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A178325
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G.f.: A(x) = Sum_{n>=0} x^n/(1-x)^(n^2).
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9
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1, 1, 2, 6, 21, 83, 363, 1730, 8889, 48829, 284858, 1755325, 11374092, 77208275, 547261631, 4039201624, 30967330941, 246084049137, 2023030659970, 17175765057532, 150367445873108, 1355528352031358, 12566899017130088
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OFFSET
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0,3
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COMMENTS
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Equals the row sums of triangle A214398.
a(n) is the number of weak compositions of n such that if the first part is equal to k then there are a total of k^2 + 1 parts. A weak composition is an ordered partition of the integer n into nonnegative parts. a(3) = 6 because we have: 1+2, 2+0+0+0+1, 2+0+0+1+0, 2+0+1+0+0, 2+1+0+0+0, 3+0+0+0+0+0+0+0+0+0. - Geoffrey Critzer, Oct 09 2013
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LINKS
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FORMULA
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a(n) = Sum_{k=0..n} C((n-k)^2 + k-1, k).
G.f.: A(x) = Sum_{n>=0} (x-x^2)^n*Product_{k=1..n} ((1-x)^(4*k-3) - x)/((1-x)^(4*k-1) - x) due to a q-series identity.
Let q = 1/(1-x), then g.f. A(x) equals the continued fraction:
. A(x) = 1/(1- q*x/(1- q*(q^2-1)*x/(1- q^5*x/(1- q^3*(q^4-1)*x/(1- q^9*x/(1- q^5*(q^6-1)*x/(1- q^13*x/(1- q^7*(q^8-1)*x/(1- ...)))))))))
due to an identity of a partial elliptic theta function.
log(a(n)) ~ n*(log(n) - 2) * (1 + log(4*n) - log((log(n) - 2)*log(n))) / log(n). - Vaclav Kotesovec, Jan 10 2023
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EXAMPLE
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G.f.: A(x) = 1 + x + 2*x^2 + 6*x^3 + 21*x^4 + 83*x^5 + 363*x^6 +...
A(x) = 1 + (x-x^2)*((1-x)-x)/((1-x)^3-x) + (x-x^2)^2*((1-x)-x)*((1-x)^5-x)/(((1-x)^3-x)*((1-x)^7-x)) + (x-x^2)^3*((1-x)-x)*((1-x)^5-x)*((1-x)^9-x)/(((1-x)^3-x)*((1-x)^7-x)*((1-x)^11-x)) +...
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MATHEMATICA
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nn=22; CoefficientList[Series[Sum[x^k/(1-x)^(k^2), {k, 0, nn}], {x, 0, nn}], x] (* Geoffrey Critzer, Oct 09 2013 *)
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PROG
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(PARI) {a(n)=sum(k=0, n, binomial((n-k)^2+k-1, k))}
(PARI) {a(n)=polcoeff(sum(m=0, n, x^m/(1-x+x*O(x^n))^(m^2)), n)}
(PARI) {a(n)=polcoeff(sum(m=0, n, (x-x^2)^m*prod(k=1, m, ((1-x)^(4*k-3)-x)/((1-x)^(4*k-1)-x +x*O(x^n)))), 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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