

A085810


Number of threechoice paths along a corridor of height 5, starting from the lower side.


16



1, 2, 5, 13, 35, 96, 266, 741, 2070, 5791, 16213, 45409, 127206, 356384, 998509, 2797678, 7838801, 21963661, 61540563, 172432468, 483144522, 1353740121, 3793094450, 10628012915, 29779028189, 83438979561, 233790820762, 655067316176, 1835457822857, 5142838522138, 14409913303805
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OFFSET

1,2


COMMENTS

A threechoice path is a path whose steps lie in the set {(1,1), (1,0), (1,1)}.
Narrower corridors produce A000012, A000079, A000129, A001519, A057960. An infinitely wide corridor would produce A005773.
Diagonal sums of A114164.  Paul Barry, Nov 15 2005
C(n):= a(n)*(1)^n appears in the following formula for the nonpositive powers of rho*sigma, where rho:=2*cos(Pi/7) and sigma:=sin(3*Pi/7)/sin(Pi/7) = rho^21 are the ratios of the smaller and larger diagonal length to the side length in a regular 7gon (heptagon). See the Steinbach reference where the basis <1,rho,sigma> is used in an extension of the rational field. (rho*sigma)^(n) = C(n) + B(n)*rho + A(n)*sigma,n>=0, with B(n)= A181880(n2)*(1)^n, and A(n)= A116423(n+1)*(1)^(n+1). For the nonnegative powers see A120757(n), A122600(n1) and A181879(n), respectively. See also a comment under A052547.
a(n) is also the number of biwall directed polygons with n cells. (The definition of biwall directed polygons is given in the article on A122737.)


LINKS

G. C. Greubel, Table of n, a(n) for n = 1..1000
S. Feretic, Generating functions for biwall directed polygons, in: Proc. of the Seventh Int. Conf. on Lattice Path Combinatorics and Applications (eds. S. Rinaldi and S. G. Mohanty), Siena, 2010, 147151.
P. Steinbach, Golden fields: a case for the heptagon, Math. Mag. 70 (1997), p. 2231 (formula 5).
Roman Witula, Damian Slota and Adam Warzynski, QuasiFibonacci Numbers of the Seventh Order, J. Integer Seq., 9 (2006), Article 06.4.3.
Index entries for linear recurrences with constant coefficients, signature (4,3,1).


FORMULA

a(n) = 4*a(n1)  3*a(n2)  a(n3).
From Paul Barry, Nov 15 2005: (Start)
G.f.: (12*x)/(14*x+3*x^2+x^3).
a(n) = Sum_{k=0..floor(n/2)} (Sum_{j=0..nk} C(nk, j)*C(j+k, 2k));
a(n) = Sum_{k=0..floor(n/2)} (Sum_{j=0..nk} C(nk, k+j)*C(k, kj)*2^(n2kj));
a(n) = Sum_{k=0..floor(n/2)} (Sum_{j=0..n2*k} C(nj, n2*kj)*C(k, j)(1)^j*2^(n2*kj)). (End)
a(n1) = B(n;1) = (1/7)*((c(4)c(1))*(1c(1))^n + (c(1)c(2))*(1c(2))^n + (c(2)c(4))*(1c(4))^n), where a(1):=0, c(j):=2*cos(2*Pi*j/7). Moreover, B(n;d), n in N, d in C, denotes the respective quasiFibonacci number defined in comments to A121449 or in WitulaSlotaWarzynski's paper (see also A077998, A006054, A052975, A094789, A121442).  Roman Witula, Aug 09 2012


MATHEMATICA

LinearRecurrence[{4, 3, 1}, {1, 2, 5}, 50] (* Roman Witula, Aug 09 2012 *)
CoefficientList[Series[(1  2 x)/(1  4 x + 3 x^2 + x^3), {x, 0, 40}], x] (* Vincenzo Librandi, Sep 18 2015 *)


PROG

(MAGMA) I:=[1, 2, 5]; [n le 3 select I[n] else 4*Self(n1)3*Self(n2)Self(n3): n in [1..35]]; // Vincenzo Librandi, Sep 18 2015
(PARI) x='x+O('x^30); Vec((12*x)/(14*x+3*x^2+x^3)) \\ G. C. Greubel, Apr 19 2018


CROSSREFS

Cf. A000012, A000079, A000129, A001519, A057960, A005773.
Sequence in context: A007075 A000107 A063028 * A235611 A005773 A022855
Adjacent sequences: A085807 A085808 A085809 * A085811 A085812 A085813


KEYWORD

easy,nonn


AUTHOR

Philippe Deléham, Jul 25 2003


EXTENSIONS

Svjetlan Feretic made the following changes:
1) corrected and clarified the short description of the sequence,
2) changed offset to 1,
3) added two comments (to define a threechoice path and to say what a(n) also is),
4) added a link.
Explanation for 1): The paths under consideration "live" in a corridor like 0<=y<=5. Thus, the ordinate of a vertex of a path can take six values (0,1,2,3,4,5), but the height of the corridor is five.
Explanation for 2): a(1)=1 is the number of paths with zero steps, a(2)=2 is the number of paths with one step, a(3)=5 is the number of paths with two steps, ...


STATUS

approved



