There's Cubic Residues As Well?

x 3 1 ( m o d 2016 ) x^3\equiv 1 \pmod{2016}

How many positive integer solutions x < 2016 x<2016 does the above congruency have?

Bonus Problem : Looking ahead, what about x 3 1 ( m o d 2017 ) x^3\equiv 1 \pmod{2017} or even x 3 1 ( m o d 201 7 2017 ) x^3\equiv 1 \pmod{2017^{2017}} ?


The answer is 9.

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1 solution

Otto Bretscher
Feb 29, 2016

Relevant wiki: Chinese Remainder Theorem

Since 2016 = 7 × 9 × 32 2016=7\times 9\times 32 , we can solve the congruence x 3 1 x^3\equiv 1 modulo 7, 9, and 32 and then multiply the numbers of solutions, by the Chinese Remainder Theorem.

While there is a general theory, the numbers are so small that we can easily do this "by inspection".

Modulo 7 we find the solutions x 1 , 2 , 4 x\equiv1,2,4 , and modulo 9 we have x 1 , 4 , 7 x\equiv1,4,7 .

Note that x 8 1 ( m o d 32 ) x^8\equiv 1 \pmod{32} for odd x x . Thus, if x 3 1 ( m o d 32 ) x^3\equiv 1\pmod{32} , then x 9 x 1 ( m o d 32 ) x^9\equiv x\equiv 1 \pmod{32} , so that x 1 x\equiv1 is the only solution in this case.

The total number of solutions is 3 × 3 × 1 = 9 3\times 3\times 1=\boxed{9}

Moderator note:

Great explanation of how to use the Chinese Remainder Theorem to simplify this problem.

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