Kernel of Composition of Ring Homomorphisms
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Theorem
Let $f : A \to B$ and $g : B \to C$ be ring homomorphisms.
Let $g \circ f$ be their composition.
Then the kernel of $g \circ f$ is the preimage under $f$ of the kernel of $g$:
- $\map \ker {g \circ f} = f^{-1} \sqbrk {\ker g}$
Proof
By definition, the kernel of a ring homomorphism is its kernel when considered as a group homomorphism between the additive groups.
The result follows from Kernel of Composition of Group Homomorphisms.
$\blacksquare$