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Greatest GCD (Posted on 2014-10-12) Difficulty: 3 of 5
The sequence {S(n)} is defined by the relationship: S(n) = 100 + n2, whenever n is a positive integer.

If G(n) = gcd(S(n), S(n+1)), then find the maximum value of G(n).

No Solution Yet Submitted by K Sengupta    
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Solution re: Generalised approach - tying up the loose ends. | Comment 4 of 5 |
(In reply to Generalised approach by broll)

Completing the picture:

Clearly, the function n^2 is cyclic mod any prime, P, with the cycle repeating after an interval of P. Similarly, the function n^2+k is normally just the same cycle, with an offset, except for the case already described. So if (4k+1) is prime, as in the example, it is not only the largest but actually the only prime having the desired property. It follows at once that no multiple of (4k+1) could divide {S(n), S(n+1)}, since this would imply that the function n^2+k was non-cyclic mod at least 1 other prime, which we already know not to be the case.

  Posted by broll on 2014-10-12 23:17:54
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