Quantum Computing.
11, ƒ = ƒ 11 = 11¹ because the first bit is 1, ƒ = ƒ² = 11² = 121, ƒ = ƒ mod 21 = 121 mod 21 = 16, ƒ = ƒ m = 16 x 11 = 176 because the second bit is 1, ƒ = ƒ² = 176² = 30976, nothing to do because the 3rd bit is 0, ƒ = ƒ² = 30976² = 959512576, ƒ = ƒ mod 21 = 959512576 mod 21 = 1 and ƒ = ƒ m = 11