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A360308
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Number T(n,k) of permutations of [n] whose descent set is the k-th finite subset of positive integers in Gray order; triangle T(n,k), n>=0, 0<=k<=ceiling(2^(n-1))-1, read by rows.
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1
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1, 1, 1, 1, 1, 2, 1, 2, 1, 3, 3, 5, 3, 1, 5, 3, 1, 4, 6, 9, 11, 4, 16, 9, 6, 9, 1, 4, 16, 9, 11, 4, 1, 5, 10, 14, 26, 10, 35, 19, 26, 40, 5, 19, 61, 35, 40, 14, 10, 26, 19, 35, 5, 1, 14, 10, 35, 61, 14, 40, 40, 26, 19, 5, 1, 6, 15, 20, 50, 20, 64, 34, 71, 111
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OFFSET
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0,6
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COMMENTS
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The list of finite subsets of the positive integers in Gray order begins: {}, {1}, {1,2}, {2}, {2,3}, {1,2,3}, {1,3}, {3}, ... cf. A003188, A227738, A360287.
The descent set of permutation p of [n] is the set of indices i with p(i)>p(i+1), a subset of [n-1].
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LINKS
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EXAMPLE
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T(5,5) = 4: there are 4 permutations of [5] with descent set {1,2,3} (the 5th subset in Gray order): 43215, 53214, 54213, 54312.
Triangle T(n,k) begins:
1;
1;
1, 1;
1, 2, 1, 2;
1, 3, 3, 5, 3, 1, 5, 3;
1, 4, 6, 9, 11, 4, 16, 9, 6, 9, 1, 4, 16, 9, 11, 4;
...
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MAPLE
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a:= proc(n) a(n):= `if`(n<2, n, Bits[Xor](n, a(iquo(n, 2)))) end:
b:= proc(u, o, t) option remember; `if`(u+o=0, x^a(t),
add(b(u-j, o+j-1, t), j=1..u)+
add(b(u+j-1, o-j, t+2^(o+u-1)), j=1..o))
end:
T:= n-> (p-> seq(coeff(p, x, i), i=0..degree(p)))(b(n, 0$2)):
seq(T(n), n=0..7);
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MATHEMATICA
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a[n_] := a[n] = If[n<2, n, BitXor[n, a[Quotient[n, 2] ]]];
b[u_, o_, t_] := b[u, o, t] = If[u + o == 0, x^a[t], Sum[b[u - j, o + j - 1, t], {j, 1, u}] + Sum[b[u + j - 1, o - j, t + 2^(o + u - 1)], {j, 1, o}]];
T[n_] := CoefficientList[b[n, 0, 0], x];
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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