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 A108440 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 having k u=(2,1) steps among the steps leading to the first d step. 1
 1, 1, 1, 5, 4, 1, 33, 25, 7, 1, 249, 184, 54, 10, 1, 2033, 1481, 446, 92, 13, 1, 17485, 12620, 3863, 846, 139, 16, 1, 156033, 111889, 34637, 7881, 1411, 195, 19, 1, 1431281, 1021424, 318812, 74492, 14102, 2168, 260, 22, 1, 13412193, 9536113, 2995228 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 LINKS Alois P. Heinz, Rows n = 0..140, flattened Emeric Deutsch, Problem 10658: Another Type of Lattice Path, American Math. Monthly, 107, 2000, 368-370. FORMULA G.f.: G(t, z)=1/(1-tzA-zA^2)-1, 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). EXAMPLE T(2,1)=4 because we have udud, udUdd, uUddd and Uuddd. Triangle begins: .1; .1,1; .5,4,1; .33,25,7,1; .249,184,54,10,1; MAPLE A:=(2/3)*sqrt((z+3)/z)*sin((1/3)*arcsin(sqrt(z)*(z+18)/(z+3)^(3/2)))-1/3: G:=1/(1-t*z*A-z*A^2): 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 # second Maple program: b:= proc(x, y, t) option remember; expand(`if`(y<0 or y>x, 0, `if`(x=0, 1, b(x-1, y-1, false)+b(x-1, y+2, t)+ b(x-2, y+1, t)*`if`(t, z, 1)))) end: T:= n-> (p-> seq(coeff(p, z, i), i=0..n))(b(3*n, 0, true)): seq(T(n), n=0..10); # Alois P. Heinz, Oct 06 2015 MATHEMATICA b[x_, y_, t_] := b[x, y, t] = Expand[If[y < 0 || y > x, 0, If[x == 0, 1, b[x - 1, y - 1, False] + b[x - 1, y + 2, t] + b[x - 2, y + 1, t]*If[t, z, 1]]]]; T[n_] := Function[p, Table[Coefficient[p, z, i], {i, 0, n}]][ b[3*n, 0, True]]; Table[T[n], {n, 0, 10}] // Flatten (* Jean-François Alcover, Jan 29 2016, after Alois P. Heinz *) CROSSREFS Row sums yield A027307. Column 0 yields A034015. Cf. A027307, A034015, A108441. Sequence in context: A329120 A152862 A348014 * A102220 A279151 A224228 Adjacent sequences: A108437 A108438 A108439 * A108441 A108442 A108443 KEYWORD nonn,tabl AUTHOR Emeric Deutsch, Jun 08 2005 STATUS approved

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Last modified April 21 20:45 EDT 2024. Contains 371885 sequences. (Running on oeis4.)