Restricted product
# Restricted product

In mathematics, the restricted product is a construction in the theory of topological groups.

Let $I$ be an index set; $S$ a finite subset of $I$. If $G_{i}$ is a locally compact group for each $i\in I$, and $K_{i}\subset G_{i}$ is an open compact subgroup for each $i\in I\setminus S$, then the restricted product

    $\prod _{i}\nolimits 'G_{i}\,$

is the subset of the product of the $G_{i}$'s consisting of all elements $(g_{i})_{i\in I}$ such that $g_{i}\in K_{i}$ for all but finitely many $i\in I\setminus S$.

This group is given the topology whose basis of open sets are those of the form

    $\prod _{i}A_{i}\,,$

where $A_{i}$ is open in $G_{i}$ and $A_{i}=K_{i}$ for all but finitely many $i$.

One can easily prove that the restricted product is itself a locally compact group. The best known example of this construction is that of the adele ring and idele group of a global field.

