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Divisible and not Divisible (Posted on 2023-06-19) Difficulty: 3 of 5
Determine the minimum value of a positive integer N such that N divides 2N-2, but N does NOT divide 3N - 3.

See The Solution Submitted by K Sengupta    
Rating: 5.0000 (1 votes)

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Solution Quick solution Comment 3 of 3 |
by direct evaluation N=2 divides 3^2-3=6 and N=3 divides 3^3-3=24.  By Fermat's little theorem, all larger primes also satisfy N divides 3^N-3.  Thus no primes are solutions so N must be composite.  

The term Pseudoprime describes any composite number N that divides A^(N-1) with N coprime to A, which is only trivially different from N dividing A^N-A.  Then what we seek is for N to be a pseudoprime of base 2 but not base 3.

Then the OEIS has already done the hard work:
A001567: Fermat pseudoprimes to base 2, also called Sarrus numbers or Poulet numbers.
A005935 Pseudoprimes to base 3.
The very first number in A001567 is 341 and it is not in A005935, then that means N=341 is the minimum value of a positive integer N such that N divides 2^N-2, but N does NOT divide 3^N - 3.

  Posted by Brian Smith on 2023-06-19 16:28:31
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