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smc

number theory senior

Problem

Let be an odd integer, and let denote the number of quadruples of distinct integers with for all such that divides . There is a polynomial such that for all odd integers . What is
(A)
(B)
(C)
(D)
(E)
Solution
For a fixed value of there is a total of possible ordered quadruples Let We claim that exactly of these ordered quadruples satisfy that divides Since we conclude that is the complete residue system modulo for all integers Given any ordered quadruple in modulo it follows that exactly one of these ordered quadruples has sum modulo \begin{array}{c|c} & \\ [-2.5ex] \textbf{Ordered Quadruple} & \textbf{Sum Modulo }\boldsymbol{m} \\ [0.5ex] \hline & \\ [-2ex] (a'_1, a'_2, a'_3, a'_4) & S'+4(0) \\ [0.5ex] (a'_1+1, a'_2+1, a'_3+1, a'_4+1) & S'+4(1) \\ [0.5ex] (a'_1+2, a'_2+2, a'_3+2, a'_4+2) & S'+4(2) \\ [0.5ex] \cdots & \cdots \\ [0.5ex] (a'_1+m-1, a'_2+m-1, a'_3+m-1, a'_4+m-1) & S'+4(m-1) \\ [0.5ex] \end{array} We conclude that so By Vieta's Formulas, we get
Final answer
E