higher geometry / derived geometry
geometric little (∞,1)-toposes
geometric big (∞,1)-toposes
derived smooth geometry
Given a (typically algebraically closed) field $k$, an algebraic $k$-group is a group object in the category of $k$-varieties.
There are two important classes of algebraic groups whose intersection is trivial (the identity group): Linear algebraic groups and abelian varieties.
Any algebraic group contains a unique normal linear algebraic subgroup $H$ such that their quotient $G/H$ is an abelian variety.
An algebraic $k$-group is linear if it is a Zariski-closed subgroup of the general linear group $GL(n,k)$ for some $n$.
An algebraic group is linear iff it is affine.
An algebraic group scheme is affine if the underlying scheme is affine.
The category of affine group schemes is the opposite of the category of commutative Hopf algebras.
Another important class are commutative algebraic $k$-groups whose underlying variety is projective, namely the abelian varieties; in dimension $1$ these are precisely the elliptic curves. If $k$ is a perfect field and $G$ an algebraic $k$-group, the theorem of Chevalley says that there is a unique linear subgroup $H\subset G$ such that $G/H$ is an abelian variety.
An abelian variety of dimension $1$ is called an elliptic curve.
Some of the definitions of the following classes exist more generally for group schemes.
(…)
(See also more generally unipotent group scheme.)
An element $x$ of an affine algebraic group is called unipotent if its associated right translation operator $r_x$ on the affine coordinate ring? $A[G]$ of $G$ is locally unipotent as an element of the ring of linear endomorphism of $A[G]$ where ”locally unipotent” means that its restriction to any finite dimensional stable subspace of $A[G]$ is unipotent as a ring object.
(Jordan-Chevalley decomposition?) Any commutative linear algebraic group over a perfect field is the product of a unipotent and a semisimple algebraic group.
The group objects in the category of algebraic schemes and formal schemes are called (algebraic) group schemes and formal groups, respectively.
Among group schemes are ‘the infinite-dimensional algebraic groups’ of Shafarevich.
Algebraic analogues of loop groups are in the category of ind-schemes. All linear algebraic $k$-groups are affine.
The affine line $\mathbb{A}^1$ comes canonically with the structure of a group under addition: the additive group $\mathbb{G}_a$.
The affine line without its origin, $\mathbb{A}^1 - \{0\}$ comes canonically with the structure of a group under multiplication: the multiplicative group $\mathbb{G}_m$.
The standard references are
M. Demazure, P. Gabriel, Groupes algebriques, tome 1 (later volumes never appeared), Mason and Cie, Paris 1970
M. Artin, J. E. Bertin, M. Demazure, P. Gabriel, A. Grothendieck, M. Raynaud, J.-P. Serre, Schemas en groupes, i.e. SGA III-1, III-2, III-3
A. Borel, Linear algebraic groups, Springer (2nd edition much expanded)
W. Waterhouse, Introduction to affine group schemes, GTM 66, Springer 1979.
S. Lang, Abelian varieties, Springer 1983.
D. Mumford, Abelian varieties, 1970, 1985.
J. C. Jantzen, Representations of algebraic groups, Acad. Press 1987 (Pure and Appl. Math. vol 131); 2nd edition AMS Math. Surveys and Monog. 107 (2003; reprinted 2007)
T. Springer, Linear algebraic groups, Progress in Mathematics 9, Birkhäuser Boston (2nd ed. 1998, reprinted 2008)