Which of the following functions f:R→Rf: \mathbb{R} \to \mathbb{R}f:R→R is an involution?
f(x)=x+1f(x) = x+1f(x)=x+1
f(x)=1/xf(x) = 1/xf(x)=1/x
f(x)=1−xf(x) = 1-xf(x)=1−x
f(x)=x2f(x) = x^2f(x)=x2