Question 26
The function $f$ satisfies the functional equation $$
f(x)+f(y)=f(x+y)-xy-1
$$ for every pair $x,$ $y$ of real numbers. If $f(1)=1,$ then the number of integers $n\ne1$ for which $f(n)=n$ is
f(x)+f(y)=f(x+y)-xy-1
$$ for every pair $x,$ $y$ of real numbers. If $f(1)=1,$ then the number of integers $n\ne1$ for which $f(n)=n$ is