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G.f.: Sum_{n>=0} x^n * Sum_{k=0..n} binomial(n,k)^3 * x^(2*k).
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%I #13 Oct 15 2014 21:55:52

%S 1,1,1,2,9,28,66,153,433,1345,3952,10991,30954,90988,271845,804153,

%T 2361457,6979690,20842285,62493914,187274712,561448399,1688263179,

%U 5093148285,15393417178,46570446829,141063389488,427979185898,1300470246165,3956367018001,12048354848013,36728336040306

%N G.f.: Sum_{n>=0} x^n * Sum_{k=0..n} binomial(n,k)^3 * x^(2*k).

%C Limit a(n)/a(n+1) = 1 - t = t^3 = 0.3176721961... where t = ((sqrt(93)+9)/18)^(1/3) - ((sqrt(93)-9)/18)^(1/3).

%H Paul D. Hanna, <a href="/A248658/b248658.txt">Table of n, a(n) for n = 0..500</a>

%F G.f.: A(x) = Sum_{n>=0} (3*n)!/(n!)^3 * x^(4*n) / (1-x-x^3)^(3*n+1).

%F a(n) = Sum_{k=0..[n/3]} C(n-2*k,k)^3.

%F G.f.: A(x) = G( x^4/(1-x-x^3)^3 )/(1-x-x^3) where G(x) satisfies:

%F * G(x^3) = G( x*(1+3*x+9*x^2)/(1+6*x)^3 )/(1+6*x) and G(x) is the g.f. of A006480.

%e G.f. A(x) = 1 + x + x^2 + 2*x^3 + 9*x^4 + 28*x^5 + 66*x^6 + 153*x^7 +...

%e which equals the series:

%e A(x) = 1/(1-x-x^3) + 3!/1!^3*x^4/(1-x-x^3)^4 + 6!/2!^3*x^8/(1-x-x^3)^7 + 9!/3!^3*x^12/(1-x-x^3)^10 + 12!/4!^3*x^16/(1-x-x^3)^13 +...

%e The g.f. also equals the series:

%e A(x) = 1 +

%e x*(1 + x^2) +

%e x^2*(1 + 2^3*x^2 + x^4) +

%e x^3*(1 + 3^3*x^2 + 3^3*x^4 + x^6) +

%e x^4*(1 + 4^3*x^2 + 6^3*x^4 + 4^3*x^6 + x^8) +

%e x^5*(1 + 5^3*x^2 + 10^3*x^4 + 10^3*x^6 + 5^3*x^8 + x^10) +...

%t Table[Sum[Binomial[n-2*k,k]^3,{k,0,Floor[n/3]}],{n,0,20}] (* _Vaclav Kotesovec_, Oct 15 2014 *)

%o (PARI) {a(n)=local(A=1); A=sum(m=0, n, x^m*sum(k=0, m, binomial(m, k)^3*x^(2*k)) +x*O(x^n)); polcoeff(A, n)}

%o for(n=0, 40, print1(a(n), ", "))

%o (PARI) {a(n)=polcoeff(sum(m=0, n,x^(4*m)/(1-x-x^3 +x*O(x^n))^(3*m+1)*(3*m)!/(m!)^3), n)}

%o for(n=0, 40, print1(a(n), ", "))

%o (PARI) {a(n)=sum(k=0, n\3, binomial(n-2*k, k)^3)}

%o for(n=0, 40, print1(a(n), ", "))

%Y Cf. A181545, A181543.

%K nonn

%O 0,4

%A _Paul D. Hanna_, Oct 10 2014