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 A056807 #55 Aug 18 2025 05:40:33
%S A056807 1,3,7,10,28,36,67,81,147,483,643,1020,1900,2620,10453,27720,52824,
%T A056807 105589,111988,618853,665829
%N A056807 Numbers k such that 3*10^k + 1 is prime.
%H A056807 S. W. Golomb, <a href="http://www.jstor.org/stable/2005337">Properties of the sequence 3.2^n+1</a>, Math. Comp., 30 (1976), 657-663.
%H A056807 S. W. Golomb, <a href="/A004119/a004119.pdf">Properties of the sequence 3.2^n+1</a>, Math. Comp., 30 (1976), 657-663. [Annotated scanned copy]
%H A056807 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/30001.htm#prime">Prime numbers of the form 300...001</a>.
%H A056807 Sabin Tabirca and Kieran Reynolds, <a href="https://web.archive.org/web/20070211053803/http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%F A056807 a(n) = A101823(n) + 1.
%e A056807 k = 3 gives (3*10^3+1) = 3000+1 = 3001, which is prime.
%t A056807 Do[ If[ PrimeQ[ 3*10^k + 1], Print[ k ]], {k, 0, 20000}]
%o A056807 (PARI) is(k)=isprime(3*10^k+1) \\ _Charles R Greathouse IV_, Feb 17 2017
%Y A056807 Cf. A056797, A062339, A101823, A199683, A259866.
%K A056807 nonn,more,hard
%O A056807 1,2
%A A056807 _Robert G. Wilson v_, Aug 22 2000
%E A056807 a(13)-a(14) from Julien Peter Benney (jpbenney(AT)ftml.net), Nov 23 2004
%E A056807 a(15) from _Hugo Pfoertner_, Jan 18 2005
%E A056807 a(16)-a(17) from _Robert G. Wilson v_, Jan 18 2005
%E A056807 a(18) from Roman Makarchuk, Dec 05 2008 confirmed as next term by _Ray Chandler_, Mar 02 2012
%E A056807 a(19) from _Alexander Gramolin_, Feb 24 2012 confirmed as next term by _Ray Chandler_, Mar 02 2012
%E A056807 a(20)-a(21) from Kamada data by _Robert Price_, Jan 26 2015