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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
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.
LINKS
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: A091620 A208738 A144686 * A144685 A085584 A369614
KEYWORD
nonn
AUTHOR
Christian G. Bower, Jun 03 2005
STATUS
approved