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Let $G_1$ and $G_2$ be two abelian groups with respective subgroups $N_1$ and $N_2$. and let $f:G_1 \to G_2$ be a homomorphism. Under what conditions is the induced map $f':G_1/N_1 \to G_2/N_2$ well defined?

I think the only thing to verify is that given any $g_1\in N_1$ we must have $f(g_1)\in N_2$. is there any other condition?

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  • $\begingroup$ Sub-groups of abelian groups are normal. So i think, this should be defined anyway. $\endgroup$ – user9413 May 4 '11 at 9:53
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No other conditions. This works not just for abelian groups, but for any groups $G_1$ and $G_2$. Also, $N_2$ is not very important here. One way of thinking about this in general is: a map from $G_1$ factors through $G_1/N_1$ if and only if the normal subgroup $N_1$ is in its kernel. Now given $f:G_1\rightarrow G_2$ and a normal subgroup $N_2$ of $G_2$, you can always compose with the quotient map to get $$ f:G_1\rightarrow G_2\rightarrow G_2/N_2, $$ and $N_1$ is in its kernel if and only if $f(N_1)\leq N_2$.

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