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%I #16 Jun 13 2015 00:52:34
%S 1,121,12121,1212121,121212121,12121212121,1212121212121,
%T 121212121212121,12121212121212121,1212121212121212121,
%U 121212121212121212121,12121212121212121212121,1212121212121212121212121,121212121212121212121212121,12121212121212121212121212121
%N 1 concatenated with n 21's.
%H Vincenzo Librandi, <a href="/A137466/b137466.txt">Table of n, a(n) for n = 0..100</a>
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (101,-100).
%F From _Colin Barker_, May 31 2015: (Start)
%F a(n) = (2^(3+2*n)*5^(1+2*n)-7)/33.
%F a(n) = 100*a(n-1) + 21.
%F a(n) = 101*a(n-1) - 100*a(n-2).
%F G.f.: (20*x+1) / ((x-1)*(100*x-1)).
%F (End)
%e a(7) = 1 21 21 21 21 21 21 21.
%t Table[FromDigits[PadRight[{1},n,{1,2}]],{n,1,31,2}] (* or *) LinearRecurrence[{101,-100},{1,121},30] (* _Harvey P. Dale_, Aug 17 2013 *)
%o (PARI) Vec((20*x+1)/((x-1)*(100*x-1)) + O(x^30)) \\ _Colin Barker_, May 31 2015
%K easy,nonn,base
%O 0,2
%A _Ctibor O. Zizka_, Apr 19 2008
%E More terms from _Harvey P. Dale_, Aug 17 2013