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

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

Cf. A212584, A212585, A212586, A164392, A212589.

Sequence in context: A011797 A051172 A032192 * A338223 A118425 A097809

Adjacent sequences: A212584 A212585 A212586 * A212588 A212589 A212590

Keyword

nonn,walk

Author

David Scambler, May 22 2012

Status

approved