Which function is periodic?
f(x)=x2f(x) = x^2f(x)=x2
f(x)=exf(x) = e^xf(x)=ex
f(x)=sin(x)f(x) = \sin(x)f(x)=sin(x)
f(x)=ln(x)f(x) = \ln(x)f(x)=ln(x)