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smc

number theory senior

Problem

The number is prime. Let . What is the remainder when is divided by
(A)
(B)
(C)
(D)
Solution
Note that . We have for Therefore This is simply an alternating series of triangular numbers that goes like this: After finding the first few sums of the series, it becomes apparent that and Obviously, falls in the second category, so our desired value is
Final answer
C