A245067 Number of three-dimensional random walks with 2n steps in the wedge region x >= y >= z, beginning and ending at the origin without crossing the wedge boundary.

1, 2, 12, 120, 1610, 25956, 474012, 9475752, 202921290, 4587734580, 108376022040, 2654745191280, 67043341981980, 1737717447946200, 46062204663294000, 1245096242017227360, 34239776369652506970, 956050033694583839220

Offset: 0

Author

Jean-François Alcover, Nov 12 2014

Keywords

• nonn
• walk
• easy

Examples

			For 2n=4, the 12 acceptable walks are:
(0, 0, -1), (0, -1, -1), (0, 0, -1), (0 ,0, 0);
(0, 0, -1), (0, 0, 0), (0, 0, -1), (0 ,0, 0);
(0, 0, -1), (0, 0, 0), (1, 0, 0), (0 ,0, 0);
(0, 0, -1), (1, 0, -1), (0, 0, -1), (0 ,0, 0);
(0, 0, -1), (1, 0, -1), (1, 0, 0), (0 ,0, 0);
(1, 0, 0), (0, -1, -1), (0, 0, -1), (0 ,0, 0);
(1, 0, 0), (0, 0, 0), (0, 0, -1), (0 ,0, 0);
(1, 0, 0), (0, 0, 0), (1, 0, 0), (0 ,0, 0);
(1, 0, 0), (1, 0, -1), (0, 0, -1), (0 ,0, 0);
(1, 0, 0), (1, 0, -1), (1, 0, 0), (0 ,0, 0);
(1, 0, 0), (1, 1, 0), (0, 0, -1), (0 ,0, 0);
(1, 0, 0), (1, 1, 0), (1, 0, 0), (0 ,0, 0).
		

References

• Steven R. Finch, Mathematical Constants, Cambridge University Press, 2003, Section 5.9 Polya's random walk constants, p. 326.

Links

• Eric Weisstein's MathWorld, Polya's Random Walk Constants

Crossrefs

Cf. A000108, A002896, A086230, A086231.

Programs

Mathematica

a[n_] := CatalanNumber[n]*HypergeometricPFQ[{1/2, -n-1, -n}, {2, 2}, 4]; Table[a[n], {n, 0, 20}]

Formula

a(n) = sum_{k=0..n} (2n)!*(2k)!/((n-k)!*(n+1-k)!*k!^2*(k+1)!^2).

a(n) = C(n) * 3F2(1/2, -n-1, -n; 2, 2; 4) where C(n) is the n-th Catalan number and 3F2 the hypergeometric function.

a(n) ~ 2^(2*n-4) * 3^(2*n+9/2) / (Pi^(3/2) * n^(9/2)). - Vaclav Kotesovec, Nov 13 2014

Recurrence: n*(n+2)^2*a(n) = 2*(2*n-1)*(10*n^2 + 2*n - 3)*a(n-1) - 36*(n-1)*(2*n-3)*(2*n-1)*a(n-2). - Vaclav Kotesovec, May 14 2016