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 A276261 #9 Feb 16 2025 08:33:36
%S A276261 127,211,757,2521,2857,6301,8527,16381,19867,23689,24697,27847,32341,
%T A276261 37171,38431,42337,66361,68041,82237,89839,97777,103951,114661,140071,
%U A276261 152461,162751,170689,192781,204331,216217,231547,240997,284131,308827,353557,357421,385057,389089
%N A276261 Centered 21-gonal primes.
%C A276261 Primes of the form (21*k^2 + 21*k + 2)/2.
%C A276261 Numbers k such that (21*k^2 + 21*k + 2)/2 is prime: 3, 4, 8, 15, 16, 24, 28, 39, 43, 47, 48, 51, 55, 059, 60, 63, 79, 80, 88, 92, 96, 99, ...
%H A276261 OEIS Wiki, <a href="http://oeis.org/wiki/Centered_polygonal_numbers#cite_note-1">Centered polygonal numbers</a>
%H A276261 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/CenteredPolygonalNumber.html">Centered Polygonal Number</a>
%H A276261 <a href="/index/Ce#CENTRALCUBE">Index entries for sequences related to centered polygonal numbers</a>
%t A276261 Intersection[Table[(21 k^2 + 21 k + 2)/2, {k, 0, 1000}], Prime[Range[33000]]]
%o A276261 (PARI) lista(nn) = for(n=1, nn, if(isprime(p=(21*n^2 + 21*n + 2)/2), print1(p, ", "))); \\ _Altug Alkan_, Aug 26 2016
%Y A276261 Cf. A000040, A069178.
%Y A276261 Cf. similar sequences of the centered k-gonal primes: A125602 (k = 3), A027862 (k = 4), A145838 (k = 5), A002407 (k = 6), A144974 (k = 7), A090562 (k = 10), A262344 (k = 11), A262493 (k = 13), A264821 (k = 14), A264822 (k = 15), A264823 (k = 16), A264824 (k = 17), A264825 (k = 18), A264844 (k = 19), A264845 (k = 20), A201715 (k = 24).
%K A276261 nonn
%O A276261 1,1
%A A276261 _Ilya Gutkovskiy_, Aug 26 2016