Invariant of an equivalence relation

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Definition

Let $S$ be a set and let $\sim\subseteq S\times S$ be an equivalence relation on $S$, let $W$ be any set and let $f:S\rightarrow W$ be any function from $S$ to $W$. Then:

• We say "$f$ is an invariant of $\sim$" if:
  • $\forall a,b\in S[a\sim b\implies f(a)=f(b)]$ - in other words, $f$ is constant on the equivalence classes of $\sim$.

Complete invariant

With the setup of $S$, $W$, $\sim$ and $f:S\rightarrow W$ as above define a "complete invariant" as follows:

• "$f$ is a complete invariant of $\sim$" if:
  • $\forall a,b\in S[a\sim b\iff f(a)=f(b)]$ - in other words, $f$ is constant on and distinct on the equivalence classes of $\sim$.

Terminology

It's hard to be formal in English, however we may say any of the following:

• "$f$ is an invariant of $\sim$"
• "$\sim$ is invariant under $f$"
  • This makes sense as we're saying the $a\sim b$ property holds (doesn't vary) "under" (think "image of $A$ under $f$"-like terminology) $f$, that $f(a)=f(b)$

Examples and instances

• Homotopy invariance of path concatenation
  • Homotopy invariance of loop concatenation

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TODO: Create a category and start collecting

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See also

• Complete system of invariants - a finite set of complete invariants really.
• Set of canonical forms - a subset of $S$, $C\in\mathcal{P}(S)$, such that there exists a unique $c\in C$ such that $c\sim s$
  • An equivalent condition to the axiom of choice is that every partition has a set of representatives that's closely related. Be warned

References

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More on the fundamentals of mathematics would be good