Is it appropriate to state a mathematical fact with the word "whenever"?

In your specific example "if" is a reasonably reasonable synonym for "whenever".
But, in mathematics, this is not always the case.

eg "Whenever the value of X changes from negative to positive."
Whenever A decreasing exponentially = B increasing linearly ... "
"Whenever the rotating field vector is at 90 degrees to ..."

In these cases one or both of the compared variables are dynamic and the term "whenever" not only signifies coincidence, but also the fact that one or both variables are changing, and that the equality occurred at a dynamic point in time. Using "if" here also conveys coincidence but loses the time variable nuance.

Somewhat less mathematical but maybe makes the point more clearly:

• The tidal pools are filled whenever the local high-tide level is above 2.1 metres.