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 A055520 #29 Feb 16 2025 08:32:42
%S A055520 1,2,3,4,5,6,7,17,39,49,59,77,100,150,318,381,783,1731,1917,8854,
%T A055520 11244,11959,12129,18532,22717,23364,24252,24548,25323,30177,53717,
%U A055520 380975,424860
%N A055520 Numbers k such that 30*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A055520 Also numbers k such that (10^(k+1)-7)/3 is prime.
%D A055520 Martin Gardner, Strong Laws of Small Primes, in The Last Recreations, p. 194 (1997).
%H A055520 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/33331.htm#prime">Prime numbers of the form 33...331</a>.
%H A055520 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/3.html">3</a>
%H A055520 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A055520 a(n) = A055557(n) - 1. - _Robert Price_, Jan 30 2015
%t A055520 Do[ If[ PrimeQ[30*(10^n - 1)/9 + 1], Print[n]], {n, 0, 50410}]
%Y A055520 Cf. A055557.
%Y A055520 Indices of A033175 that are primes. Cf. A051200, A055557.
%K A055520 hard,nonn
%O A055520 1,2
%A A055520 _Eric W. Weisstein_
%E A055520 a(32)-a(33) from Leonid Durman, Jan 09-10 2012