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A321280
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Number T(n,k) of permutations p of [n] with exactly k descents such that the up-down signature of p has nonnegative partial sums; triangle T(n,k), n>=0, 0<=k<=max(0,floor((n-1)/2)), read by rows.
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4
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1, 1, 1, 1, 2, 1, 8, 1, 22, 22, 1, 52, 172, 1, 114, 856, 604, 1, 240, 3488, 7296, 1, 494, 12746, 54746, 31238, 1, 1004, 43628, 330068, 518324, 1, 2026, 143244, 1756878, 5300418, 2620708, 1, 4072, 457536, 8641800, 43235304, 55717312, 1, 8166, 1434318, 40298572, 309074508, 728888188, 325024572
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OFFSET
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0,5
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LINKS
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EXAMPLE
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Triangle T(n,k) begins:
1;
1;
1;
1, 2;
1, 8;
1, 22, 22;
1, 52, 172;
1, 114, 856, 604;
1, 240, 3488, 7296;
1, 494, 12746, 54746, 31238;
1, 1004, 43628, 330068, 518324;
1, 2026, 143244, 1756878, 5300418, 2620708;
1, 4072, 457536, 8641800, 43235304, 55717312;
1, 8166, 1434318, 40298572, 309074508, 728888188, 325024572;
1, 16356, 4438540, 180969752, 2026885824, 7589067592, 8460090160;
...
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MAPLE
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b:= proc(u, o, c) option remember; `if`(c<0, 0, `if`(u+o=0, 1/x,
add(expand(x*b(u-j, o-1+j, c-1)), j=1..u)+
add(b(u+j-1, o-j, c+1), j=1..o)))
end:
T:= n-> (p-> seq(coeff(p, x, i), i=0..degree(p)))(`if`(n=0, 1, b(n, 0, 1))):
seq(T(n), n=0..14);
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MATHEMATICA
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b[u_, o_, c_] := b[u, o, c] = If[c < 0, 0, If[u + o == 0, 1/x, Sum[Expand[ x*b[u - j, o - 1 + j, c - 1]], {j, 1, u}] + Sum[b[u + j - 1, o - j, c + 1], {j, 1, o}]]];
T[n_] := Function[p, Table[Coefficient[p, x, i], {i, 0, Exponent[p, x]}]][ If[n == 0, 1, b[n, 0, 1]]];
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CROSSREFS
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KEYWORD
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nonn,tabf
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AUTHOR
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STATUS
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approved
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