# Group is B-Algebra Iff All Elements Self-Inverse

## Theorem

Let $\struct {G, \circ}$ be a group whose identity element is $e$.

Then $\struct {G, \circ}$ is also a $B$-algebra if and only if:

- $\forall g \in G: g = g^{-1}$

That is, if and only if all elements of $G$ are self-inverse.

## Proof

Let $\struct {G, \circ}$ be a group such that $\forall g \in G: g = g^{-1}$.

From Group Induces B-Algebra, the algebraic structure $\struct {G, *}$ where $*$ is defined as:

- $\forall a, b \in G: a * b := a \circ b^{-1}$

is a $B$-algebra.

But as $b = b^{-1}$, it follows that:

- $\forall a, b \in G: a * b := a \circ b$

and so $\struct {G, \circ}$ is itself a $B$-algebra.

Now suppose it is not the case that $\forall g \in G: g = g^{-1}$.

Suppose $\struct {G, \circ}$ were a $B$-algebra.

By axiom $(\text A 2)$ of $B$-algebra, the identity element $e$ would be the $0$, as:

- $\forall a \in G: a \circ 0 = a$

As an Identity is Unique, there can be no other element in $G$ which can satisfy the conditions to be the $0$ of a $B$-algebra.

Now as not every element is self-inverse:

- $\exists x \in G: x \circ x \ne e$

Then by axiom $(\text A 1)$ of $B$-algebra it follows that $\struct {G, \circ}$ is not a $B$-algebra.

Hence the result.

$\blacksquare$