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A248828
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Number of 2n-length words, either empty or beginning with the first character of an n-ary alphabet, that can be built by repeatedly inserting doublets into the initially empty word.
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3
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1, 1, 3, 29, 523, 14289, 530526, 25066621, 1443039123, 98156060225, 7711583225338, 687676559089101, 68652814486950398, 7588068106131457489, 920064964125791788188, 121445943726500589053565, 17337678537189658091486851, 2661994674815094376005234945
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OFFSET
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0,3
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LINKS
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FORMULA
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EXAMPLE
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a(2) = 3: aaaa, aabb, abba (with alphabet {a,b}).
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MAPLE
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a:= n->`if`(n=0, 1, add(binomial(2*n, j)*(n-j)*(n-1)^j, j=0..n-1)/n):
seq(a(n), n = 0..20);
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MATHEMATICA
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Flatten[{1, 1, Table[Sum[Binomial[2*n, j]*(n-j)*(n-1)^j, {j, 0, n-1}]/n, {n, 2, 20}]}] (* Vaclav Kotesovec, Oct 15 2014 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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