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Indices of prime Jacobsthal numbers.
13

%I #33 Aug 28 2024 10:55:14

%S 3,4,5,7,11,13,17,19,23,31,43,61,79,101,127,167,191,199,313,347,701,

%T 1709,2617,3539,5807,10501,10691,11279,12391,14479,42737,83339,95369,

%U 117239,127031,138937,141079,267017,269987,374321,986191,4031399

%N Indices of prime Jacobsthal numbers.

%C All terms are prime except a(1) = 4. All prime terms are listed in A000978. - _Alexander Adamchuk_, Oct 03 2006

%F a(n) = A000978(n-1) for n >= 3. - _Alexander Adamchuk_, Oct 03 2006

%o (PARI) is(n)=ispseudoprime((2^n - (-1)^n) / 3) \\ _Charles R Greathouse IV_, Apr 28 2015

%Y Cf. A000978, A001045, A001605.

%K nonn,more

%O 1,1

%A _Paul Barry_, May 09 2005

%E More terms from _Alexander Adamchuk_, Oct 03 2006

%E a(41)-a(42) from _Bill McEachen_, Aug 28 2024