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%I #24 Apr 08 2019 08:31:55
%S 1,10,100,990,9640,91900,855100,7754050,68545360,592095160,5020469200,
%T 42054532750,350538754600,2926602465940,24587635740040,
%U 208406304739450,1784567064858400,15453880256710000,135459380264937760,1202295227210127910,10804306958861721400
%N Number of n-letter strings over a ten letter alphabet where no letter appears exactly three times.
%C species is SEQ_10(SET_(!=3)(Z))
%H Math Stackexchange, Marko Riedel et al., <a href="http://math.stackexchange.com/questions/1765530/">N digit numbers with no digit appearing exactly twice.</a>
%F E.g.f.: (exp(z)-z^3/3!)^10.
%e a(3) = 10^3-10 because all 3-letter strings qualify except the strings containing only one type of letter.
%p a := n->n!*coeftayl((exp(z)-z^3/3!)^10, z=0, n);
%t With[{nn=20},CoefficientList[Series[(Exp[x]-x^3/3!)^10,{x,0,nn}],x] Range[ 0,nn]!] (* _Harvey P. Dale_, Apr 08 2019 *)
%Y Cf. A265325, A272502, A272503.
%K nonn,base
%O 0,2
%A _Marko Riedel_, May 01 2016
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