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A216212 Number of n step walks (each step +/-1 starting from 0) which are never more than 4 or less than -4. 5
1, 2, 4, 8, 16, 30, 60, 110, 220, 400, 800, 1450, 2900, 5250, 10500, 19000, 38000, 68750, 137500, 248750, 497500, 900000, 1800000, 3256250, 6512500, 11781250, 23562500, 42625000, 85250000, 154218750, 308437500, 557968750, 1115937500, 2018750000, 4037500000 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..34.

FORMULA

a(n) = A068913(4,n).

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

a(n) = 5*a(n-2) - 5*a(n-4), a(0) = 1, a(1)= 2, a(2) = 4, a(3) = 8, a(4) = 16.

a(2*n+1) = 2*A039717(n+1), a(2*n+2) = 4*A039717(n+1).

MATHEMATICA

nn=30; CoefficientList[Series[(1+x-x^2)^2/(1-5x^2+5x^4), {x, 0, nn}], x] (* Geoffrey Critzer, Jan 14 2014 *)

CROSSREFS

Cf. A068911, A068912, A068913.

Sequence in context: A027423 A140410 A213368 * A164263 A283836 A244825

Adjacent sequences:  A216209 A216210 A216211 * A216213 A216214 A216215

KEYWORD

nonn,walk

AUTHOR

Philippe Deléham, Mar 13 2013

STATUS

approved

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Last modified October 15 15:14 EDT 2019. Contains 328030 sequences. (Running on oeis4.)