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FORMULA
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a(n)= (3^(2*n))*(6*risefac(2/3, n)*risefac(1/3, n) - 4*n!*risefac(2/3, n) + risefac(4/3, n)*n!)/4!, with risefac(x, n)=Pochhammer(x, n).
E.g.f.: (6*hypergeom([2/3, 1/3], [], 9*x) - 4*hypergeom([1, 2/3], [], 9*x) + hypergeom([4/3, 1], [], 9*x) - 3)/4!.
a(n)= (6*fac3(3*n-2)*fac3(3*n-1)-4*fac3(3*n-1)*fac3(3*n)+fac3(3*n)*fac3(3*n+1))/4!, n>=2, with fac3(n)=A007661(n) (triple factorials). Rewritten from eq.12 of the Blasiak et al. reference given in A091534 for r=5, s=2, k=4.
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