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 A056802 #19 Sep 02 2025 22:48:44
%S A056802 0,1,2,3,8,15,27,30,66,90,160,348,391,411,626,727,2740,3921,6048,6891,
%T A056802 8860,21978,32411,41390,52553,82948,95907,264840,537363,543056,587616,
%U A056802 638931,673271,1561518
%N A056802 Numbers k such that 2*9^k + 1 is prime.
%t A056802 Do[ If[ PrimeQ[ 2*9^n + 1], Print[ n ]], {n, 0, 1000}]
%o A056802 (PARI) is(n)=ispseudoprime(2*9^n+1) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y A056802 Cf. A003306 (k such that 2*3^k + 1 is prime).
%K A056802 nonn,hard,more,changed
%O A056802 1,3
%A A056802 _Robert G. Wilson v_, Aug 22 2000
%E A056802 More terms from _Rick L. Shepherd_, Apr 22 2003
%E A056802 a(21)-a(31) from _David Broadhurst_, Feb 22 2010
%E A056802 a(32)-a(34) from _Paul S. Vanderveen_, Feb 09 2020