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Just proved if $G$ is isomorphic to $G_1$, $H$ is isomorphic to $H_1$, then $G \times H$ is isomorphic to $G_1 \times H_1$. I am wondering could this be an if and only if statement. Which means, If $G\times H$ is isomorphic to $G _1\times H_1$, must $G$ be isomorphic to either $G_1$ or $H_1$?

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    Awesome. I was trying to use {1} to make some counterexample, but failed. Yours is very good one.2012-02-16

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This isn't even true for integers (for example $4 \times 4 = 2 \times 8$). The corresponding group-theoretic example is that $(C_2^2) \times (C_2^2) \cong C_2 \times (C_2^3)$.

A modified version of this question asks whether $G \cong G_1$ implies $H \cong H_1$. This turns out to be true if $G$ is finite; see Hirshon's On cancellation in groups. This result is clearly false in general for infinite groups (it suffices to find a nontrivial group $G$ such that $G \cong G^2$, and for example the direct sum of countably many copies of any given group works).

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    For the result on cancelling, I give a finitely presented example here: http://www.artofproblemsolving.com/Forum/viewtopic.php?f=61&t=3516372012-02-16
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Direct products are associative in the sense that $(A\times B)\times C \;\cong A\times(B\times C)$ are isomorphic. So we could pick $A=\mathbb{Z}/p\mathbb{Z},\,B=\mathbb{Z}/q\mathbb{Z},\,C=\mathbb{Z}/r\mathbb{Z}$ for distinct primes $p,q,r$ for an easy counterexample.