A186766 - OEIS

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A186766

Triangle read by rows: T(n,k) is the number of permutations of {1,2,...,n} having k nonincreasing odd cycles (0<=k<=floor(n/3)). A cycle (b(1), b(2), ...) is said to be increasing if, when written with its smallest element in the first position, it satisfies b(1)<b(2)<b(3)<... . A cycle is said to be odd if it has an odd number of entries. For example, the permutation (152)(347)(6)(8) has 1 nonincreasing odd cycle.

1, 1, 2, 5, 1, 20, 4, 77, 43, 472, 238, 10, 2585, 2385, 70, 21968, 16504, 1848, 157113, 189695, 15792, 280, 1724064, 1591082, 310854, 2800, 15229645, 21449481, 3100614, 137060, 204738624, 213397204, 59267252, 1583120, 15400, 2151199429, 3347368503, 676271024, 51981644, 200200

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OFFSET

0,3

COMMENTS

Row n has 1+floor(n/3) entries.

Sum of entries in row n is n!.

T(n,0) = A186767(n).

Sum(k*T(n,k),k>=0) = A186768(n).

LINKS

Alois P. Heinz, Rows n = 0..200, flattened

FORMULA

E.g.f.: G(t,z)=exp((1-t)sinh z)*(1+z)^{(t-1)/2}/(1-z)^{(t+1)/2}.

The 5-variate e.g.f. H(x,y,u,v,z) of permutations with respect to size (marked by z), number of increasing odd cycles (marked by x), number of increasing even cycles (marked by y), number of nonincreasing odd cycles (marked by u), and number of nonincreasing even cycles (marked by v), is given by

H(x,y,u,v,z)=exp(((x-u)sinh z + (y-v)(cosh z - 1))*(1+z)^{(u-v)/2}/(1-z)^{(u+v)/2}.

We have: G(t,z)=H(1,1,t,1,z).

EXAMPLE

T(3,1)=1 because we have (132).

T(4,1)=4 because we have (1)(243), (143)(2), (142)(3), and (132)(4).

Triangle starts:

5,1;

20,4;

77,43;

MAPLE

g := exp((1-t)*sinh(z))*(1+z)^((t-1)*1/2)/(1-z)^((t+1)*1/2): gser := simplify(series(g, z = 0, 16)): for n from 0 to 13 do P[n] := sort(expand(factorial(n)*coeff(gser, z, n))) end do: for n from 0 to 13 do seq(coeff(P[n], t, k), k = 0 .. floor((1/3)*n)) end do; # yields sequence in triangular form

# second Maple program:

b:= proc(n) option remember; expand(

`if`(n=0, 1, add(b(n-j)*binomial(n-1, j-1)*

`if`(j::even, (j-1)!, 1+x*((j-1)!-1)), j=1..n)))

end:

T:= n-> (p-> seq(coeff(p, x, i), i=0..degree(p)))(b(n)):

seq(T(n), n=0..14); # Alois P. Heinz, Apr 13 2017

MATHEMATICA

b[n_] := b[n] = Expand[If[n == 0, 1, Sum[b[n-j]*Binomial[n-1, j-1]*If[ EvenQ[j], (j-1)!, 1+x*((j-1)!-1)], {j, 1, n}]]];

T[n_] := Function[p, Table[Coefficient[p, x, i], {i, 0, Exponent[p, x]}]][ b[n]];

Table[T[n], {n, 0, 14}] // Flatten (* Jean-François Alcover, May 03 2017, after Alois P. Heinz *)

CROSSREFS