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 A108449 Number of paths from (0,0) to (3n,0) that stay in the first quadrant (but may touch the horizontal axis), consisting of steps u=(2,1),U=(1,2), or d=(1,-1) and having no pyramids (a pyramid is a sequence u^pd^p or U^pd^(2p) for some positive integer p, starting at the x-axis). 4
 1, 0, 4, 32, 252, 2112, 18484, 166976, 1545548, 14583808, 139774180, 1356966240, 13316740764, 131890671680, 1316627340564, 13234192747648, 133829733962732, 1360586260341248, 13898403178004420, 142578916276009632 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Column 0 of A108445. LINKS Vaclav Kotesovec, Table of n, a(n) for n = 0..100 Emeric Deutsch, Problem 10658: Another Type of Lattice Path, American Math. Monthly, 107, 2000, 368-370. FORMULA G.f.=(1-z)/[1+z-z(1-z)A(1+A)], where A=1+zA^2+zA^3=(2/3)*sqrt((z+3)/z)*sin((1/3)*arcsin(sqrt(z)*(z+18)/(z+3)^(3/2)))-1/3 (the g.f. of A027307). G.f. y(x) satisfies: -1 + 3*x - 3*x^2 + x^3 + y + 3*x*y - 9*x^2*y + 5*x^3*y - 5*x*y^2 - x^2*y^2 + 5*x^3*y^2 + x^4*y^2 - x*y^3 + 9*x^2*y^3 - 3*x^3*y^3 + 3*x^4*y^3 = 0. - Vaclav Kotesovec, Mar 18 2014 a(n) ~ (11+5*sqrt(5))^n * sqrt(1738885 + 811683*sqrt(5)) / (961*sqrt(5*Pi) *n^(3/2)*2^(n+3/2)). - Vaclav Kotesovec, Mar 18 2014 EXAMPLE a(2)=4 because the paths uUddd, Ududd, UdUddd and Uuddd have no pyramids. MAPLE A:=(2/3)*sqrt((z+3)/z)*sin((1/3)*arcsin(sqrt(z)*(z+18)/(z+3)^(3/2)))-1/3: g:=(1-z)/(1+z-z*(1-z)*A*(1+A)): gser:=series(g, z=0, 24): 1, seq(coeff(gser, z^n), n=1..21); PROG (PARI) {a(n)=local(y=1); for(i=1, n, y = -(-1 + 3*x - 3*x^2 + x^3 + 3*x*y - 9*x^2*y + 5*x^3*y - 5*x*y^2 - x^2*y^2 + 5*x^3*y^2 + x^4*y^2 - x*y^3 + 9*x^2*y^3 - 3*x^3*y^3 + 3*x^4*y^3) + (O(x^n))^4); polcoeff(y, n)} for(n=0, 20, print1(a(n), ", ")) \\ Vaclav Kotesovec, Mar 18 2014 CROSSREFS Cf. A027307, A108445. Sequence in context: A301402 A303416 A007278 * A213413 A317512 A300177 Adjacent sequences:  A108446 A108447 A108448 * A108450 A108451 A108452 KEYWORD nonn AUTHOR Emeric Deutsch, Jun 11 2005 STATUS approved

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Last modified September 21 16:31 EDT 2021. Contains 347598 sequences. (Running on oeis4.)