%I #14 Jun 06 2021 06:28:16
%S 1,19,361,6859,130321,2476099,47045881,148305659,893871739,2817807521,
%T 4234136149,10350100679,16983563041,53538342899,80448586831,
%U 196651912901,322687697779,815211156289,1017228515081,1432001198261,1528523149789
%N Numbers k such that k divides 20^k - 1.
%C 19 divides a(n) for n > 1. All powers of 19 are terms. a(n) = 19^(n-1) for all to n < 8, while a(8) = A128356(8) = 148305659 = 410819*19^2.
%C Prime divisors of a(n) in the order of appearance are {19, 410819, 617311, 1508981, ...}. - _Alexander Adamchuk_, May 16 2010
%o (PARI) is(n)=Mod(20,n)^n==1 \\ _Charles R Greathouse IV_, Nov 04 2016
%Y Cf. A001029, A128358, A014960, A014956, A014951, A014949, A014946, A014945, A067945, A128357, A128356, A128400, A177920.
%K nonn
%O 1,2
%A _Alexander Adamchuk_, Mar 02 2007
%E a(9)-a(11) from _Stefan Steinerberger_, May 09 2007
%E a(12)-a(15) from _Alexander Adamchuk_, May 16 2010
%E Edited and a(16)-a(21) added by _Max Alekseyev_, Oct 02 2010