Determine all functions $f : R_+ \to R$ such that:$$f(x)f(y) = f(xy) + \frac{1}{x} + \frac{1}{y}$$for all $x, y$ positive reals.
Source: 2005 Cuba MO 2.4
Tags: algebra, functional
Determine all functions $f : R_+ \to R$ such that:$$f(x)f(y) = f(xy) + \frac{1}{x} + \frac{1}{y}$$for all $x, y$ positive reals.