OFFSET
0,2
COMMENTS
a(n) is divisible by (n+1): a(n)/(n+1) = A123681(n).
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..365
FORMULA
a(n) = Pochhammer(n, n + 1)*subfactorial(-2*n - 1) + (-1)^n*subfactorial(-n) where subfactorial(n) = exp(-1)*Gamma(n + 1, -1). - Peter Luschny, Oct 18 2017
a(n) ~ 2^(2*n - 1/2) * n^n / exp(n). - Vaclav Kotesovec, Nov 27 2017
EXAMPLE
Since a(n) = Sum_{k=0..n} k! * [x^k] 1/(1-x)^n, to get a(4),
list coefficients of x^0 through x^4 in 1/(1-x)^4, [1,4,10,20,35],
then dot product with factorials 0! through 4!, [0!,1!,2!,3!,4! ],
so that a(4) = 1*0! + 4*1! + 10*2! + 20*3! + 35*4! = 985.
MAPLE
subfactorial := n -> simplify(exp(-1)*GAMMA(n+1, -1)):
a := n -> pochhammer(n, n+1)*subfactorial(-2*n-1)+(-1)^n*subfactorial(-n):
seq(simplify(evalc(a(n))), n=0..18); # Peter Luschny, Oct 18 2017
MATHEMATICA
Table[Sum[Binomial[n + k - 1, k]*k!, {k, 0, n}], {n, 0, 20}] (* G. C. Greubel, Oct 18 2017 *)
PROG
(PARI) a(n)=sum(k=0, n, binomial(n+k-1, k)*k!)
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Oct 05 2006
EXTENSIONS
Definition corrected Oct 27 2006
STATUS
approved