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 A289723 #14 Jun 17 2025 00:37:16
%S A289723 7,61,223,709,59029,177127,1162261447,3486784381,94143178807,
%T A289723 847288609423,2541865828309,7509466514979724803946715958257527,
%U A289723 147808829414345923316083210206383297581,26183890704263137277674192438430182020124327
%N A289723 Primes of the form 3^k - 20.
%H A289723 Robert Price, <a href="/A289723/b289723.txt">Table of n, a(n) for n = 1..20</a>
%H A289723 F. Firoozbakht, M. F. Hasler, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL13/Hasler/hasler2.html">Variations on Euclid's formula for Perfect Numbers</a>, JIS 13 (2010) #10.3.1.
%H A289723 Henri & Renaud Lifchitz, <a href="http://www.primenumbers.net/prptop">PRP Records</a>
%H A289723 OpenPFGW Project, <a href="http://sourceforge.net/projects/openpfgw/">Primality Tester</a>
%F A289723 a(n) = 3^A219041(n) - 20. - _Elmo R. Oliveira_, Nov 09 2023
%t A289723 Select[Table[3^k - 20, {k, 3, 100}], PrimeQ[#] &]
%Y A289723 Cf. A000040, A219041 (corresponding k's).
%K A289723 nonn
%O A289723 1,1
%A A289723 _Robert Price_, Sep 02 2017