Hello.
Recently, I was solving a problem to which I simplified it to evaluating this summation:
C is n choose k. N is up to 1000, and K is up to 2^30.
Answer appears to be I guess {K * (N - 1)K}
I can't get however how to arrive at this, or perhaps compute the original summation in a smart way, I tried messing expanding nCk to factorials, but didn't get anywhere. <.<