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 A101395 #21 Jul 02 2025 16:52:09
%S A101395 0,1,3,9,39,2323
%N A101395 Numbers k such that 4*10^k+7 is prime.
%C A101395 No further terms < 50000.
%C A101395 a(7) > 2*10^5. - _Robert Price_ May 16 2015
%H A101395 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40007.htm#prime">Prime numbers of the form 400...007</a>.
%H A101395 Sabin Tabirca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%H A101395 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A101395 a(n) = A101714(n-1) + 1.
%e A101395 n = 1, 3, 9 are members since 47, 4007 and 4000000007 are primes.
%t A101395 Do[ If[ PrimeQ[4*10^n + 7], Print[n]], {n, 0, 10000}]
%o A101395 (PARI) is(n)=ispseudoprime(4*10^n+7) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y A101395 Cf. A056806, A100501, A088274, A101714.
%K A101395 hard,nonn
%O A101395 1,3
%A A101395 Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005