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A326007
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G.f.: Sum_{n>=0} (n+1)*(n+2)/2 * x^n * ((1+x)^n + 1)^n / (1 + x*(1+x)^n)^(n+3).
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3
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1, 3, 9, 22, 81, 285, 1339, 6264, 33567, 186811, 1116105, 7001244, 46150265, 318158826, 2286494076, 17088720336, 132492477111, 1063527470481, 8822541504319, 75512660179788, 665878308902676, 6041491458457319, 56330651731617333, 539160888285121116, 5292067580412471801, 53218232521845617886, 547833354998854396224, 5768212264434778469998, 62074688689939991197548
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OFFSET
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0,2
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COMMENTS
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More generally, the following sums are equal:
(1) Sum_{n>=0} binomial(n+k-1, n) * r^n * (q^n + p)^n / (1 + p*q^n*r)^(n+k),
(2) Sum_{n>=0} binomial(n+k-1, n) * r^n * (q^n - p)^n / (1 - p*q^n*r)^(n+k),
for any fixed integer k; here, k = 3 and q = (1+x), p = 1, r = x. See other examples for k = 1 (A323680), k = 2 (A326006), k = 4 (A326008).
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LINKS
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Table of n, a(n) for n=0..28.
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FORMULA
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G.f.: Sum_{n>=0} (n+1)*(n+2)/2 * x^n * ((1+x)^n + 1)^n / (1 + x*(1+x)^n)^(n+3).
G.f.: Sum_{n>=0} (n+1)*(n+2)/2 * x^n * ((1+x)^n - 1)^n / (1 - x*(1+x)^n)^(n+3).
G.f.: Sum_{n>=0} (n+1)*(n+2)/2 * x^n * Sum_{k=0..n} binomial(n,k) * ( (1+x)^n - (1+x)^k )^(n-k).
G.f.: Sum_{n>=0} (n+1)*(n+2)/2 * x^n * Sum_{k=0..n} binomial(n,k) * ( (1+x)^n + (1+x)^k )^(n-k) * (-1)^k.
G.f.: Sum_{n>=0} (n+1)*(n+2)/2 * x^n * Sum_{k=0..n} binomial(n,k) * Sum_{j=0..n-k} (-1)^j * binomial(n-k,j) * (1 + x)^((n-j)*(n-k)).
FORMULAS INVOLVING TERMS.
a(n) = Sum_{i=0..n} (n-i+1)*(n-i+2)/2 * Sum_{j=0..n-i} Sum_{k=0..n-i-j} (-1)^k * binomial(n-i,j) * binomial(n-i-j,k) * binomial((n-i-j)*(n-i-k),i).
a(n) = Sum_{i=0..n} (n-i+1)*(n-i+2)/2 * Sum_{j=0..n-i} Sum_{k=0..n-i-j} binomial((n-i-j)*(n-i-k),i) * (-1)^j * (n-i)! / ((n-i-j-k)!*j!*k!).
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EXAMPLE
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G.f.: A(x) = 1 + 3*x + 9*x^2 + 22*x^3 + 81*x^4 + 285*x^5 + 1339*x^6 + 6264*x^7 + 33567*x^8 + 186811*x^9 + 1116105*x^10 + 7001244*x^11 + 46150265*x^12 + ...
such that
A(x) = 1/(1+x)^3 + 3*x*((1+x) + 1)/(1 + x*(1+x))^4 + 6*x^2*((1+x)^2 + 1)^2/(1 + x*(1+x)^2)^5 + 10*x^3*((1+x)^3 + 1)^3/(1 + x*(1+x)^3)^6 + 15*x^4*((1+x)^4 + 1)^4/(1 + x*(1+x)^4)^7 + 21*x^5*((1+x)^5 + 1)^5/(1 + x*(1+x)^5)^8 + 28*x^6*((1+x)^6 + 1)^6/(1 + x*(1+x)^6)^9 + 36*x^7*((1+x)^7 + 1)^7/(1 + x*(1+x)^7)^10 + ...
also,
A(x) = 1/(1-x)^3 + 3*x*((1+x) - 1)/(1 - x*(1+x))^4 + 6*x^2*((1+x)^2 - 1)^2/(1 - x*(1+x)^2)^5 + 10*x^3*((1+x)^3 - 1)^3/(1 - x*(1+x)^3)^6 + 15*x^4*((1+x)^4 - 1)^4/(1 - x*(1+x)^4)^7 + 21*x^5*((1+x)^5 - 1)^5/(1 - x*(1+x)^5)^8 + 28*x^6*((1+x)^6 - 1)^6/(1 - x*(1+x)^6)^9 + 36*x^7*((1+x)^7 - 1)^7/(1 - x*(1+x)^7)^10 + ...
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PROG
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(PARI) {a(n) = my(A = sum(m=0, n+1, (m+1)*(m+2)/2 * x^m*((1+x +x*O(x^n) )^m - 1)^m/(1 - x*(1+x +x*O(x^n) )^m )^(m+3) )); polcoeff(A, n)}
for(n=0, 35, print1(a(n), ", "))
(PARI) {a(n) = sum(i=0, n, (n-i+1)*(n-i+2)/2 * sum(j=0, n-i, sum(k=0, n-i-j, (-1)^k * binomial(n-i, j) * binomial(n-i-j, k) * binomial((n-i-j)*(n-i-k), i) )))}
for(n=0, 35, print1(a(n), ", "))
(PARI) {a(n) = sum(i=0, n, (n-i+1)*(n-i+2)/2 * sum(j=0, n-i, sum(k=0, n-i-j, (-1)^j * binomial((n-i-j)*(n-i-k), i) * (n-i)! / ((n-i-j-k)!*j!*k!) )))}
for(n=0, 35, print1(a(n), ", "))
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CROSSREFS
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Cf. A323680, A326006, A326008.
Sequence in context: A203454 A197666 A146009 * A272559 A270991 A029512
Adjacent sequences: A326004 A326005 A326006 * A326008 A326009 A326010
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KEYWORD
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nonn
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AUTHOR
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Paul D. Hanna, Jun 02 2019
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STATUS
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approved
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