d/dx f(x) = f(x)

Are there any other functions from R to R whose derivative everywhere equals itself, beside x -> a*e^x for constant a in R?

My calc 2 class basically started with d/dx f(x) = f(x) then did some shit with some properties and showed that the solution was e^(x).  So yeah its just e^(x).

If you only care about magnitudes and allow complex numbers, e^(sqrt(-1)*t) would qualify.  Its magnitude is 1 everywhere (just changed direction in the complex plane) and its derivative acts the same, just 90 degrees off.