Analogy between quotient groups and quotient topology

The quotient topology is the unique topology on the set \$X/{sim}\$ of equivalence classes such that for any topological space \$Y\$ and any map \$f:X/{sim}to Y\$, \$f\$ is continuous iff \$varphicirc f\$ is continuous. (Note that in the case where you take \$Y=X/{sim}\$ and \$f\$ to be the identity map, this says in particular that \$varphi\$ itself is continuous.) The analogous characterization also works for the quotient group structure, and pretty much any other quotient structure you’ll ever run into.

Another way to phrase this in the case of topological spaces is that the quotient topology is the finest topology that makes \$varphi\$ continuous. Indeed, for any topology that makes \$varphi\$ continuous, \$varphicirc f\$ will be continuous whenever \$f\$ is continuous. So assuming \$varphi\$ is continuous, the condition above is just that \$f\$ is continuous whenever \$varphicirc f\$ is: that is, there are as many continuous maps out of \$X/{sim}\$ as possible (given the condition that \$varphi\$ is continuous). More continuous maps out of a space means a finer topology, so this is saying it’s the finest topology for which \$varphi\$ is continuous.

Both these quotients have the following universal property:
Let \$X\$ be a group (topological space). In the case of groups, you can associate a equivalence relation to every normal subgroup, which is \$xsim y \$ if they belong to the same coset. Then \$G/N\$ is just \$G/sim \$ as a set.

So now we just view the common problem of going mod an equivalence relation. The quotient structure on \$X/sim \$ is one that makes the quotient map \$p: Xto X/sim \$ have the universal property that given a map (continous or group homomorphism respectively) \$g:Xto Y\$ with \$f(x)=f(y)\$ for all \$xsim y\$ then there exists a unique \$bar{g}:X/sim to Y\$ such that \$g=pcircbar{g}\$.

You can try and workout that in the case of groups, this universal property makes sure that the quotient group structure on \$G/N\$ is the only one where \$Gto G/sim\$ is a group homomorphism. But for groups we can’t go mod arbitrary equivalence relations only those arising from normal subgroups.

On the other hand for topological spaces, there can be other topological space structures on \$X/sim \$ which make \$Xto X/sim\$ continuous, but only the quotient map has the universal property.