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A108469 Main diagonal of A090806. 4
1, 1, 2, 4, 9, 20, 45, 100, 220, 478, 1025, 2169, 4534, 9364, 19125, 38646, 77306, 153173, 300765, 585518, 1130612, 2166284, 4120062, 7780817, 14595364, 27201794, 50383690, 92768457, 169835952, 309223286, 560036477, 1009124256 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

REFERENCES

Alter, Ronald; Curtz, Thaddeus B.; Wang, Chung C. Permutations with fixed index and number of inversions. Proceedings of the Fifth Southeastern Conference on Combinatorics, Graph Theory and Computing (Florida Atlantic Univ., Boca Raton, Fla., 1974), pp. 209-228. Congressus Numerantium, No. X, Utilitas Math., Winnipeg, Man., 1974.   From N. J. A. Sloane, Mar 20 2012

LINKS

Vaclav Kotesovec, Table of n, a(n) for n = 0..90

MATHEMATICA

nmax = 20; p = 1; Do[Do[p = Expand[p*(1 - x^i*y^j)]; p = Select[p, (Exponent[#, x] <= nmax) && (Exponent[#, y] <= nmax) &], {i, 1, nmax}], {j, 1, nmax}]; p = Expand[Normal[Series[1/p, {x, 0, nmax}, {y, 0, nmax}]]]; p = Select[p, Exponent[#, x] == Exponent[#, y] &]; Flatten[{1, Table[Coefficient[p, x^n*y^n], {n, 1, nmax}]}] (* Vaclav Kotesovec, Apr 08 2016 *)

CROSSREFS

Sequence in context: A219229 A091620 A208738 * A085584 A080019 A052534

Adjacent sequences:  A108466 A108467 A108468 * A108470 A108471 A108472

KEYWORD

nonn

AUTHOR

Christian G. Bower, Jun 03 2005

STATUS

approved

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Last modified October 16 18:03 EDT 2019. Contains 328102 sequences. (Running on oeis4.)