1,2

a(n) is a subset of A067945(n) = {1, 2, 4, 8, 16, 20, 32, 40, 64, 80, 100, 128, 160, 200, 220, ...} Numbers n such that n divides 3^n-1.

2 divides a(n) for n>1. 2^2 divides a(n) for n>2. 5 divides a(n) for n>3. 11 divides a(n) for n = {5,7,8,9,10,12,13,14,15,16,17,18,19,20,22,23,24,26,27,28,29,30,31,31,33,34,35,...}. 11^2 divides a(n) for n = {7,12,14,15,26,27,29,30,31,33,34,...}. Prime factors of a(n) in the order of their appearance in a(n) are {2,5,11,61,23,67,1181,661,47,1321,367,3851,5501,727,461,269,...}. - Alexander Adamchuk (alex(AT)kolmogorov.com), Jan 11 2007

Select[Range[30000], IntegerQ[(PowerMod[3, #, #^2 ]-1)/#^2 ]&]

Cf. A127100, A127101, A127102, A127104, A123105, A127106, A127107, A127092. Cf. A067945 = numbers n such that n divides 3^n-1.

Sequence in context: A110371 A120388 A061348 this_sequence A059831 A064493 A059588

Adjacent sequences: A127100 A127101 A127102 this_sequence A127104 A127105 A127106

hard,nonn

Alexander Adamchuk (alex(AT)kolmogorov.com), Jan 05 2007

More terms from Ryan Propper (rpropper(AT)stanford.edu) and Alexander Adamchuk (alex(AT)kolmogorov.com), Jan 05 2007