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A124921
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Distributes the number of permutations in the alternating group; cf. A060351.
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2
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1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 3, 1, 1, 3, 1, 1, 1, 2, 5, 2, 5, 8, 5, 2, 2, 5, 8, 5, 2, 5, 2, 1, 1, 2, 8, 4, 9, 18, 12, 6, 8, 19, 31, 17, 12, 21, 9, 3, 2, 10, 19, 14, 18, 30, 21, 6, 4, 14, 17, 10, 6, 6, 3, 0
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OFFSET
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0,11
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COMMENTS
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The symmetric group distribution of permutation descents is summarized in Table A008292; for example 1 57 302 302 57 1 sums the following A060351 values:
1.......5.......10.......10.......5.......1
.......14.......35.......35.......14.......
.......19.......26.......40.......19.......
.......14.......40.......19.......14.......
........5.......61.......26.......5.......
................26.......61..............
................19.......40..............
................40.......26..............
................35.......35..............
................10.......10..............
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LINKS
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EXAMPLE
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The distribution is based on the frequency of descents; for example, when permuting four symbols the 12 patterns are ddd ddu dud udu dud duu udd udu dud udu uud and uuu yielding the frequency distribution 1 1 3 1 1 3 1 1.
Triangle T(n,k) begins:
1;
1;
1, 0;
1, 1, 1, 0;
1, 1, 3, 1, 1, 3, 1, 1;
1, 2, 5, 2, 5, 8, 5, 2, 2, 5, 8, 5, 2, 5, 2, 1;
...
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MAPLE
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b:= proc(u, o, t, h) option remember; expand(`if`(u+o=0, h,
add(b(u-j, o+j-1, t+1, irem(h+u-j, 2))*x^floor(2^(t-1)), j=1..u)+
add(b(u+j-1, o-j, t+1, irem(h+u+j-1, 2)), j=1..o)))
end:
T:= n-> (p-> seq(coeff(p, x, i), i=0..ceil(2^(n-1)-1)))(b(n, 0$2, 1)):
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MATHEMATICA
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b[u_, o_, t_, h_] := b[u, o, t, h] = Expand[If[u+o == 0, h,
Sum[b[u-j, o+j-1, t+1, Mod[h+u-j, 2]]*x^Floor[2^(t-1)], {j, 1, u}]+
Sum[b[u+j-1, o-j, t+1, Mod[h+u+j-1, 2]], {j, 1, o}]]];
T[n_] := With[{p = b[n, 0, 0, 1]}, Table[Coefficient[p, x, i],
{i, 0, Ceiling[2^(n-1)-1]}]];
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CROSSREFS
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KEYWORD
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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