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%I #22 Jul 18 2021 04:54:11
%S 3,5,7,9,11,20,28,62,161,204,471,505,881,1810,1812,2506,3321,6809,
%T 9272,15131,17449,25250,27989,36082,53309,64970,66354,69646
%N Numbers k such that 3^k (mod 2^k) is prime.
%F A002380(a(n)) = A178985(n). - _Amiram Eldar_, Jul 18 2021
%t fQ[n_] := PrimeQ@ PowerMod[3, n, 2^n]; k = 1; lst = {}; While[k < 15001, If[fQ@ k, AppendTo[lst, k]]; k++]; lst
%o (PARI) for(n=1, 10^5, if(ispseudoprime((3^n)%(2^n)), print1(n, ", "))) \\ _Felix Fröhlich_, Jun 05 2014
%Y Cf. A002380, A178985.
%K nonn,more
%O 1,1
%A _Robert G. Wilson v_, Jan 03 2011
%E a(20)-a(23) from _Felix Fröhlich_, Jun 06 2014
%E a(24)-a(28) from _Amiram Eldar_, Jul 18 2021
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