

A337869


The number of random walks on the simple square lattice that return to the origin (0,0) after 2n steps and do not pass through (0,0) or (1,0) at intermediate steps.


3



3, 13, 106, 1073, 12142, 147090, 1865772, 24463905, 328887346, 4508608610, 62781858592, 885513974674, 12624162072740, 181611275997040, 2633023723495116, 38431604042148681, 564258290166041298, 8327627696761062714, 123471550301117915892
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OFFSET

1,1


COMMENTS

The number of walks on the simple square lattice that take one of the four directions U, D, R, L at each step and return to zero is zero if the number of steps is odd. If the number of steps is even, the sequence counts walks that start at (0,0), return to (0,0) and never pass through (0,0) or (1,0) in between.
The ordinary generating function is a mix of inverses of sums and differences of the hypergeometric generating functions in A002894 and A060150. See Maple.


LINKS

Table of n, a(n) for n=1..19.
R. J. Mathar, Random Walk on the Square Lattice: Return to (0,0) with or without passing (1,0) (Sep 2020)


EXAMPLE

Example: a(1)=3 counts the walks UD, DU, LR (but not RL which would pass (1,0)) of 2 steps that return to the origin.


MAPLE

g002894 := hypergeom([1/2, 1/2], [1], 16*x^2) ;
g060150 := x*hypergeom([1, 3/2, 3/2], [2, 2], 16*x^2) ;
11/2/(g002894+g060150)1/2/(g002894g060150) ;
taylor(%, x=0, 40);
gfun[seriestolist](%) ; # includes zeros of odd steps


CROSSREFS

Cf. A002894, A060150, A337870.
Sequence in context: A098027 A182104 A073587 * A333736 A220704 A061377
Adjacent sequences: A337866 A337867 A337868 * A337870 A337871 A337872


KEYWORD

nonn,walk


AUTHOR

R. J. Mathar, Sep 27 2020


STATUS

approved



