Natural Numbers under Multiplication form Subsemigroup of Integers
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Theorem
Let $\struct {\N, \times}$ denote the set of natural numbers under multiplication.
Let $\struct {\Z, \times}$ denote the set of integers under multiplication.
Then $\struct {\N, \times}$ is a subsemigroup of $\struct {\Z, \times}$.
Proof
We have from Natural Numbers under Multiplication form Semigroup that $\struct {\N, \times}$ forms a semigroup.
We have from Integers under Multiplication form Semigroup that $\struct {\Z, \times}$ forms a semigroup.
From Natural Numbers are Non-Negative Integers, we have that $\N \subseteq \Z$.
From the definition of integer multiplication it follows that $\times: \Z \to \Z$ is an extension of $\times: \N \to \N$.
Hence the result.
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): $\S 32$ Identity element and inverses