This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A079363 #19 Sep 08 2022 08:45:08
%S A079363 5,17,53,4373,13121,1062881,6973568801,188286357653,15251194969973,
%T A079363 100063090197999413,1046695266054721074427023041,
%U A079363 763040848953891663257299797617,556256778887387022514571552463521
%N A079363 Primes of the form 2*3^k - 1.
%C A079363 Sum of reciprocals = 0.2779972845973183835923785945..
%H A079363 Vincenzo Librandi, <a href="/A079363/b079363.txt">Table of n, a(n) for n = 1..20</a>
%p A079363 select(isprime,[2*3^k-1$k=0..200]); # _Muniru A Asiru_, Nov 24 2018
%t A079363 Select[2*3^Range[100]-1, PrimeQ]
%o A079363 (PARI) \\ Primes in the sequence of sums of alternating powers of 3
%o A079363 pseq3(n) = { j=a=1; p=1; sr=0; while(j<=n, a = a + 3^(p); if(isprime(a),print1(a", "); sr+=1.0/a; ); a = a+3^(p-1); if(isprime(a),print1(a", "); sr+=1.0/a; ); p+=1; j+=2; ); print(); print(sr); }
%o A079363 (Magma) [a: n in [1..200] | IsPrime(a) where a is 2*3^n-1 ]; // _Vincenzo Librandi_, Dec 09 2011
%Y A079363 Cf. A003306 (n such that 2*3^n+1 is prime), A003307 (n such that 2*3^n-1 is prime).
%K A079363 nonn
%O A079363 1,1
%A A079363 _Cino Hilliard_, Feb 15 2003