The minimum positive integer p such that 3p modulo 17 = 1 is

2007

The minimum positive integer p such that 3p modulo 17 = 1 is

  1. A.

    5

  2. B.

    8

  3. C.

    12

  4. D.

    16

Attempted by 10 students.

Show answer & explanation

Correct answer: D

Key idea: by Fermat's little theorem, 3^16 ≡ 1 (mod 17), so the multiplicative order of 3 modulo 17 must divide 16.

  1. Check the proper divisors of 16: 1, 2, 4, 8.

  2. Compute powers modulo 17: 3^1 ≡ 3; 3^2 ≡ 9; 3^4 = (3^2)^2 ≡ 9^2 = 81 ≡ 13 (mod 17); 3^8 = (3^4)^2 ≡ 13^2 = 169 ≡ 16 (mod 17). None of these are 1.

  3. Since no smaller divisor of 16 gives 3^d ≡ 1, the order of 3 modulo 17 is 16, so the minimal positive p with 3^p ≡ 1 (mod 17) is p = 16.

Answer: 16

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