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 A097860 Triangle read by rows: T(n,k) is the number of Motzkin paths of length n with k peaks (n>=0, 0<=k<=floor(n/2)). 0
 1, 1, 1, 1, 2, 2, 4, 4, 1, 8, 10, 3, 17, 24, 9, 1, 37, 58, 28, 4, 82, 143, 81, 16, 1, 185, 354, 231, 60, 5, 423, 881, 653, 205, 25, 1, 978, 2204, 1824, 676, 110, 6, 2283, 5534, 5058, 2164, 435, 36, 1, 5373, 13940, 13946, 6756, 1631, 182, 7, 12735, 35213, 38262, 20710 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS Row sums are the Motzkin numbers (A001006). Column 0 gives A004148. This triangle is the Motzkin path equivalent to the Narayana numbers (A001263). - Dan Drake, Feb 17 2011 LINKS Dan Drake and Ryan Gantner, Generating functions for plateaus in Motzkin paths, J. of the Chungcheong Math. Soc., Vol 25, No. 3, p. 475, August 2012. Zhuang, Yan. A generalized Goulden-Jackson cluster method and lattice path enumeration, Discrete Mathematics 341.2 (2018): 358-379. Also arXiv: 1508.02793v2. FORMULA G.f. G = G(t, z) satisfies G = 1+z*G+z^2*G*(G-1+t). G.f. has explicit form G(x,t) = (w-sqrt(w^2-4*x^2))/(2*x^2) with w = 1-x+x^2-x^2*t. (Drake and Ganter, Th. 6) - Peter Luschny, Nov 14 2014 EXAMPLE Triangle starts: 1; 1; 1, 1; 2, 2; 4, 4, 1; 8, 10, 3; 17, 24, 9, 1; Row n has 1+floor(n/2) terms. T(4,1)=4 because (UD)HH, H(UD)H, HH(UD) and U(UD)D are the only Motzkin paths of length 4 with 1 peak (here U=(1,1), H=(1,0) and D=(1,-1)); peaks are shown between parentheses. MAPLE eq:=G=1+z*G+z^2*G*(G-1+t):sol:=solve(eq, G): G:=sol[2]: Gser:=simplify(series(G, z=0, 16)): P[0]:=1: for n from 1 to 13 do P[n]:=sort(coeff(Gser, z^n)) od: seq(seq(coeff(t*P[n], t^k), k=1..1+floor(n/2)), n=0..13); # Alternatively A097860_row := proc(n) local w, f, p, i; w := 1-x+x^2-x^2*t; f := (w-sqrt(w^2-4*x^2))/(2*x^2); p := simplify(coeff(series(f, x, 3+2*n), x, n)); seq(coeff(p, t, i), i=0..iquo(n, 2)) end: seq(print(A097860_row(n)), n=0..7); # Peter Luschny, Nov 14 2014 MATHEMATICA gf = With[{w = 1 - x + x^2 - x^2*t}, (w - Sqrt[w^2 - 4*x^2])/(2*x^2)]; cx[n_] := cx[n] = SeriesCoefficient[gf, {x, 0, n}]; T[n_, k_] := SeriesCoefficient[cx[n], {t, 0, k}]; Table[T[n, k], {n, 0, 14}, {k, 0, n/2}] // Flatten (* Jean-François Alcover, Dec 04 2017, after Peter Luschny *) CROSSREFS Cf. A001006, A004148, A001263. Sequence in context: A089886 A071511 A119922 * A098979 A071928 A165207 Adjacent sequences:  A097857 A097858 A097859 * A097861 A097862 A097863 KEYWORD nonn,tabf AUTHOR Emeric Deutsch, Sep 01 2004 STATUS approved

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Last modified January 18 20:57 EST 2019. Contains 319282 sequences. (Running on oeis4.)