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 A101394 #32 Jul 02 2025 15:53:37
%S A101394 0,2,4,5,8,9,28,191,196,2038,34414,39266,50579,94286,108412,130480,
%T A101394 178091,185355
%N A101394 Numbers k such that 4*10^k+9 is prime.
%C A101394 a(19) > 2*10^5. - _Robert Price_, May 24 2015
%H A101394 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40009.htm#prime">Prime numbers of the form 400...009</a>.
%H A101394 Sabin Tabirca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%F A101394 a(n) = A101715(n-1) + 1.
%e A101394 n = 2, 4, 5, 8, 9 are members since 409, 40009, 400009, 400000009 and 4000000009 are all prime.
%t A101394 Do[ If[ PrimeQ[4*10^n + 9], Print[n]], {n, 0, 10000}]
%o A101394 (PARI) is(n)=ispseudoprime(4*10^n+9) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y A101394 Cf. A088275, A056806, A101715.
%K A101394 nonn
%O A101394 1,2
%A A101394 Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005
%E A101394 a(10)=2038 from Joao da Silva (zxawyh66(AT)yahoo.com), Sep 30 2005
%E A101394 a(11)-a(12) from Kamada data by _Robert Price_, Dec 13 2010
%E A101394 Edited by _Ray Chandler_, Dec 23 2010
%E A101394 a(13)-a(18) from _Robert Price_, May 24 2015