Which of the following functions is an involution on R∖{0}\mathbb{R} \setminus \{0\}R∖{0}?
f(x)=x2f(x) = x^2f(x)=x2
f(x)=1xf(x) = \frac{1}{x}f(x)=x1
f(x)=2xf(x) = 2xf(x)=2x
f(x)=x+1f(x) = x+1f(x)=x+1