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 A056215 #21 Aug 02 2023 01:50:55
%S A056215 281,521,1031,1951,2281,2311,2591,3671,5471,5711,6791,7481,8111,8681,
%T A056215 8761,9281,9551,10601,11321,12401,13151,13591,14831,14951,15671,16111,
%U A056215 16361,18671,21191,21521,21881,24281,24551,25391,25801,25841,26161
%N A056215 Primes p whose reciprocal has period (p-1)/10.
%C A056215 Cyclic numbers of the tenth degree (or tenth order): the reciprocals of these numbers belong to one of ten different cycles. Each cycle has (p-1)/10 digits.
%H A056215 Amiram Eldar, <a href="/A056215/b056215.txt">Table of n, a(n) for n = 1..10000</a>
%H A056215 <a href="/index/1#1overn">Index entries for sequences related to decimal expansion of 1/n</a>
%t A056215 f[n_Integer] := Block[{ds = Divisors[n - 1]}, (n - 1)/Take[ ds, Position[ PowerMod[ 10, ds, n], 1] [[1, 1]]] [[ -1]]]; Select[ Prime[ Range[4, 3000]], f[ # ] == 10 &]
%Y A056215 Cf. A097443, A055628, A056157, A056210, A056211, A056212, A056213, A056214, A056216, A056217, A098680, A135073.
%K A056215 nonn,base
%O A056215 1,1
%A A056215 _Robert G. Wilson v_, Sep 15 2004
%E A056215 Entry revised by _N. J. A. Sloane_, Apr 30 2007