%I #14 Mar 16 2024 05:27:45
%S 21,2121,212121,21212121,2121212121,212121212121,21212121212121,
%T 2121212121212121,212121212121212121,21212121212121212121,
%U 2121212121212121212121,212121212121212121212121,21212121212121212121212121,2121212121212121212121212121
%N Repeat 21.
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (101,-100).
%F a(n) = (10^(2*n)-1)*21/99.
%p a:= n-> parse(cat(21$n)):
%p seq(a(n), n=1..20); # _Alois P. Heinz_, Jun 16 2021
%o (PARI) g2(n) = (10^(2*n)-1)*1*21/99 for(x=1,25,print1(g2(x),","))
%Y Cf. A177090.
%K nonn,easy
%O 1,1
%A _Cino Hilliard_, Jan 02 2009