A098076 Triangle read by rows: T(n,k) is the number of peakless Motzkin paths of length n and height k (can be easily expressed using RNA secondary structure terminology).
1, 1, 1, 1, 1, 1, 3, 1, 6, 1, 1, 11, 5, 1, 20, 15, 1, 1, 36, 38, 7, 1, 64, 91, 28, 1, 1, 113, 211, 89, 9, 1, 199, 477, 255, 45, 1, 1, 350, 1059, 690, 172, 11, 1, 615, 2321, 1797, 572, 66, 1, 1, 1080, 5037, 4555, 1754, 295, 13, 1, 1896, 10847, 11320, 5098, 1118, 91, 1, 1, 3328
Offset: 0
Examples
Triangle starts: 1; 1; 1; 1, 1; 1, 3; 1, 6, 1; 1, 11, 5; 1, 20, 15, 1; 1, 36, 38, 7; 1, 64, 91, 28, 1; 1, 113, 211, 89, 9; 1, 199, 477, 255, 45, 1; Row n >0 has ceil(n/2) terms. T(6,2) = 5 because the peakless Motzkin paths of length 6 and height 2 are HUUHDD, UHUHDD, UUHHDD, UUHDDH, UUHDHD, where U=(1,1), H=(1,0) and D=(1,-1).
Links
- Alois P. Heinz, Rows n = 0..200, flattened
- I. L. Hofacker, P. Schuster and P. F. Stadler, Combinatorics of RNA secondary structures, Discrete Appl. Math., 88, 1998, 207-237.
- P. R. Stein and M. S. Waterman, On some new sequences generalizing the Catalan and Motzkin numbers, Discrete Math., 26 (1979), 261-272.
- M. Vauchassade de Chaumont and G. Viennot, Polynômes orthogonaux et problèmes d'énumération en biologie moléculaire, Sem. Loth. Comb. B08l (1984) 79-86.
Programs
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Maple
P[0]:=1: P[1]:=sort(1-z): for j from 2 to 30 do P[j]:=sort(expand((1-z+z^2)*P[j-1]-z^2*P[j-2])) od: G:=1+sum(t^i*z^(2*i+1)/P[i]/P[i+1],i=0..25): Gser:=simplify(series(G,z=0,21)): Q[0]:=1: for m from 1 to 18 do Q[m]:=sort(coeff(Gser,z^m)) od: 1,seq(seq(coeff(t*Q[n],t^k),k=1..ceil(n/2)),n=1..16);
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Mathematica
max = 16; p[0] = 1; p[1] := 1 - z; p[j_] := p[j] = (1 - z + z^2)*p[j - 1] - z^2*p[j - 2]; gf = 1 + Sum[t^j*z^(2*j + 1)/(p[j]*p[j + 1]), {j, 0, max}]; se = Series[gf, {t, 0, max}, {z, 0, max}]; CoefficientList[se, {z, t}] // DeleteCases[#, 0, 2] & // Flatten (* Jean-François Alcover, Jun 25 2013 *)
Formula
G.f.: 1+sum(t^j*z^(2j+1)/[P(j)*P(j+1)], j=0..infinity), where P(j) are polynomials in z defined by P(0)=1, P(1)=1-z, P(j)=(1-z+z^2)P(j-1) -z^2*P(j-2), j=2, 3, ... .
Comments