%I #11 Aug 04 2015 14:15:33
%S 221,703,793,3781,7363,9061,11647,13051,17767,19669,19909,22681,27133,
%T 30073,31021,38503,41353,43213,58829,79081,80137,83569,88357,92509,
%U 96049,104653,107213,121987,127697,128627,129889,130561,133141,139679,139897
%N Strong pseudoprimes to base 21.
%H R. J. Mathar, <a href="/A020247/b020247.txt">Table of n, a(n) for n = 1..204</a>
%H <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>
%K nonn
%O 1,1
%A _David W. Wilson_