# Distribution on a linear algebraic group

In algebraic geometry, given a linear algebraic group G over a field k, a distribution on it is a linear functional $k[G]\to k$ satisfying some support condition. A convolution of distributions is again a distribution and thus they form the Hopf algebra on G, denoted by Dist(G), which contains the Lie algebra Lie(G) associated to G. Over a field of characteristic zero, Cartier's theorem says that Dist(G) is isomorphic to the universal enveloping algebra of the Lie algebra of G and thus the construction gives no new information. In the positive characteristic case, the algebra can be used as a substitute for the Lie group–Lie algebra correspondence and its variant for algebraic groups in the characteristic zero; for example, this approach taken in (Jantzen 1987).

## Construction
### The Lie algebra of a linear algebraic group

Let k be an algebraically closed field and G a linear algebraic group (that is, affine algebraic group) over k. By definition, Lie(G) is the Lie algebra of all derivations of k[G] that commute with the left action of G. As in the Lie group case, it can be identified with the tangent space to G at the identity element.

### Enveloping algebra

There is the following general construction for a Hopf algebra. Let A be a Hopf algebra. The finite dual of A is the space of linear functionals on A with kernels containing left ideals of finite codimensions. Concretely, it can be viewed as the space of matrix coefficients.

## Distributions on an algebraic group
### Definition

Let X = Spec A be an affine scheme over a field k and let I$_x$ be the kernel of the restriction map $A\to k(x)$, the residue field of x. By definition, a distribution f supported at x'' is a k-linear functional on A such that $f(I_{x}^{n})=0$ for some n. (Note: the definition is still valid if k is an arbitrary ring.)
Now, if G is an algebraic group over k, we let Dist(G) be the set of all distributions on G supported at the identity element (often just called distributions on G). If f, g are in it, we define the product of f and g, demoted by f * g, to be the linear functional
    $k[G]{\overset {\Delta }{\to }}k[G]\otimes k[G]{\overset {f\otimes g}{\to }}k\otimes k=k$
where Δ is the comultiplication that is the homomorphism induced by the multiplication $G\times G\to G$. The multiplication turns out to be associative (use $1\otimes \Delta \circ \Delta =\Delta \otimes 1\circ \Delta$) and thus Dist(G) is an associative algebra, as the set is closed under the muplication by the formula:
    (*) $\Delta (I_{1}^{n})\subset \sum _{r=0}^{n}I_{1}^{r}\otimes I_{1}^{n-r}.$
It is also unital with the unity that is the linear functional $k[G]\to k,\phi \mapsto \phi (1)$, the Dirac's delta measure.
The Lie algebra Lie(G) sits inside Dist(G). Indeed, by definition, Lie(G) is the tangent space to G at the identity element 1; i.e., the dual space of $I_{1}/I_{1}^{2}$. Thus, a tangent vector amounts to a linear functional on I$_1$ that has no constant term and kills the square of I$_1$ and the formula (*) implies $[f,g]=f*g-g*f$ is still a tangent vector.
Let ${\mathfrak {g}}=\operatorname {Lie} (G)$ be the Lie algebra of G. Then, by the universal property, the inclusion ${\mathfrak {g}}\hookrightarrow \operatorname {Dist} (G)$ induces the algebra homomorphism:
    $U({\mathfrak {g}})\to \operatorname {Dist} (G).$
When the base field k has characteristic zero, this homomorphism is an isomorphism.

### Examples
#### Additive group

Let $G=\mathbb {G} _{a}$ be the additive group; i.e., G(R) = R for any k-algebra R. As a variety G is the affine line; i.e., the coordinate ring is k[t] and I$^n$ $_0$ = (t$^n$).

#### Multiplicative group

Let $G=\mathbb {G} _{m}$ be the multiplicative group; i.e., G(R) = R$^*$ for any k-algebra R. The coordinate ring of G is k[t, t$^{−1}$] (since G is really GL$_1$(k).)

### Correspondence

* For any closed subgroups H, 'K of G, if k is perfect and H is irreducible, then
       $H\subset K\Leftrightarrow \operatorname {Dist} (H)\subset \operatorname {Dist} (K).$
* If V is a G-module (that is a representation of G), then it admits a natural structure of Dist(G)-module, which in turns gives the module structure over ${\mathfrak {g}}$.
* Any action G on an affine algebraic variety X induces the representation of G on the coordinate ring k[G]. In particular, the conjugation action of G induces the action of G on k[G]. One can show I$^n$ $_1$ is stable under G and thus G acts on (k[G]/I$^n$ $_1$)$^*$ and whence on its union Dist(G). The resulting action is called the adjoint action of G.

## The case of finite algebraic groups

Let G be an algebraic group that is "finite" as a group scheme; for example, any finite group may be viewed as a finite algebraic group. There is an equivalence of categories between the category of finite algebraic groups and the category of finite-dimensional cocommutative Hopf algebras given by mapping G to k[G]$^*$, the dual of the coordinate ring of G. Note that Dist(G) is a (Hopf) subalgebra of k[G]$^*$.