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A097681 E.g.f.: (1/(1-x^6))*exp( 6*sum_{i>=0} x^(6*i+1)/(6*i+1) ) for an order-6 linear recurrence with varying coefficients. 5
1, 6, 36, 216, 1296, 7776, 47376, 314496, 2612736, 28740096, 368395776, 4796983296, 60300205056, 750367328256, 10151357239296, 164475953381376, 3110937349718016, 61410199093641216, 1174438559356747776 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Limit_{n->inf} n*n!/a(n) = 6*c = 0.1140186893... where c = 6*exp(psi(1/6)+EulerGamma) = 0.0190031148...(A097671) and EulerGamma is the Euler-Mascheroni constant (A001620) and psi() is the Digamma function (see Mathworld link).

REFERENCES

Mohammad K. Azarian, Problem 1218, Pi Mu Epsilon Journal, Vol. 13, No. 2, Spring 2010, p. 116.  Solution published in Vol. 13, No. 3, Fall 2010, pp. 183-185.

A. M. Odlyzko, Linear recurrences with varying coefficients, in Handbook of Combinatorics, Vol. 2, R. L. Graham, M. Grotschel and L. Lovasz, eds., Elsevier, Amsterdam, 1995, pp. 1135-1138.

LINKS

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

Benoit Cloitre, On a generalization of Euler-Gauss formula for the Gamma function, preprint 2004.

Andrew Odlyzko, Asymptotic enumeration methods, in Handbook of Combinatorics, vol. 2, 1995, pp. 1063-1229.

Eric Weisstein's World of Mathematics, Digamma Function.

FORMULA

For n>=6: a(n) = 6*a(n-1) + n!/(n-6)!*a(n-6); for n<6: a(n)=6^n. E.g.f.: 1/(1-x^6)*(1+x)/(1-x)*sqrt((1+x+x^2)/(1-x+x^2))* exp(sqrt(3)*atan(sqrt(3)*x/(1-x^2))).

EXAMPLE

The sequence {1, 6, 36/2!, 216/3!, 1296/4!, 7776/5!, 47376/6!,...} is generated by a recursion described by Benoit Cloitre's generalized Euler-Gauss formula for the Gamma function (see Cloitre link).

PROG

(PARI) {a(n)=n!*polcoeff(1/(1-x^6)*exp(6*sum(i=0, n, x^(6*i+1)/(6*i+1)))+x*O(x^n), n)}

(PARI) a(n)=if(n<0, 0, if(n==0, 1, 6*a(n-1)+if(n<6, 0, n!/(n-6)!*a(n-6))))

CROSSREFS

Cf. A097671, A097677-A097680, A097682-A097682.

Sequence in context: A215748 A000400 A238936 * A050736 A196869 A172489

Adjacent sequences:  A097678 A097679 A097680 * A097682 A097683 A097684

KEYWORD

nonn,changed

AUTHOR

Paul D. Hanna, Sep 01 2004

STATUS

approved

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Last modified March 8 20:14 EST 2021. Contains 341953 sequences. (Running on oeis4.)