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Determine the number of "good" p-tuples

Source: 2014 CMO #3

May 11, 2014
number theory proposednumber theory

Problem Statement

Let pp be a fixed odd prime. A pp-tuple (a1,a2,a3,,ap)(a_1,a_2,a_3,\ldots,a_p) of integers is said to be good if
[*] (i) 0aip10\le a_i\le p-1 for all ii, and [*] (ii) a1+a2+a3++apa_1+a_2+a_3+\cdots+a_p is not divisible by pp, and [*] (iii) a1a2+a2a3+a3a4++apa1a_1a_2+a_2a_3+a_3a_4+\cdots+a_pa_1 is divisible by pp.
Determine the number of good pp-tuples.