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A227997
Triangular array read by rows. T(n,k) is the number of square lattice walks that start and end at the origin after 2n steps having k primitive loops; n>=1, 1<=k<=n.
1
4, 20, 16, 176, 160, 64, 1876, 1808, 960, 256, 22064, 22048, 13248, 5120, 1024, 275568, 282528, 182528, 83456, 25600, 4096, 3584064, 3747456, 2542464, 1284096, 481280, 122880, 16384, 47995476, 50981136, 35851968, 19365120, 8186880, 2617344, 573440, 65536, 657037232, 707110432, 511288256, 290053120, 133084160, 48799744, 13647872, 2621440, 262144, 9150655216, 9958458656, 7363711104, 4338317824, 2113592320, 851398656, 276856832, 68943872, 11796480, 1048576
OFFSET
1,1
COMMENTS
The walk consists of steps in the four directions NW,NE,SW,SE. A primitive loop is a walk that starts and ends at the origin but does not otherwise touch the origin.
Row sums are A002894.
Column 1 is A054474
LINKS
Philippe Flajolet and Robert Sedgewick, Analytic Combinatorics, Cambridge Univ. Press, 2009, page 90.
FORMULA
G.f.: 1/( 1 - y*(1 - 1/A(x)) ) where A(x) is the o.g.f. for A002894.
EXAMPLE
4,
20, 16,
176, 160, 64,
1876, 1808, 960, 256,
22064, 22048, 13248, 5120, 1024,
275568, 282528, 182528, 83456, 25600, 4096
MATHEMATICA
nn=6; a=Sum[Binomial[2n, n]^2x^n, {n, 0, nn}]; Map[Select[#, #>0&]&, Drop[CoefficientList[Series[1/(1-y(1-1/a)), {x, 0, nn}], {x, y}], 1]]//Grid
CROSSREFS
Sequence in context: A213822 A182456 A196380 * A130316 A131745 A261755
KEYWORD
nonn,walk,tabl
AUTHOR
Geoffrey Critzer, Oct 04 2013
STATUS
approved