OFFSET
0,3
COMMENTS
LINKS
Alois P. Heinz, Rows n = 0..150, flattened
Toufik Mansour, Statistics on Dyck Paths, Journal of Integer Sequences, Vol. 9 (2006), Article 06.1.5.
FORMULA
G.f.: C(z*(1-t*z-z^2+t*z^2)/(1-t*z-z^3+t*z^3)), where C(u) = (1-sqrt(1-4*u))/(2*u) is the Catalan function. See Corollary 2.2 in the Mansour reference.
EXAMPLE
Row 4 is 11, 2, 1; indeed in the 14 Dyck paths of semilength 4 ududu occurs only once in ududuudd, once in uudududd, and twice in udududud.
Triangle starts:
1;
1;
2;
4, 1;
11, 2, 1;
31, 8, 2, 1;
...
MAPLE
C := proc (u) options operator, arrow: (1/2-(1/2)*sqrt(1-4*u))/u end proc: G := C(z*(1-t*z-z^2+t*z^2)/(1-t*z-z^3+t*z^3)): Gser := simplify(series(G, z = 0, 20)): T := proc (n, k) options operator, arrow: coeff(coeff(Gser, z, n), t, k) end proc: 1; 1; for n from 2 to 12 do seq(T(n, k), k = 0 .. n-2) end do; # yields sequence in triangular form
# second Maple program:
b:= proc(x, y, t) option remember; `if`(y<0 or y>x, 0,
`if`(x=0, 1, expand(b(x-1, y+1, [2, 2, 4, 2, 4][t])*
`if`(t=5, z, 1) +b(x-1, y-1, [1, 3, 1, 5, 1][t]))))
end:
T:= n->(p->seq(coeff(p, z, i), i=0..degree(p)))(b(2*n, 0, 1)):
seq(T(n), n=0..15); # Alois P. Heinz, Sep 10 2014
MATHEMATICA
b[x_, y_, t_] := b[x, y, t] = If[y<0 || y>x, 0, If[x == 0, 1, Expand[b[x-1, y+1, {2, 2, 4, 2, 4}[[t]] ]*If[t == 5, z, 1] + b[x-1, y-1, {1, 3, 1, 5, 1}[[t]] ]]]]; T[n_] := Function[{p}, Table[Coefficient[p, z, i], {i, 0, Exponent[p, z]}]][b[2*n, 0, 1]]; Table[T[n], {n, 0, 15}] // Flatten (* Jean-François Alcover, May 27 2015, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Emeric Deutsch, Sep 10 2014
STATUS
approved