A212587 - OEIS

A212587

Walks of length n on the x-axis using steps {1,0,-1} and visiting no point more than twice.

1, 4, 12, 30, 66, 148, 316, 682, 1438, 3052, 6428, 13574, 28594, 60284, 127004, 267634, 563870, 1188084, 2503180, 5274062, 11112002, 23412164, 49327516, 103929178, 218970366, 461352988, 972033500, 2047996278, 4314962930, 9091278956, 19154591420, 40357179234

(list; graph; refs; listen; history; text; internal format)

OFFSET

0,2

COMMENTS

Note: a 0 step counts as a visit and a step but does not contribute to the length of the walk.

LINKS

Table of n, a(n) for n=0..31.

FORMULA

G.f. (-1-2*x-2*x^2-2*x^3+2*x^4-2*x^5-2*x^6-2*x^7+3*x^8) / ((-1+x^2)^2 * (-1+2*x+x^4)).

MATHEMATICA

CoefficientList[Series[ (-1-2*x-2*x^2-2*x^3+2*x^4-2*x^5-2*x^6 -2*x^7+3*x^8)/((-1+x^2)^2 * (-1+2*x+x^4)), {x, 0, 45}], x]

CROSSREFS