Inverse of Identity Element is Itself

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Theorem

Let $\struct {S, \circ}$ be an algebraic structure with an identity element $e$.

Let the inverse of $e$ be $e^{-1}$.

Then:

$e^{-1} = e$

That is, $e$ is self-inverse.


Proof

From Identity Element is Idempotent:

$e \circ e = e$

Hence the result by definition of identity element.

$\blacksquare$


Sources