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A249940 E.g.f.: 1 + Sum_{n>=1} 2*exp(n^2*x) / 2^n. 3
3, 12, 300, 18732, 2183340, 408990252, 112366270380, 42565371881772, 21262618727925420, 13542138653027381292, 10710751184977536812460, 10299377679212761538176812, 11833116484296581890602595500, 16008903039376673969944510156332, 25190248259800264134073495741338540 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..14.

FORMULA

E.g.f.: 3/(5 - 4*cosh(x)) = Sum_{n>=0} a(n)*x^(2*n)/(2*n)!.

a(n) = Sum_{k=0..2*n} 4*k! * Stirling2(2*n, k) for n>0 with a(0)=3.

a(n) = Sum_{k=1..[(2*n+1)/3]} 2*(3*k)! * Stirling2(2*n+1,3*k) / k for n>0 with a(0)=3, after Vladimir Kruchinin in A242858.

a(n) ~ 2 * (2*n)! / (log(2))^(2*n+1). - Vaclav Kotesovec, Nov 29 2014

EXAMPLE

E.g.f.: A(x) = 3 + 12*x + 300*x^2/2! + 18732*x^3/3! + 2183340*x^4/4! +...

where the e.g.f. equals the infinite series:

A(x) = 1 + 2*exp(x)/2 + 2*exp(4*x)/2^2 + 2*exp(9*x)/2^3 + 2*exp(16*x)/2^4 + 2*exp(25*x)/2^5 + 2*exp(36*x)/2^6 +...

We also have the following series expansion:

3/(5 - 4*cosh(x)) = 3 + 12*x^2/2! + 300*x^4/4! + 18732*x^6/6! + 2183340*x^8/8! + 408990252*x^10/10! +...

MATHEMATICA

nmax=20; Table[(CoefficientList[Series[3/(5-4*Cosh[x]), {x, 0, 2*nmax}], x] * Range[0, 2*nmax]!)[[n]], {n, 1, 2*nmax+2, 2}] (* Vaclav Kotesovec, Nov 29 2014 *)

PROG

(PARI) /* E.g.f.: 3/(5 - 4*cosh(x)): */

{a(n) = local(X=x+O(x^(2*n+1))); (2*n)!*polcoeff( 3/(5 - 4*cosh(X)), 2*n)}

for(n=0, 20, print1(a(n), ", "))

(PARI) /* E.g.f.: 1 + Sum_{n>=1} 2*exp(n^2*x)/2^n */

\p100 \\ set precision

{a(n) = round( n!*polcoeff(1+2*sum(m=1, 500, exp(m^2*x +x*O(x^n))/2^m*1.), n))}

for(n=0, 20, print1(a(n), ", "))

(PARI) /* Formula for a(n): */

{Stirling2(n, k)=n!*polcoeff(((exp(x+x*O(x^n))-1)^k)/k!, n)}

{a(n) = if(n==0, 3, sum(k=0, 2*n, 4*k! * Stirling2(2*n, k) ))}

for(n=0, 20, print1(a(n), ", "))

(PARI) /* Formula for a(n): */

{Stirling2(n, k)=n!*polcoeff(((exp(x+x*O(x^n))-1)^k)/k!, n)}

{a(n) = if(n==0, 3, 2*sum(k=1, (2*n+1)\3, (3*k)! * Stirling2(2*n+1, 3*k) / k))}

for(n=0, 20, print1(a(n), ", "))

CROSSREFS

Cf. A242858, A249941.

Sequence in context: A280086 A102945 A300532 * A132515 A279122 A246957

Adjacent sequences:  A249937 A249938 A249939 * A249941 A249942 A249943

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Nov 19 2014

STATUS

approved

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Last modified May 31 15:41 EDT 2020. Contains 334748 sequences. (Running on oeis4.)