Intersection of Ring Ideals is Largest Ideal Contained in all Ideals
Theorem
Let $\struct {R, +, \circ}$ be a ring
Let $\mathbb L$ be a non-empty set of ideals of $R$.
Then the intersection $\bigcap \mathbb L$ of the members of $\mathbb L$ is the largest ideal of $R$ contained in each member of $\mathbb L$.
Proof
Let $L = \bigcap \mathbb L$.
From Intersection of Ring Ideals is Ideal $L$ is indeed an ideal of $R$.
Let $L = \bigcap \mathbb L$.
From Intersection of Subrings is Largest Subring Contained in all Subrings, we have that $L$ is the largest subring of $R$ contained in each member of $\mathbb L$.
As $L$ is the largest subring of $R$ contained in each member of $\mathbb L$, and it is an ideal of $R$, there can be no larger ideal as it would then not be a subring.
So $L$ is the largest ideal of $R$ contained in each member of $\mathbb L$.
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {IV}$: Rings and Fields: $22$. New Rings from Old: Theorem $22.4$