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A135389
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Number of walks of length 2*n+2 from origin to (1,1) in a square lattice.
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6
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2, 24, 300, 3920, 52920, 731808, 10306296, 147232800, 2127513960, 31031617760, 456164781072, 6749962774464, 100445874620000, 1502052155856000, 22557604697766000, 340044833169460800, 5143178101688094600
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OFFSET
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0,1
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COMMENTS
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a(n) is the number of walks of length 2n+2 in an infinite square lattice that begin at the origin and end at (1,1) using steps (1,0), (-1,0), (0,1), (0,-1).
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LINKS
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FORMULA
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a(n) = binomial(2n+2,n) * binomial(2n+2,n+1) = A001791(n+1)*A000984(n+1).
D-finite with recurrence n*(n+2)*a(n) -4*(2*n+1)^2*a(n-1)=0. - R. J. Mathar, Jul 14 2013
E.g.f.: Sum_{n>0} a(n-1) * x^(2*n)/(2*n)! = BesselI(1, 2*x)^2. - Michael Somos, Oct 17 2019
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EXAMPLE
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G.f. = 2 + 24*x + 300*x^2 + 3920*x^3 + 731808*x^4 + 10306296*x^5 + ... - Michael Somos, Oct 17 2019
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MAPLE
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series( 2*hypergeom([3/2, 3/2], [3], 16*x), x=0, 20); # Mark van Hoeij, Apr 06 2013
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MATHEMATICA
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Table[Binomial[2n + 2, n] Binomial[2n + 2, n + 1], {n, 0, 19}] (* Alonso del Arte, Apr 06 2013 *)
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CROSSREFS
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KEYWORD
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easy,nonn,walk
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AUTHOR
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Stefan Hollos (stefan(AT)exstrom.com), Dec 11 2007
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STATUS
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approved
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