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COMMENTS
| All a(n), except a(2) = 5, are of the form (2^p + 1)/3 - the Wagstaff primes A000979[n] = {3,11,43,683,2731,43691,174763,...}.
Indices of prime Jacobsthal numbers are listed in A107036[n] = {3,4,5,7,11,13,17,19,23,31,43,61,...}.
For n>1 A107036[n] = A000978[n] Numbers n such that (2^n + 1)/3 is prime. - Alexander Adamchuk (alex(AT)kolmogorov.com), Oct 10 2006
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MATHEMATICA
| Select[Table[(2^n + (-1)^(n - 1))/3, {n, 200}], PrimeQ] (*From Vladimir Joseph Stephan Orlovsky, Mar 29 2011*)
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