# Infinite products of topological groups

While studying for a topological groups course, I wondered if we could define the product of uncountably many topological groups such that the product is still a topological group. That is: let $G_i$ be a topological group with product law $p_i$ for each $i \in I$ (with $I$ uncountable). We can give $G = \prod_{i \in I} G_i$ the (Tychonoff) product topology and define the product law of $G$ by:

$\pi_i \circ p = p_i$ for all $i \in I$.

However, when trying to prove that this mapping is continuous end up needing $I$ to be at most countable or that the topologies of $G_i$ be discrete.

Is there any way to get around this?

Thanks.

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The tag [topological-groups] already exists. I edited your question for you though. –  Harry Gindi Feb 13 '10 at 18:11
It is true though that some nice properties of topological spaces are lost by taking uncountable products. For instance, given a nonempty family of topological spaces, uncountably many of which are not endowed with the trivial topology, the product is not first-countable. In particular, metrizability is lost in uncountable products. –  Pete L. Clark Feb 13 '10 at 23:12
"...nonempty family of NONEMPTY topological spaces" :) –  Pete L. Clark Feb 14 '10 at 1:48
... not that $G_i$ are compact implies that the product is compact. It is often an issue that this is not true for locally compact. here, one prefers the restricted product. –  plusepsilon.de Jan 22 '12 at 15:31

You can define the product of an arbitrary family $(G_i)_{i \in I}$ of topological groups $G_i$ by equipping the group-theoretic product $G = \prod_{i \in I} G_i$ with the product topology; the product topology is indeed compatible with the group structure (confer Bourbaki, General topology, III.2.9, but it's pretty obvious actually).
Perhaps your problem is the product topology? Note that a basis for the product topology are the sets $(U_i)_{i \in I}$ where $U_i \subseteq G_i$ is open and $U_i = G_i$ for all but finitely many $i \in I$. (confer wiki for the product topology).