A393387 - OEIS

%I #34 Feb 18 2026 23:27:34

%S 1,3,9,15,18,31,34,37,51,73,78,216,219,304,318,342,712,834,1095,1171,

%T 1368,1642,1845,2179,3502,3595,3762,3861,6717,9135,9624,10171,11161,

%U 15399,22099,23056,33424,34602,37654,47854,48843,51423,64216,67533,68463,86557,104241

%N Numbers k such that 39*2^k + 5 is prime.

%e 9 is in this sequence because 39*2^9 + 5 = 19973 is prime.

%e Initial terms and associated primes:

%e   a(1) = 1, 83;

%e   a(2) = 3, 317;

%e   a(3) = 9, 19973;

%e   a(4) = 15, 1277957;

%e   a(5) = 18, 10223621;

%e   a(6) = 31, 83751862277, etc.

%o (PARI) is(k) = ispseudoprime(39*2^k + 5)

%Y Cf. A002269.

%K nonn

%O 1,2

%A _Zhicheng Wei_, Feb 13 2026

%E a(35) onward from _Michael S. Branicky_, Feb 14 2026