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 A056806 #42 May 03 2024 12:38:19
%S A056806 0,1,2,3,13,229,242,309,957,1473,1494,3182,3727,4177,23210,25719,
%T A056806 32835,36990,103958,789955,1038890
%N A056806 Numbers n such that 4*10^n + 1 is prime.
%C A056806 Some of the results were computed using the PrimeFormGW (PFGW) primality-testing program. - _Hugo Pfoertner_, Nov 14 2019
%H A056806 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40001.htm#prime">Prime numbers of the form 400...001</a>.
%H A056806 Sabin Tabirca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%F A056806 a(n) = A101712(n-1) + 1.
%t A056806 Do[ If[ PrimeQ[ 4*10^n + 1], Print[ n ]], {n, 0, 30000}]
%o A056806 (PARI) is(n)=isprime(4*10^n+1) \\ _Charles R Greathouse IV_, Feb 17 2017
%Y A056806 Cf. A056797 (9*10^n+1 is prime), A101712.
%K A056806 more,nonn
%O A056806 1,3
%A A056806 _Robert G. Wilson v_, Aug 22 2000
%E A056806 a(12)-a(16) using PrimeForm from _Hugo Pfoertner_, Jul 08 2004
%E A056806 32835 from _Ray Chandler_, Aug 30 2010
%E A056806 36990 from Peter Benson, Aug 23 2003 confirmed as next term by _Ray Chandler_, Sep 07 2010
%E A056806 103958 from Peter Benson, Dec 31 2004 confirmed as next term by _Ray Chandler_, Feb 18 2012
%E A056806 a(20)-a(21) from Kamada data by _Tyler Busby_, May 03 2024