Normality satisfies transfer condition

This article gives the statement, and possibly proof, of a subgroup property satisfying a subgroup metaproperty
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Statement

Verbal statement

If a subgroup is normal in the group, its intersection with any other subgroup is normal in that subgroup.

Symbolic statement

Let $H\le G$ be a normal subgroup and let $K$ be any subgroup of $G$. Then, $H\cap K◃K$.

Property-theoretic statement

The subgroup property of being normal satisfies the transfer condition.

Definitions used

Normal subgroup

A subgroup $H$ of a group $G$ is said to be normal if for any $g\in G$ and $h\in H$, $gh{g}^{-1}\in H$.

Transfer condition

A subgroup property $p$ is said to satisfy transfer condition if whenever $H,K$ are subgroups of $G$ and $H$ has property $p$ in $G$, $H\cap K$ has property $p$ in $K$.

Related facts

• Second isomorphism theorem: This result equates the quotient of the non-normal subgroup, by the intersection, with the quotient of the product of subgroups, by the normal subgroup.

Generalizations

Stronger metaproperties satisfied by normality

• Normality satisfies inverse image condition

Proof

Hands-on proof

Given: A group $G$, a normal subgroup $H◃G$ and a subgroup $K\le G$

To prove: $H\cap K◃K$. In other words, we need to prove that given any $g\in K$ and $h\in H\cap K$, $gh{g}^{-1}\in H\cap K$.

Proof: Since $h\in H\cap K$, we in particular have $h\in H$. Since $H◃G$ (viz $H$ is normal in $G$), $gh{g}^{-1}\in H$.

But we also have that $g\in K$ and $h\in K$. Since $K$ is a subgroup, $gh{g}^{-1}\in K$.

Combining these two facts, $gh{g}^{-1}\in H\cap K$.

References

Textbook references

• Abstract Algebra by David S. Dummit and Richard M. Foote, 10-digit ISBN 0471433349, 13-digit ISBN 978-0471433347, ^{More info}, Page 88, Exercise 24
• Topics in Algebra by I. N. Herstein, ^{More info}, Page 53, Problem 5