Let $f$ is a function and not constant.Suppose $x,y\in \mathbb{R}$ and $$f(x + y) = f(x) + f(y) - 2f(xy)$$
What is $f(x)$?
Let $f(x + y) = f(x) + f(y) - 2f(xy)$. What is $f(x)$?
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calculus
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1Here http://math.stackexchange.com/questions/1957137/values-of-f1986-knowing-f1-1-and-functional-equation-fa-b-fa is the same question for integer arguments. – 2017-02-15