# Intersection of Subrings Containing Subset is Smallest

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## Theorem

Let $\struct {R, +, \circ}$ be a ring.

Let $S \subseteq R$ be a subset of $R$.

Let $L$ be the intersection of the set of all subrings of $R$ containing $S$.

Then $L$ is the smallest subring of $R$ containing $S$.

## Proof

From Intersection of Subrings is Subring, $L$ is indeed a subring of $R$.

Let $T$ be a subring of $R$ containing $S$.

Let $x, y \in L$.

By the Subring Test, we have that:

\(\displaystyle x - y\) | \(\in\) | \(\displaystyle L\) | |||||||||||

\(\displaystyle x \circ y\) | \(\in\) | \(\displaystyle L\) |

By Intersection is Largest Subset, it follows that $x, y \in T$.

But $T$ is also a subring of $R$.

So, by the Subring Test again, we have that:

\(\displaystyle x - y\) | \(\in\) | \(\displaystyle T\) | |||||||||||

\(\displaystyle x \circ y\) | \(\in\) | \(\displaystyle T\) |

So by definition of subset, $L \subseteq T$.

$\blacksquare$

## Sources

- 1965: Seth Warner:
*Modern Algebra*... (previous) ... (next): $\S 22$: Theorem $22.4$