Additive Group of Integers is Normal Subgroup of Complex
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Theorem
Let $\struct {\Z, +}$ be the additive group of integers.
Let $\struct {\C, +}$ be the additive group of complex numbers.
Then $\struct {\Z, +}$ is a normal subgroup of $\struct {\C, +}$.
Proof
From Additive Group of Integers is Normal Subgroup of Reals, $\struct {\Z, +} \lhd \struct {\R, +}$.
From Additive Group of Reals is Subgroup of Complex, $\struct {\R, +} \lhd \struct {\C, +}$.
Thus $\struct {\Z, +} \le \struct {\C, +}$.
From Complex Numbers under Addition form Infinite Abelian Group, $\struct {\C, +}$ is abelian.
From Subgroup of Abelian Group is Normal, it follows that $\struct {\Z, +} \lhd \struct {\C, +}$.
$\blacksquare$