Linear Operator on General Logarithm

Theorem
Let $\phi: \R^\R \to \R^\R, y \mapsto \map \phi y$ be a linear operator on the space of functions from $\R\to\R$.

Let $y$ be a real function such that:


 * $\forall x \in \R: \map y x > \map \bszero x = 0$.

Let $\log_a y$ be the logarithm of $y$ to base $a$.

Then:


 * $\map \phi {\log_a y} = \dfrac 1 {\ln a} \paren {\map \phi {\ln y} }$

where $\ln$ is the natural logarithm.