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A229451 G.f.: exp( Sum_{n>=1} (3*n)!/n!^3 * x^n/n ). 3
1, 6, 63, 866, 13899, 246366, 4676768, 93322596, 1934035965, 41286407510, 902562584556, 20119266633060, 455832458083577, 10470568749165246, 243361203186769659, 5714294570067499930, 135377464019074334826, 3232534121305720233264, 77726654423445817800164 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

The sixth root of the o.g.f. A(x)^(1/6) = 1 + x + 8*x^2 + 101*x^3 + 1569*x^4 + 27445*x^5 + ... appears to have integer coefficients. See A229452. More generally, if A(m,x) := exp( Sum_{n >= 1} (m*n)!/n!^m * x^n/n ), m = 1,2,3,..., then it can be shown that the expansion of A(m,x) has integer coefficients. We conjecture that the expansion of A(m,x)^(1/m!) also has integer coefficients. - Peter Bala, Feb 16 2020

LINKS

Table of n, a(n) for n=0..18.

FORMULA

a(n) ~ c * 3^(3*n)/n^2, where c = 0.406436... - Vaclav Kotesovec, Dec 25 2013

EXAMPLE

G.f.: A(x) = 1 + 6*x + 63*x^2 + 866*x^3 + 13899*x^4 + 246366*x^5 +...

where

log(A(x)) = 6*x + 90*x^2/2 + 1680*x^3/3 + 34650*x^4/4 + 756756*x^5/5 +...+ A006480(n)*x^n/n +...

MATHEMATICA

CoefficientList[Series[Exp[6*x*HypergeometricPFQ[{1, 1, 4/3, 5/3}, {2, 2, 2}, 27*x]], {x, 0, 20}], x] (* Vaclav Kotesovec, Dec 25 2013 *)

PROG

(PARI) {a(n)=polcoeff(exp(sum(k=1, n, (3*k)!/k!^3*x^k/k) +x*O(x^n)), n)}

for(n=0, 25, print1(a(n), ", "))

CROSSREFS

Cf. A229452, A006480 (De Bruijn's S(3,n)), A333042, A333043.

Sequence in context: A234465 A231552 A302103 * A132078 A113669 A340912

Adjacent sequences:  A229448 A229449 A229450 * A229452 A229453 A229454

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Sep 23 2013

STATUS

approved

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Last modified June 18 18:17 EDT 2021. Contains 345120 sequences. (Running on oeis4.)