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 A020137 #37 Jun 18 2021 17:03:43 %S A020137 9,21,45,63,65,105,117,133,153,231,273,341,481,511,561,585,645,651, %T A020137 861,949,1001,1105,1281,1365,1387,1417,1541,1649,1661,1729,1785,1905, %U A020137 2047,2169,2465,2501,2701,2821,3145,3171,3201,3277,3605,3641,4005,4033,4097 %N A020137 Pseudoprimes to base 8. %C A020137 This sequence is a subsequence of the sequence A122785. In fact the terms are odd composite terms of A122785. Theorem: If both numbers q and 2q-1 are primes (q is in the sequence A005382) and n=q*(2q-1) then 8^(n-1)==1 (mod n) (n is in the sequence) iff q is of the form 12k+1. 2701,18721,49141,104653,226801,665281,721801,... is the related subsequence. This subsequence is also a subsequence of the sequence A122785. - _Farideh Firoozbakht_, Sep 15 2006 %C A020137 Composite numbers k such that 8^(k-1) == 1 (mod k). - _Michel Lagneau_, Feb 18 2012 %C A020137 If q and 3q-2 are odd primes, then q*(3q-2) is in the sequence. - _Davide Rotondo_, May 25 2021 %H A020137 Amiram Eldar, <a href="/A020137/b020137.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..613 from R. J. Mathar, terms 614..1000 from T. D. Noe) %H A020137 <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a> %t A020137 Select[Range[4100], ! PrimeQ[ # ] && PowerMod[8, (# - 1), # ] == 1 &] (* _Farideh Firoozbakht_, Sep 15 2006 *) %Y A020137 Cf. A001567 (pseudoprimes to base 2), A005382, A122783, A122785. %K A020137 nonn %O A020137 1,1 %A A020137 _David W. Wilson_