Modular exponentiaion. See https://en.wikipedia.org/wiki/Modular_exponentiation.
PowerMod
PowerMod[2, 10000000, 3]
PowerMod[3, -2, 10]
PowerMod[0, -1, 2]
PowerMod[5, 2, 0]
PowerMod does not support rational coefficients (roots) yet.