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FORMULA
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G.f.: x/(1-19*x+19*x^2-x^3) = x/((1-x)*(1-18*x+x^2).
a(n+1) = sum(S(k, 18), k=0..n) with n>=0, S(k, 18) = U(k, 9) = A049660(k+1).
a(n) = 19*a(n-1)-19*a(n-2)+a(n-3), n>=3, a(0)=0, a(1)=1, a(2)=19.
a(n) = 18*a(n-1)-a(n-2)+1, n>=2, a(0)=0, a(1)=1.
a(n+1) = (S(n+1, 18)-S(n, 18) -1)/16, n>=0.
a(n) = -1/16-(1/80)*(9-4*sqrt(5))^n*sqrt(5)+(1/32)*(9-4*sqrt(5))^n+(1/80)*sqrt(5)*(9+4 *sqrt(5))^n+(1/32)*(9+4*sqrt(5))^n. - Paolo P. Lava, Oct 03 2008
a(n) = 1/8*sum(Fibonacci(6*k), k=0..n). - Gary Detlefs, Dec 07 2010
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