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 A102975 #29 Jul 13 2025 19:17:23
%S A102975 1,2,12,15,51,194,498,2136,2822,3762,7428,22740,30452,37952,55254
%N A102975 Numbers k such that 3*10^k + 6*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A102975 Also numbers k such that (11*10^k+1)/3 is prime.
%C A102975 a(16) > 10^5. - _Robert Price_, Jan 29 2015
%H A102975 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/36667.htm#prime">Prime numbers of the form 366...667</a>.
%H A102975 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A102975 a(n) = A101840(n) + 1.
%t A102975 Do[ If[ PrimeQ[(11*10^n+1)/3], Print[n]], {n, 0, 10000}]
%o A102975 (Magma) [n: n in [0..300] | IsPrime((11*10^n+1) div 3)]; // _Vincenzo Librandi_, Dec 01 2015
%Y A102975 Cf. A002275, A101840.
%K A102975 more,nonn
%O A102975 1,2
%A A102975 _Robert G. Wilson v_, Dec 17 2004
%E A102975 Addition of a(12) from Kamada data by _Robert Price_, Dec 13 2010
%E A102975 a(13)-a(14) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E A102975 a(15) from Kamada data by _Robert Price_, Jan 29 2015