The question and solution are both attached. I am stuck at the inequality between the sum and the integral right before stating that the integral's sum is 1/(4k^4). I am confused as to how I should compare a sum and an integral. I know Integrals are defined as Riemann sums but, to be honest, I forgot about that stuff completely. Basically, what's the logic behind the comparison? Any input would be greatly appreciated! Thanks in advance!