# [seqfan] Re: Help with A002793 needed/Karol A. Penson

Max Alekseyev maxale at gmail.com
Mon Jul 5 21:16:45 CEST 2010

```The exponential generating function for A002793 is

( E_1(1) - E_1(1/(1-x)) ) * exp(1/(1-x)) / (1-x)
= ( Ei(-1/(1-x)) - Ei(-1) ) * exp(1/(1-x)) / (1-x)

where E_1(x) and Ei(x) are the exponential integrals
http://mathworld.wolfram.com/ExponentialIntegral.html

Max

On Mon, Jul 5, 2010 at 1:02 PM, Karol PENSON <penson at lptl.jussieu.fr> wrote:
> Can anyone help with the following three questions concerning A002793:
>
>     1.  is the formula for a(n) known ?
>     2.  is any generating function (ogf , egf ... etc. ) of a(n) known ?
>     3.  in one of my calculations the following splitting of a(n)'s
> appear :
>
>                   a(2)=4    =3+1
>                   a(3)=20  =11+8+1
>                   a(4)=124=50+58+15+1
>                   a(5)=920=274+444+177+24+1,
>                         etc.
>         I would be happy to obtain the formula for the triangle.
>
>       Thanks in advance ,    Karol A. Penson
>
>
>
>
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>

```