Subgroup of Subgroup with Prime Index

Theorem
Let $\struct {G, \circ}$ be a group.

Let $H$ be a subgroup of $G$.

Let $K$ be a subgroup of $H$.

Let:
 * $\index G K = p$

where:
 * $p$ denotes a prime number
 * $\index G K$ denotes the index of $K$ in $G$.

Then either:
 * $H = K$

or:
 * $H = G$

Proof
From the Tower Law for Subgroups:


 * $\index G K = \index G H \index H K$

As $\index G K = p$ is prime, either $\index G H = p$ or $\index H K = p$.

Thus either $\index G H = 1$ or $\index H K = 1$.

The result follows from Index is One iff Subgroup equals Group.