# ind-variety

Let $\operatorname{\mathbb{K}}$ be a field. An ind-variety over $\operatorname{\mathbb{K}}$ is a set $X$ along with a filtration  :

 $X_{0}\subset X_{1}\subset\cdots X_{n}\subset\cdots$

such that

1. 1.

$X=\bigcup\limits_{j\geq 0}X_{j}$

2. 2.

Each $X_{i}$ is a finite dimensional algebraic variety over $\operatorname{\mathbb{K}}$

3. 3.

The ring of regular functions  on an ind-variety $X$ is defined to be $\operatorname{\mathbb{K}}[X]:=\varprojlim\operatorname{\mathbb{K}}[X_{j}]$ where the limit is taken with respect to the family of maps $\left\{i_{j}^{*}\colon\operatorname{\mathbb{K}}[X_{j+1}]\to\operatorname{% \mathbb{K}}[X_{j}]\right\}_{j\geq 0}$.

This ring is given the structure  of a topological ring by letting each $\operatorname{\mathbb{K}}[X_{j}]$ have the discrete topology and $\operatorname{\mathbb{K}}[X]$ have the induced inverse limit topology   , i.e. the topology induced from the canonical inclusion $\varprojlim\operatorname{\mathbb{K}}[X_{j}]\subset\prod_{j}\operatorname{% \mathbb{K}}[X_{j}]$ and the product topology on $\prod_{j}\operatorname{\mathbb{K}}[X_{j}]$.

An ind-variety is called affine (resp. projective) if each $X_{j}$ is affine (resp. projective).

The notion of an ind-variety goes back to Igor Shafarevich in  and .

## Examples

Let $\mathcal{K}:=\operatorname{\mathbb{K}}((t))$ be the ring of formal Laurant series over $\operatorname{\mathbb{K}}$ and $\mathcal{O}:=\operatorname{\mathbb{K}}[[t]]$ be its ring of integers, the formal Taylor series  . Let $V=\operatorname{\mathbb{K}}^{n}$. Then the set $X$ of $\mathcal{O}$-lattices ($\mathcal{O}$-submodules of maximal rank) in $V\otimes_{\operatorname{\mathbb{K}}}\mathcal{K}$ is an example of a (non-finite dimensional) projective ind-variety using the filtration

 $X_{i}:=\left\{L\in X\mid t^{i}L_{0}\subset L\subset t^{-i}L_{0},\dim_{% \operatorname{\mathbb{K}}}L/t^{i}L_{0}=in\right\}$

where $L_{0}:=V\otimes_{\operatorname{\mathbb{K}}}\mathcal{O}$.

## References

• 1 George Lusztig, Singularities, character formulas, and a q-analog of weight multiplicities, Astérisque 101-102 (1983), pp. 208-229.
• 2 Shrawan Kumar, Kac-Moody Groups, their Flag Varieties and Representation Theory. Progress in Mathematics Vol. 204. Birkhauser, 2002.
• 3 Igor Shafarevich, On some infinite-dimensional groups. II Math USSR Izvestija 18 (1982), pp. 185 - 194.
• 4 Igor Shafarevich, Letter to the editors: ”On some infinite-dimensional groups. II“ Izv. Ross. Akad. Nauk. Ser. Mat. 59 (1995), pp. 224 - 224.
Title ind-variety Indvariety 2013-03-22 15:30:56 2013-03-22 15:30:56 benjaminfjones (879) benjaminfjones (879) 7 benjaminfjones (879) Definition msc 14A10 msc 14L15 ind-variety