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 A101397 #36 Jul 02 2025 15:22:19
%S A101397 0,1,3,7,10,40,419,449,1737,2245,3131,3813,5345,5659,5681,8410,9097,
%T A101397 11293,21061
%N A101397 Numbers k such that 4*10^k+3 is prime.
%C A101397 See Kamada link for search limit and prime vs. PRP status.
%C A101397 a(20) > 2*10^5. - _Robert Price_, Jul 17 2015
%H A101397 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40003.htm#prime">Prime numbers of the form 400...003</a>.
%H A101397 Sabin Tabirca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%F A101397 a(n) = A101713(n-1) + 1.
%e A101397 n = 1, 3, 7, 10 are members since 43, 4003, 40000003 and 40000000003 are prime numbers.
%t A101397 Do[ If[ PrimeQ[4*10^n + 3], Print[n]], {n, 0, 10000}] (* _Robert G. Wilson v_, Jan 18 2005 *)
%o A101397 (PARI) is(n)=ispseudoprime(4*10^n+3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A101397 Cf. A049054, A081677, A056806, A101713.
%K A101397 nonn
%O A101397 1,3
%A A101397 Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005
%E A101397 a(18)-a(19) from Kamada data by _Robert Price_, Dec 10 2010