Natural Numbers under Addition form Commutative Monoid

Theorem
The algebraic structure $\struct {\N, +}$ consisting of the set of natural numbers $\N$ under addition $+$ is a commutative monoid whose identity is zero.

Proof
Consider the natural numbers $\N$ defined as the naturally ordered semigroup.

From the definition of the naturally ordered semigroup, it follows that $\struct {\N, +}$ is a commutative semigroup.

From the definition of zero, $\struct {\N, +}$ has $0 \in \N$ as the identity, hence is a monoid.

Also see

 * Natural Numbers under Addition do not form Group