A function links an input value to an output value. Functions are written in function notation with the name of the function (usually f or g), a variable written in brackets and an expression ...

This is called function notation and is just another way of expressing the relationship between two variables. Therefore, in general y = f (x).

Leo A. Aroian, The Probability Function of the Product of Two Normally Distributed Variables, The Annals of Mathematical Statistics, Vol. 18, No. 2 (Jun., 1947), pp. 265-271 ...