when a function is defined from Q to Q,why is it necessary to find if it's true for N,Z etc.is it necessary for other domains and codomains also?
in many solutions there are only specific solutions.like the exp below:
\[f(x+y)+f(x)f(y)=f(xy)+f(x)+f(y)\]
the sol said:notice that f(x)=0 and f(x)=2 are obviously the solutions.then do i need to find the sol by thinking only in certain equations?
