Subgroup of Integers is Ideal
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Theorem
Let $\struct {\Z, +}$ be the additive group of integers.
Every subgroup of $\struct {\Z, +}$ is an ideal of the ring $\struct {\Z, +, \times}$.
Corollary
Every subring of $\struct {\Z, +, \times}$ is an ideal of the ring $\struct {\Z, +, \times}$.
Proof
Let $H$ be a subgroup of $\struct {\Z, +}$.
Let $n \in \Z, h \in H$.
Then from the definition of cyclic group and Negative Index Law for Monoids:
- $n h = n \cdot h \in \gen h \subseteq H$
The result follows.
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {IV}$: Rings and Fields: $25$. Cyclic Groups and Lagrange's Theorem: Theorem $25.2$
- 1969: C.R.J. Clapham: Introduction to Abstract Algebra ... (previous) ... (next): Chapter $5$: Rings: $\S 21$. Ideals: Example $34$