complex number

Powers of i cycle in groups of 4:

i^0 = 1

i^1 = i

i^2 = -1

i^3 = -i

$i^{11} + i^{16} + i^{21} + i^{28} + i^{31} = -i + 1 + i + 0 - i = \boxed{1-i}$