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 A101392 #30 Jul 02 2025 16:52:12
%S A101392 0,1,5,25,455,761,9205,13561,15955,26669,113941
%N A101392 Numbers k such that 2*10^k+9 is prime.
%C A101392 761 and 9205 only probably prime. No others less than 10000.
%C A101392 a(12) > 2*10^5. - _Robert Price_, Jun 06 2015
%H A101392 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/20009.htm#prime">Prime numbers of the form 200...009</a>.
%H A101392 Sabin Tabirca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%F A101392 a(n) = A101952(n-1) + 1 for n>1.
%e A101392 n = 1, 5 are members since 29 and 200009 are primes.
%t A101392 Do[ If[ PrimeQ[2*10^n + 9], Print[n]], {n, 0, 10000}]
%o A101392 (PARI) is(n)=ispseudoprime(2*10^n+9) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y A101392 Cf. A081677, A088275, A101952.
%K A101392 hard,nonn
%O A101392 1,3
%A A101392 Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005
%E A101392 a(8)-a(9) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E A101392 a(10)-a(11) from Kamada data by _Robert Price_, Dec 13 2010
%E A101392 Prepended a(1)=0 by _Robert Price_, Jun 06 2015