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A326006 G.f.: Sum_{n>=0} (n+1) * x^n * ((1+x)^n + 1)^n / (1 + x*(1+x)^n)^(n+2). 3
1, 2, 5, 10, 35, 110, 484, 2090, 10449, 54526, 306394, 1817190, 11357273, 74488856, 510662890, 3649746072, 27120964497, 209070202378, 1668632739832, 13763654492458, 117143535623438, 1027274860573646, 9269762507422848, 85969437563219710, 818533062206441679, 7992935182604293880, 79974062362514607510, 819199175961485536540, 8583780760842171037456 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

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 = 2 and q = (1+x), p = 1, r = x. See other examples for k = 1 (A323680), k = 3 (A326007), k = 4 (A326008).

LINKS

Table of n, a(n) for n=0..28.

FORMULA

G.f.: Sum_{n>=0} (n+1) * x^n * ((1+x)^n + 1)^n / (1 + x*(1+x)^n)^(n+2).

G.f.: Sum_{n>=0} (n+1) * x^n * ((1+x)^n - 1)^n / (1 - x*(1+x)^n)^(n+2).

G.f.: Sum_{n>=0} (n+1) * x^n * Sum_{k=0..n} binomial(n,k) * ( (1+x)^n - (1+x)^k )^(n-k).

G.f.: Sum_{n>=0} (n+1) * 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) * 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) * 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) * 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!).

EXAMPLE

G.f.: A(x) = 1 + 2*x + 5*x^2 + 10*x^3 + 35*x^4 + 110*x^5 + 484*x^6 + 2090*x^7 + 10449*x^8 + 54526*x^9 + 306394*x^10 + 1817190*x^11 + 11357273*x^12 + ...

such that

A(x) = 1/(1+x)^2 + 2*x*((1+x) + 1)/(1 + x*(1+x))^3 + 3*x^2*((1+x)^2 + 1)^2/(1 + x*(1+x)^2)^4 + 4*x^3*((1+x)^3 + 1)^3/(1 + x*(1+x)^3)^5 + 5*x^4*((1+x)^4 + 1)^4/(1 + x*(1+x)^4)^6 + 6*x^5*((1+x)^5 + 1)^5/(1 + x*(1+x)^5)^7 + 7*x^6*((1+x)^6 + 1)^6/(1 + x*(1+x)^6)^8 + 8*x^7*((1+x)^7 + 1)^7/(1 + x*(1+x)^7)^9 + ...

also,

A(x) = 1/(1-x)^2 + 2*x*((1+x) - 1)/(1 - x*(1+x))^3 + 3*x^2*((1+x)^2 - 1)^2/(1 - x*(1+x)^2)^4 + 4*x^3*((1+x)^3 - 1)^3/(1 - x*(1+x)^3)^5 + 5*x^4*((1+x)^4 - 1)^4/(1 - x*(1+x)^4)^6 + 6*x^5*((1+x)^5 - 1)^5/(1 - x*(1+x)^5)^7 + 7*x^6*((1+x)^6 - 1)^6/(1 - x*(1+x)^6)^8 + 8*x^7*((1+x)^7 - 1)^7/(1 - x*(1+x)^7)^9 + ...

PROG

(PARI) {a(n) = my(A = sum(m=0, n+1, (m+1) * x^m*((1+x +x*O(x^n) )^m - 1)^m/(1 - x*(1+x +x*O(x^n) )^m )^(m+2) )); polcoeff(A, n)}

for(n=0, 35, print1(a(n), ", "))

(PARI) {a(n) = sum(i=0, n, (n-i+1) * 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) * 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), ", "))

CROSSREFS

Cf. A323680, A326007, A326008.

Sequence in context: A343167 A138190 A056300 * A144636 A320430 A018418

Adjacent sequences:  A326003 A326004 A326005 * A326007 A326008 A326009

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Jun 02 2019

STATUS

approved

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Last modified June 18 06:02 EDT 2021. Contains 345098 sequences. (Running on oeis4.)