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 A177506 #18 Sep 08 2022 08:45:53
%S A177506 5,41,401,4001,40000000000001
%N A177506 Primes of the form 4*10^n+1.
%H A177506 Vincenzo Librandi, <a href="/A177506/b177506.txt">Table of n, a(n) for n = 1..9</a>
%H A177506 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40001.htm#prime">Prime numbers of the form 400...001</a>.
%t A177506 Select[Table[4 10^n + 1, {n, 0, 500}], PrimeQ] (* _Vincenzo Librandi_ Jan 02 2014 *)
%o A177506 (Magma) [a: n in [0..250] | IsPrime(a) where a is 4*10^n+1];
%Y A177506 Cf. A056806.
%K A177506 nonn
%O A177506 1,1
%A A177506 _Vincenzo Librandi_, Dec 11 2010