OFFSET
1,2
COMMENTS
The generating function is simply derived from the generating function for directed animals. A triangular lattice version exists.
LINKS
M. Bousquet-Mélou and A. Rechnitzer, Lattice animals and heaps of dimers
M. Bousquet-Mélou and A. Rechnitzer, Lattice animals and heaps of dimers, Discrete Math. 258 (2002), no. 1-3, 235-274.
Florian Schager and Michael Wallner, A Bijection between Stacked Directed Polyominoes and Motzkin Paths with Alternative Catastrophes, arXiv:2406.16417 [math.CO], 2024. See p. 5.
FORMULA
G.f.: ((1-2x)(1-3x)-(1-4x)sqrt((1-3x)(1+x)))/(2x(2-7x)).
G.f. A(x) satisfies 0 = f(x, A(x)) where f(x ,y) = (7*x^2 - 2*x) * y^2 + (6*x^2 - 5*x + 1) * y + (3*x^2 - x). - Michael Somos, Apr 17 2012
0 = (105*n^2 + 861*n) * a(n) + (40*n^2 + 433*n + 672) * a(n+1) - (55*n^2 + 586*n + 1200) * a(n+2) + (10*n^2 + 112*n + 288) * a(n+3). - Michael Somos, Apr 17 2012
EXAMPLE
x + 2*x^2 + 6*x^3 + 19*x^4 + 63*x^5 + 213*x^6 + 729*x^7 + ...
MAPLE
gf := ((1-2*x)*(1-3*x)-(1-4*x)*sqrt((1-3*x)*(1+x)))/(2*x*(2-7*x)): s := series(gf, x, 50): for i from 1 to 100 do printf(`%d, `, coeff(s, x, i)) od:
MATHEMATICA
CoefficientList[ ((1-2*x)*(1-3*x)-(1-4*x)*Sqrt[(1-3*x)*(1+x)])/(2*x*(2-7*x)) + O[x]^30, x] // Rest (* Jean-François Alcover, Jun 19 2015 *)
PROG
(PARI) {a(n) = local(A); if( n<1, 0, A = O(x); for( k=1, ceil(n/2), A = 1/( 1/x - 2 - (2 - 7*x) / (1 - 3*x) * A)); polcoeff(A, n))} /* Michael Somos, Apr 17 2012 */
CROSSREFS
KEYWORD
nonn
AUTHOR
Mireille Bousquet-Mélou, Feb 08 2001
EXTENSIONS
More terms from James A. Sellers, Feb 09 2001
STATUS
approved