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A212585
Walks of length n on the x-axis using steps {1,-1} and visiting no point more than twice.
5
1, 2, 4, 8, 12, 18, 26, 40, 56, 82, 112, 162, 222, 316, 432, 608, 834, 1166, 1602, 2228, 3066, 4252, 5858, 8108, 11178, 15454, 21316, 29450, 40632, 56114, 77434, 106914, 147550, 203696, 281134, 388082, 535636, 739368, 1020506, 1408626, 1944266, 2683674, 3704184
OFFSET
0,2
FORMULA
G.f. (-1-x-2*x^3+3*x^5-2*x^7+x^8)/((-1+x^2)^2 * (-1+x+x^4)).
a(0)=1; a(n) = 2*(A212584(n) + sum(j,1-mod(n,2) .. n-3, step+2, A212584(j))).
MATHEMATICA
CoefficientList[Series[(-1-x-2*x^3+3*x^5-2*x^7+x^8)/((-1+x^2)^2 * (-1+x+x^4)), {x, 0, 45}], x]
CROSSREFS
KEYWORD
nonn,walk
AUTHOR
David Scambler, May 22 2012
STATUS
approved