OFFSET
0,2
COMMENTS
A weighted sum of quotients of double factorials.
a(n) are the row sum of triangle A126063.
LINKS
P. Luschny, Variants of Variations.
FORMULA
a(n) = (2n)!/(n! 2^n) Sum(k=0..n, 4^k k!/(2k)!)
a(n) = 2^n Gamma(n+1/2) Sum(k=0..n, 1/Gamma(k+1/2))
a(n) = Sum(k=0..n, 2^k n!!/k!!) [n!! defined as A001147(n), Gottfried Helms]
a(n) = Sum(k=0..n, 2^(2k-n)((n+1)! Catalan(n))/((k+1)! Catalan(k))) [Catalan(n) A000108]
a(n) = Sum(k=0..n, 2^(2k-n) QuadFact(n)/QuadFact(k)) [QuadFact(n) A001813]
Recursive form: a(n) = (2n-1)*a(n-1) + 2^n; a(0) = 1 [Gottfried Helms]
Note: The following constants will be used in the next formulas.
K = (1-exp(1)*Gamma(1/2,1))/Gamma(1/2)
M = sqrt(2)(1+exp(1)(Gamma(1/2)-Gamma(1/2,1)))
Generalized form: For x>0
a(x) = 2^x(exp(1)*Gamma(x+1/2,1) + K*Gamma(x+1/2))
Asymptotic formula:
a(n) ~ 2^n*(1+(exp(1)+K)*(n-1/2)!)
a(n) ~ M(2exp(-1)(n-1/(24*n+19/10*1/n)))^n
MAPLE
a := n -> `if`(n=0, 1, (2*n-1)*a(n-1)+2^n);
MATHEMATICA
a[n_] := Sum[2^k*((2*n-1)!!/(2*k-1)!!), {k, 0, n}]; Table[a[n], {n, 0, 14}] (* Jean-François Alcover, Jun 28 2013 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Peter Luschny, Feb 26 2007
STATUS
approved