta} G \times_S .... \times_S G \to G$.

Because $G$ is a commutative group scheme, this is a map in the category of group schemes. The $[p]$ torsion, $G[p]$, is then the pullback along the identity section of the multiplication by $[p]$ map.

The fiber of $G[p]$ at a given $s \in S$ is a group. (Its a group scheme over the residue field of $s$). And it is the p-torsion in the fiber of $G$ at $s$.

The notions of $S_p$, (as an Ind-scheme) and $T_p$ (as a scheme) readily generalize using this notion of $p$-torsion.

(*the* $p$-adic Tate module)

Let $G$ be a commutative group scheme over a field $k$ with separable closure $k^{sep}$.

Then $T_p(G(k^{sep}))$ is called *the $p$-adic Tate module of $G$*.

This Tate module enters the Tate conjecture.

If $G$ is an abelian variety $T_p(G(k^{sep}))$ is equivalently the first homology group of $G$.

(main article: p-divisible group)

Sometimes the information encoded in the colimit $T_p(G)=colim_n G[p^n]$ (we passed a contravariant functor from rings to schemes) is considered to be not sufficient and one wants more generally to study the codirected system

$0\to G[p]\stackrel{p}{\to}G[p^2]\stackrel{p}{\to}\dots \stackrel{p}{\to}G[p^{n}]\stackrel{p}{\to}G[p^{n+1}]\stackrel{p}{\to}\dots$

itself. This system is called *$p$-divisible group of $G$*. Here $p$ denotes the multiplication-with-$p$ map.

We have

(1) The $G[p^i]$ are finite group schemes.

(2) The sequences of the form

$0\to ker\, p^j\xhookrightarrow{\iota_j} ker p^{j+k}\stackrel{p^j}{\to}ker p^k\to 0$

are exact.

(3) $G=\cup_j ker\, p^j\cdot id_G$

We have as cardinality (in group theory also called “rank”) of the first item of the sequence $card \ker \,p=p^h$ for some natural number $h$. By pars pro toto we call $p^h$ also the rank of the whole sequence and $h$ we call its *height*.

Conversely we can define a $p$-divisible group to be a codirected diagram

$G_1\stackrel{i_1}{\to}G_2\stackrel{i_2}{\to}\dots$

satisfying (1)(2)(3).

- see the references at p-divisible group, in particular the notes by Richard Pink. Shatz Group Schemes, Formal Groups, and p-Divisible Groups is also very readable.

Last revised on June 6, 2020 at 16:29:37. See the history of this page for a list of all contributions to it.