Radical of Ideal Preserves Inclusion
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Theorem
Let $A$ be a commutative ring with unity.
Let $\mathfrak a \subseteq \mathfrak b \subseteq A$ be an ideals.
Then we have an inclusion of their radicals:
- $\operatorname{Rad} \left({\mathfrak a}\right) \subseteq \operatorname{Rad} \left({\mathfrak b}\right)$
Proof
Let $x \in \operatorname{Rad} \left({\mathfrak a}\right)$.
We show that $x \in \operatorname{Rad} \left({\mathfrak b}\right)$.
By definition of radical, there exists $n \in \N$ such that the power $x^n \in \mathfrak a$.
Because $\mathfrak a \subseteq \mathfrak b$, also $x^n \in \mathfrak b$.
Thus $x \in \operatorname{Rad} \left({\mathfrak b}\right)$.
$\blacksquare$