Let $X : \Omega \to \mathcal{X}$ and $Y : \Omega \to \mathcal{Y}$ each be a D5723: Simple random variable.
Let $a \in (0, \infty) \setminus \{ 1 \}$ be a D5407: Positive real number.

Then \begin{equation} I_a(X ; Y) = H_a(X) + H_a(Y) - H_a(X, Y) \end{equation}