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A108445
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Triangle read by rows: T(n,k) is number of paths from (0,0) to (3n,0) that stay in the first quadrant (but may touch the horizontal axis), consisting of steps u=(2,1),U=(1,2), or d=(1,-1) and have k pyramids (a pyramid is a sequence u^pd^p or U^pd^(2p) for some positive integer p, starting at the x-axis).
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4
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1, 0, 2, 4, 2, 4, 32, 18, 8, 8, 252, 146, 60, 24, 16, 2112, 1186, 496, 176, 64, 32, 18484, 10146, 4148, 1488, 480, 160, 64, 166976, 90162, 36216, 12792, 4160, 1248, 384, 128, 1545548, 824114, 326828, 113960, 36720, 11104, 3136, 896, 256, 14583808, 7699394
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OFFSET
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0,3
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COMMENTS
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Row sums yield A027307. Column 0 yields A108449. Number of pyramids in all paths from (0,0) to (3n,0) is given by A108450
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LINKS
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FORMULA
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G.f. =(1-z)/[1+z-2tz-z(1-z)A(1+A)], where A=1+zA^2+zA^3=(2/3)*sqrt((z+3)/z)*sin((1/3)*arcsin(sqrt(z)*(z+18)/(z+3)^(3/2)))-1/3 (the g.f. of A027307).
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EXAMPLE
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T(2,1)=2 because we have uudd and UUdddd.
Triangle begins:
1;
0,2;
4,2,4;
32,18,8,8;
252,146,60,24,16;
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MAPLE
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A:=(2/3)*sqrt((z+3)/z)*sin((1/3)*arcsin(sqrt(z)*(z+18)/(z+3)^(3/2)))-1/3: G:=(1-z)/(1+z-2*t*z-z*(1-z)*A*(1+A)): Gser:=simplify(series(G, z=0, 12)): P[0]:=1: for n from 1 to 9 do P[n]:=coeff(Gser, z^n) od: for n from 0 to 9 do seq(coeff(t*P[n], t^k), k=1..n+1) od; # yields sequence in triangular form
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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