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 A126222 Triangle read by rows: T(n,k) is the number of 2-Motzkin paths (i.e., Motzkin paths with blue and red level steps) without red level steps on the x-axis, having length n and k level steps (0 <= k <= n). 4
 1, 0, 1, 1, 0, 1, 0, 4, 0, 1, 2, 0, 11, 0, 1, 0, 15, 0, 26, 0, 1, 5, 0, 69, 0, 57, 0, 1, 0, 56, 0, 252, 0, 120, 0, 1, 14, 0, 364, 0, 804, 0, 247, 0, 1, 0, 210, 0, 1800, 0, 2349, 0, 502, 0, 1, 42, 0, 1770, 0, 7515, 0, 6455, 0, 1013, 0, 1, 0, 792, 0, 11055, 0, 27940, 0, 16962, 0 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,8 COMMENTS Row sums are the Catalan numbers (A000108). A166073 appears to be a variant of A126222 where zeros are sorted to the start of each row. - R. J. Mathar, Aug 21 2010 LINKS Alois P. Heinz, Rows n = 0..140, flattened FORMULA T(2n,0) = C(2n,n)/(n+1) (the Catalan numbers; A000108). Sum_{k=0..n} k*T(n,k) = A126223(n). G.f.: G = G(t,z) satisfies z(t + z - t^2*z)G^2 - G + 1 = 0. EXAMPLE T(3,1)=4 because we have BUD, UBD, URD and UDB, where U=(1,1), D=(1,-1), B=blue (1,0), R=red (1,0). Triangle starts: 1 0,1 1,0,1 0,4,0,1 2,0,11,0,1 0,15,0,26,0,1 5,0,69,0,57,0,1 0,56,0,252,0,120,0,1 14,0,364,0,804,0,247,0,1 0,210,0,1800,0,2349,0,502,0,1 42,0,1770,0,7515,0,6455,0,1013,0,1 0,792,0,11055,0,27940,0,16962,0,2036,0,1 132,0,8217,0,57035,0,95458,0,43086,0,4083,0,1 0,3003,0,62062,0,257257,0,305812,0,106587,0,8178,0,1 429,0,37037,0,381381,0,1049685,0,931385,0,258153,0,16369,0,1 0,11440,0,328328,0,2022384,0,3962140,0,2723280,0,614520,0,32752,0,1 1430,0,163592,0,2341976,0,9591764,0,14051660,0,7699800,0,1441928,0,65519,0,1 0,43758,0,1665456,0,14275716,0,41666184,0,47352820,0,21167312,0,3342489,0,131054,0,1 4862,0,712062,0,13527852,0,77161980,0,168567444,0,152915748,0,56818743,0,7667883,0,262125,0,1 ... MAPLE G:=(1-sqrt(1-4*z*t-4*z^2+4*z^2*t^2))/2/z/(t+z-t^2*z): Gser:=simplify(series(G, z=0, 15)): for n from 0 to 12 do P[n]:=sort(expand(coeff(Gser, z, n))) od: for n from 0 to 12 do seq(coeff(P[n], t, j), j=0..n) od; # yields sequence in triangular form # second Maple program: b:= proc(x, y) option remember; `if`(y>x or y<0, 0,       `if`(x=0, 1, expand(b(x-1, y)*`if`(y=0, 1, 2)*z+        b(x-1, y+1) +b(x-1, y-1))))     end: T:= (n, k)-> coeff(b(n, 0), z, k): seq(seq(T(n, k), k=0..n), n=0..15);  # Alois P. Heinz, May 20 2014 MATHEMATICA b[x_, y_] := b[x, y] = If[y>x || y<0, 0, If[x == 0, 1, Expand[b[x-1, y]*If[y == 0, 1, 2]*z + b[x-1, y+1] + b[x-1, y-1]]]]; T[n_, k_] := Coefficient[b[n, 0], z, k]; Table[Table[T[n, k], {k, 0, n}], {n, 0, 15}] // Flatten (* Jean-François Alcover, Feb 19 2015, after Alois P. Heinz *) CROSSREFS Cf. A000108, A126223. Sequence in context: A294583 A283675 A294653 * A071637 A141277 A198637 Adjacent sequences:  A126219 A126220 A126221 * A126223 A126224 A126225 KEYWORD nonn,tabl AUTHOR Emeric Deutsch, Dec 28 2006 STATUS approved

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