Number theoryDifficulty 4.7AIMEFind the answerUnited States
Problem: Let P=i=0∏2016(i3−i−1)2 The remainder when P is divided by the prime 2017 is not zero. Compute this remainder.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Solution: Let Q(x)=x3−x−1=(x−a)(x−b)(x−c), for a,b,c∈Fp3. Then, we can write P=i=0∏2016(i−a)(i−b)(i−c) If we consider each root separately, then P=−(a2017−a)(b2017−b)(c2017−c) The key observation is that a2017,b2017,c2017 is some nontrivial cycle of a,b,c. This is because by Frobenius' identity, (a+b)p=ap+bp. So, if P(x)=0,P(x2017)=0. But, x2017=x, since x∈/Fp. This implies the claim. So, in the end, we wish to compute (b−a)2(c−b)2(a−c)2 Note that P′(x)=3x2−1=(x−b)(x−c)+(x−a)(x−b)+(x−c)(x−a). So it suffices to compute (1−3a2)(1−3b2)(1−3c2)=−27P(31)P(−31) We can compute this is equal to −27(1−332)(1+332)=−23.
Alternatively, this is the discriminant of P(x). We utilize the well-known formula that the discriminant of x3+ax+b is −4a3−27b2=−23. So, the answer is 1994.
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