f(a,b) = f(a +b)

[Steve Ingrassia]

I do not understand your question.  If the function "f" is a function of
two variables, as on the left-hand side of your equal sign, then the
right-hand side, "f(a+b)" is undefined.  If "f" is a function of a single
variable, then "f(a,b)" makes no sense.