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%I #7 Jan 02 2017 01:57:35
%S 7,19,73,163,487,1459,2593,17497,39367,52489,71119,80191,87211,97687,
%T 135433,139483,209953,262657,379081
%N Primes which are divisors of numbers of the form (2^phi(3^k) - 1)/3^k.
%C The primes in this sequence have the property that with the exception of 7 they are congruent to 1 mod 18 and with the exception of 7, 19, 73 are congruent to 1 mod 54.
%t a = {}; Do[k = ((2^EulerPhi[3^(w + 1)] - 1)/3^(w + 1))/((2^EulerPhi[3^w] - 1)/3^w); Do[If[Mod[k, Prime[n]] == 0, AppendTo[a, Prime[n]]; Print[Prime[n]]], {n, PrimePi[2], PrimePi[379081]}], {w, 1, 20}]; Union[a] (*Artur Jasinski*)
%Y Cf. A008776, A152007.
%K hard,nonn
%O 1,1
%A _Artur Jasinski_, Nov 19 2008
%E Edited by _N. J. A. Sloane_, Nov 26 2008
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