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 A020171 #16 Aug 17 2017 11:00:47
%S A020171 6,14,21,25,33,42,77,91,105,165,185,231,325,385,425,481,525,561,777,
%T A020171 825,861,925,973,1045,1105,1221,1541,1729,1785,1807,1825,1925,2071,
%U A020171 2425,2465,2553,2821,2849,3145,3281,3439,3781,3885,4033,4417,4825,5005,5565,6105
%N A020171 Pseudoprimes to base 43.
%C A020171 Composite numbers n such that 43^(n-1) == 1 (mod n).
%H A020171 R. J. Mathar, <a href="/A020171/b020171.txt">Table of n, a(n) for n=1..1234</a>
%H A020171 <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>
%t A020171 base = 43; t = {}; n = 1; While[Length[t] < 100, n++; If[! PrimeQ[n] && PowerMod[base, n-1, n] == 1, AppendTo[t, n]]]; t (* _T. D. Noe_, Feb 21 2012 *)
%t A020171 Select[Range[7000],!PrimeQ[#]&&PowerMod[43,#-1,#]==1&] (* _Harvey P. Dale_, Aug 17 2017 *)
%Y A020171 Cf. A001567 (pseudoprimes to base 2).
%K A020171 nonn
%O A020171 1,1
%A A020171 _David W. Wilson_