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 A099412 #35 Apr 30 2024 05:26:57
%S A099412 0,2,4,11,28,55,94,475,2080,4835,5845,12338,20194,49529,56989,71546,
%T A099412 77309,284330
%N A099412 0 together with numbers k such that 4*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099412 Also numbers k such that abs((4*10^k - 31))/9 is prime.
%C A099412 a(18) > 10^5. - _Robert Price_ Sep 06 2014
%H A099412 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44441.htm#prime">Prime numbers of the form 44...441</a>.
%H A099412 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099412 a(n) = A056681(n-1) + 1.
%t A099412 Do[ If[ PrimeQ[ 4(10^n - 1)/9 - 3], Print[n]], {n, 0, 7000}]
%o A099412 (PARI)
%o A099412 for(n=0,10^4,if(ispseudoprime(abs(4*(10^n-1)/9-3)),print1(n,", "))) \\ _Derek Orr_, Sep 06 2014
%Y A099412 Cf. A002275, A056681, A104659.
%K A099412 nonn
%O A099412 1,2
%A A099412 _Robert G. Wilson v_, Oct 14 2004
%E A099412 a(12)-a(17) from Kamada data by _Robert Price_, Sep 06 2014
%E A099412 a(18) from Kamada data by _Tyler Busby_, Apr 30 2024