# Odd Integers do not form Integral Domain

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## Theorem

Let $S$ denote the set of odd integers.

Then $\struct {S, +, \times}$ is not an integral domain.

## Proof

Consider the odd integers $1$ and $3$.

We have that $1 + 3 = 4$.

But $4$ is not odd.

Thus addition on $\struct {S, +, \times}$ is not closed.

Hence $\struct {S, +, \times}$ is not even a ring, let alone an integral domain.

$\blacksquare$