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A244617
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G.f.: Sum_{n>=0} (4 + x^n)^n * x^n / (1-x)^(n+1).
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5
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1, 5, 26, 127, 636, 3153, 15727, 78406, 391494, 1955563, 9772721, 48847892, 244196337, 1220857221, 6103941997, 30518746918, 152591088797, 762948154799, 3814720881833, 19073550187976, 95367603900506, 476837621600990, 2384187034951204, 11920932287085421, 59604653684027019
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OFFSET
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0,2
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COMMENTS
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What is limit ( a(n) - 5^n )^(1/n) ? (Value is near 2.5659 at n=3000.)
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LINKS
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FORMULA
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G.f.: Sum_{n>=0} x^(n*(n+1)) / (1-x - 4*x^(n+1))^(n+1).
G.f.: Sum_{n>=0} x^n * Sum_{k=0..n} binomial(n,k) * (4 + x^k)^k.
G.f.: Sum_{n>=0} x^n * Sum_{k=0..n} binomial(n,k) * (1 + 4*x^k)^(n-k) * x^(k^2).
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EXAMPLE
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G.f.: A(x) = 1 + 5*x + 26*x^2 + 127*x^3 + 636*x^4 + 3153*x^5 + 15727*x^6 +...
where we have the series identity:
A(x) = 1/(1-x) + (4+x)*x/(1-x)^2 + (4+x^2)^2*x^2/(1-x)^3 + (4+x^3)^3*x^3/(1-x)^4 + (4+x^4)^4*x^4/(1-x)^5 +...+ (4 + x^n)^n * x^n / (1-x)^(n+1) +...
A(x) = 1/(1-5*x) + x^2/(1-x-4*x^2)^2 + x^6/(1-x-4*x^3)^3 + x^12/(1-x-4*x^4)^4 + x^20/(1-x-4*x^5)^5 + x^30/(1-x-4*x^6)^6 +...+ x^(n*(n+1)) / (1-x - 4*x^(n+1))^(n+1) +...
as well as the binomial identity:
A(x) = 1 + x*(1 + (4+x)) + x^2*(1 + 2*(4+x) + (4+x^2)^2) + x^3*(1 + 3*(4+x) + 3*(4+x^2)^2 + (4+x^3)^3) + x^4*(1 + 4*(4+x) + 6*(4+x^2)^2 + 4*(4+x^3)^3 + (4+x^4)^4) + x^5*(1 + 5*(4+x) + 10*(4+x^2)^2 + 10*(4+x^3)^3 + 5*(4+x^4)^4 + (4+x^5)^5) +...+ x^n * Sum_{k=0..n} binomial(n,k) * (4+x^k)^k +...
A(x) = 1 + x*(5 + x) + x^2*(5^2 + 2*(1+4*x)*x + x^4) + x^3*(5^3 + 3*(1+4*x)^2*x + 3*(1+4*x^2)*x^4 + x^9) + x^4*(5^4 + 4*(1+4*x)^3*x + 6*(1+4*x^2)^2*x^4 + 4*(1+4*x^3)*x^9 + x^16) + x^5*(5^5 + 5*(1+4*x)^4*x + 10*(1+4*x^2)^3*x^4 + 10*(1+4*x^3)^2*x^9 + 5*(1+4*x^4)*x^16 + x^25) +...+ x^n * Sum_{k=0..n} binomial(n,k) * (1+4*x^k)^(n-k) * x^(k^2) +...
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MATHEMATICA
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Table[SeriesCoefficient[Sum[x^(j*(j+1))/(1-x-4*x^(j+1))^(j+1), {j, 0, n}], {x, 0, n}], {n, 0, 25}] (* Vaclav Kotesovec, Jul 02 2014 *)
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PROG
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(PARI) {a(n)=local(A); A=sum(m=0, n, (4 + x^m)^m * x^m / (1-x +x*O(x^n) )^(m+1) ); polcoeff(A, n)}
for(n=0, 40, print1(a(n), ", "))
(PARI) {a(n)=local(A); A=sum(m=0, sqrtint(n+1), x^(m*(m+1)) / (1-x - 4*x^(m+1) +x*O(x^n) )^(m+1) ); polcoeff(A, n)}
for(n=0, 40, print1(a(n), ", "))
(PARI) {a(n)=polcoeff(sum(m=0, n, x^m*sum(k=0, m, binomial(m, k)*(4+x^k)^k) +x*O(x^n)), n)}
for(n=0, 40, print1(a(n), ", "))
(PARI) {a(n)=polcoeff(sum(m=0, n, x^m*sum(k=0, m, binomial(m, k)*(1+4*x^k)^(m-k)*x^(k^2)) +x*O(x^n)), n)}
for(n=0, 40, print1(a(n), ", "))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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