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%I #22 Jan 01 2022 13:43:44
%S 1,21,221,2221,22221,222221,2222221,22222221,222222221,2222222221,
%T 22222222221,222222222221,2222222222221,22222222222221,
%U 222222222222221,2222222222222221,22222222222222221,222222222222222221
%N a(n) = (10^n*2 - 11)/9.
%C a(n) are also n-1 twos followed by a one.
%C Sum of n-th row of triangle of powers of 10: 1; 10 1 10; 100 10 1 10 100; 1000 100 10 1 10 100 1000; ... - _Philippe Deléham_, Feb 24 2014
%H Vincenzo Librandi, <a href="/A165402/b165402.txt">Table of n, a(n) for n = 1..100</a>
%H Markus Tervooren, <a href="http://factordb.com/search.php?query=%282%2A10%5Ex-11%29%2F9">Factorizations of (2)w1</a>
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11, -10).
%F a(n) = 10*a(n-1) + 11, a(1) = 1. - _Philippe Deléham_, Feb 24 2014
%F a(n) = 11*a(n-1) - 10*a(n-2), a(1) = 1, a(2) = 21. - _Philippe Deléham_, Feb 24 2014
%F G.f.: x*(1+10*x)/((1-x)*(1-10*x)). - _Philippe Deléham_, Feb 24 2014
%e a(1) = 1;
%e a(2) = 10 + 1 + 10 = 21;
%e a(3) = 100 + 10 + 1 + 10 + 100 = 221;
%e a(4) = 1000 + 100 + 10 + 1 + 10 + 100 + 1000 = 2221; etc. - _Philippe Deléham_, Feb 24 2014
%t Table[FromDigits[PadLeft[{1},n,2]],{n,20}] (* or *) (2*10^Range[20]-11)/9 (* _Harvey P. Dale_, Aug 10 2011 *)
%K easy,nonn
%O 1,2
%A _Ivan Panchenko_, Sep 17 2009
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