Index of Intersection of Subgroups

Theorem
Let $G$ be a group.

Let $H, K$ be subgroups of finite index of $G$.

Then:
 * $\left[{G : H \cap K}\right] \le \left[{G : H}\right] \left[{G : K}\right]$

where $\left[{G : H}\right]$ is the index of $H$ in $G$.

Equality holds if and only if $HK = \{ hk : h \in H, k \in K \} = G$.

Note that here the symbol $\le$ is being used with its meaning less than or equal to.