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%I #25 Jul 19 2017 07:38:18
%S 1,20,301,4040,51001,620060,7352101,85656080,985263601,11225320100,
%T 126965305501,1427999420120,15990423157801,178436520564140,
%U 1985678518660501,22048354837360160,244384923399813601
%N Expansion of 1/((1-9x)(1-11x)).
%C a(n-1), n >= 0, with a(-1) = 0, is also the number of words of length n, over an alphabet of eleven letters, of which any chosen one appears an odd number of times. See the Jul 22 2003 comment in A006516 (4-letter case) and the Balakrishnan reference there. - _Wolfdieter Lang_, Jul 18 2017
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (20,-99).
%F a(n) = (11^(n+1)-9^(n+1))/2. - _Bruno Berselli_, Feb 09 2011
%F From _Vincenzo Librandi_, Feb 09 2011: (Start)
%F a(n) = 11*a(n-1)+9^n, a(0)=1.
%F a(n) = 20*a(n-1)-99*a(n-2), a(0)=1, a(1)=20. (End)
%t CoefficientList[Series[1/((1-9x)(1-11x)),{x,0,20}],x] (* or *) LinearRecurrence[{20,-99},{1,20},20] (* _Harvey P. Dale_, Jun 27 2017 *)
%o (PARI) Vec(1/((1-9*x)*(1-11*x))+O(x^99)) \\ _Charles R Greathouse IV_, Sep 26 2012
%Y Cf. A006516, A081203.
%K nonn,easy
%O 0,2
%A _N. J. A. Sloane_
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