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This page is a dire page and is in desperate need of an update.
The message is:
This must have been one of the first pages, from Feb 2015!

A bijection is a 1:1 map. A map which is both injective and surjective.

It has the useful property that for [math]f:X\rightarrow Y[/math] that [math]f^{-1}(y)[/math] is always defined, and is at most one element.

Thus [math]f^{-1}[/math] behaves as a normal function (rather than the always-valid but less useful [math]f^{-1}:Y\rightarrow\mathcal{P}(X)[/math] where [math]\mathcal{P}(X)[/math] denotes the power set of [math]X[/math])

Statements to add (to new version of the page, 'cos current version is dire)