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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.)