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 A020141 #22 Aug 22 2017 09:37:00
%S A020141 4,6,12,21,85,105,231,244,276,357,427,561,1099,1785,1891,2465,2806,
%T A020141 3605,5028,5149,5185,5565,6601,7107,8841,8911,9577,9637,10308,10585,
%U A020141 11305,12403,12621,13019,13345,13461,13685,14491,14981,15051,15505,15841,17803
%N A020141 Pseudoprimes to base 13.
%C A020141 According to _Karsten Meyer_, May 16 2006, 4, 6 and 12 should be excluded, following the strict definition in Crandall and Pomerance.
%C A020141 Composite numbers n such that 13^(n-1) == 1 (mod n).
%D A020141 R. Crandall and C. Pomerance, "Prime Numbers - A Computational Perspective", Second Edition, Springer Verlag 2005, ISBN 0-387-25282-7 Page 132 (Theorem 3.4.2. and Algorithm 3.4.3)
%H A020141 R. J. Mathar and T. D. Noe, <a href="/A020141/b020141.txt">Table of n, a(n) for n = 1..1000</a> (R. J. Mathar to 788 terms)
%H A020141 <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>
%H A020141 F. Richman, <a href="http://math.fau.edu/Richman/carm.htm">Primality testing with Fermat's little theorem</a>
%t A020141 base = 13; 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 *)
%Y A020141 Cf. A001567 (pseudoprimes to base 2).
%K A020141 nonn
%O A020141 1,1
%A A020141 _David W. Wilson_