1,2

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Select[Range[0, 100], !PrimeQ[2*3^# - 1] &] (* Vincenzo Librandi, Oct 15 2012 *)

(MAGMA) [ n: n in [0..100] | not IsPrime(2*3^n-1) ];

Cf. A003307 (Numbers n such that 2*3^n-1 is prime).

Sequence in context: A308886 A029776 A064931 * A114454 A008854 A062726

Adjacent sequences: A177100 A177101 A177102 * A177104 A177105 A177106

nonn,easy

Vincenzo Librandi

approved