A020149 - OEIS

%I #13 Aug 03 2014 14:01:10

%S 4,10,20,55,65,85,221,703,793,1045,1105,1852,2035,2465,3781,4630,5185,

%T 5473,5995,6541,7363,8695,8965,9061,10585,10945,11647,13019,13051,

%U 13981,14491,17767,18103,18721,19345,19669,19909,21667,22681,23155,24013,25465

%N Pseudoprimes to base 21.

%C Composite numbers n such that 21^(n-1) == 1 (mod n).

%H T. D. Noe, <a href="/A020149/b020149.txt">Table of n, a(n) for n = 1..1000</a>

%H <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>

%t base = 21; 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 Cf. A001567 (pseudoprimes to base 2).

%K nonn

%O 1,1

%A _David W. Wilson_