Monday, January 23, 2017

Functional Equation f(x+y)=f(x)+f(y)+f(x)f(y)

I need to find all the continuous functions from RR such that f(x+y)=f(x)+f(y)+f(x)f(y). I know, what I assume to be, the general way to attempt these problems, but I got stuck and need a bit of help. Here is what I have so far:



Try out some cases:



Let y=0: f(x)=f(x)+f(0)+f(x)f(0)0=f(0)+f(x)f(0)0=f(0)[1+f(x)]
Observe that either f(0)=0 or f(x)=1. So this gives me one solution, but I am having trouble finding the other solution(s). Somebody suggested to me that f(x)=0 is also a solution but I can't find a way to prove what they said is true. Can anyone please, without giving away the answer, give me a teeny hint? I really want to figure this out as much as I can. I've tried the case when y=x and x=y but I don't feel like those cases help me towards the solution.



Thanks in advance

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