A175934 - OEIS

%I #55 Jan 08 2023 09:46:06

%S 1,2,7,27,116,532,2554,12675,64507,334836,1765833,9434779,50962640,

%T 277839361,1526834471,8448751385,47035469902,263260232668,

%U 1480527858436,8361881707770,47409359120684,269736194796537,1539547726712437,8812663513352489,50579825742416942,291012706492224315,1678146650028389023

%N Number of lattice paths from (0,0) to (n,n) using steps S={(1,0),(0,1),(r,r)|0<r<=2} that never go above the line y=x.

%C Number of lattice paths of weight n that start in (0,0), end on the horizontal axis, never go below this axis, whose steps are of the following four kinds: h=(1,0) of weight 1, H=(1,0) of weight 2, u=(1,1) of weight 1, and d=(1,-1) of weight 0; the weight of a path is the sum of the weights of its steps. Example: a(2)=7 because we have hh, H, hud, udh, udud, uhd, and uudd. - _Emeric Deutsch_, Sep 09 2014

%H Alois P. Heinz, <a href="/A175934/b175934.txt">Table of n, a(n) for n = 0..1000</a>

%H J. P. S. Kung and A. de Mier, <a href="http://dx.doi.org/10.1016/j.jcta.2012.08.010">Catalan lattice paths with rook, bishop and spider steps</a>, Journal of Combinatorial Theory, Series A 120 (2013) 379-389. - From _N. J. A. Sloane_, Dec 27 2012

%F G.f.: (1 - x - x^2 - sqrt(x^4 + 2*x^3 - x^2 - 6*x + 1))/(2*x). - _Mark van Hoeij_, Apr 16 2013

%F G.f.: 1/(1 - x*(1 + x + 1/(1 - x*(1 + x + 1/...)))). - _Michael Somos_, Mar 30 2014

%F G.f. g = g(x) satisfies g = 1 + x*g + x^2*g + x*g^2. - _Emeric Deutsch_, Sep 09 2014

%F a(n) = Sum_{k=0..n} ((C(k)*Sum_{j=0..k+1} (binomial(k+1,j)*Sum_{i=0..n-k} (3^(j-i)*binomial(j,i)*binomial(k-j+1,n-3*k+2*j-i-2)*2^(2*(k-j)-n+1)*(-1)^(k-j+1))))), a(0)=1, a(1)=2, where C(k) = A000108(k). - _Vladimir Kruchinin_, Mar 01 2016

%F a(0) = 1, a(1) = 2; a(n) = 2 * a(n-1) + 3 * a(n-2) + Sum_{k=2..n-1} a(k) * a(n-k-1). - _Ilya Gutkovskiy_, Jul 20 2021

%F G.f.: 1/G(0), where G(k) = 1 - (2*x+x^2)/(1-x/G(k+1)) (continued fraction). - _Nikolaos Pantelidis_, Jan 08 2023

%e a(2)=7 because we can reach (2,2) in the following ways:

%e (1,0),(1,0),(0,1),(0,1);

%e (1,0),(0,1),(1,0),(0,1);

%e (1,0),(0,1),(1,1);

%e (1,0),(1,1),(0,1);

%e (1,1),(1,0),(0,1);

%e (1,1),(1,1);

%e (2,2).

%e G.f. = 1 + 2*x + 7*x^2 + 27*x^3 + 116*x^4 + 532*x^5 + 2554*x^6 + ...

%o (Sage)

%o n=20

%o M=[]

%o for posx in range(0,n+1):

%o         for posy in range(0,n+1):

%o             if posx==0:

%o                 if posy==0:

%o                     M.append([1])

%o                 else:

%o                     M.append([0])

%o             else:

%o                 if posy==0:

%o                     M[0].append(0)

%o                     M[0][posx]=M[0][posx]+M[0][posx-1]

%o                 else:

%o                     if posy>posx:

%o                         M[posy].append(0)

%o                     else:

%o                         M[posy].append(0)

%o                         M[posy][posx]=M[posy][posx-1]+M[posy][posx]

%o                         M[posy][posx]=M[posy-1][posx]+M[posy][posx]

%o                         for r in range(1,min(posx,posy,2)+1):

%o                             M[posy][posx]=M[posy-r][posx-r]+M[posy][posx]

%o L=[]

%o for k in range(0,n+1):

%o     L.append(M[k][k])

%o print(L)

%o (Maxima)

%o a(n):=if n<2 then n+1 else sum((binomial(2*k,k)*sum(binomial(k+1,j)*sum(3^(j-i)*binomial(j,i)*binomial(k-j+1,n-3*k+2*j-i-2)*(2)^(-n+2*(k-j)+1)*(-1)^(k-j+1),i,0,n-k),j,0,k+1))/(k+1),k,0,n); /* _Vladimir Kruchinin_, Mar 01 2016 */

%Y Cf. A175935, A175936, A175937, A175939.

%K nonn

%O 0,2

%A _Eric Werley_, Dec 06 2010