Re: Mathematica and infinite series

• To: mathgroup at smc.vnet.net
• Subject: [mg113197] Re: Mathematica and infinite series
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Mon, 18 Oct 2010 05:47:36 -0400 (EDT)

```The problem is that Mathematica seems to go crazy when asked to evaluate:

Sum[(Log[n]/(n^2*n!))*x^n, {n, 1, Infinity}]

E^x*Derivative[1, 0][BellB][-2, x]

The answer is given in terms of the derivative of the function BellB[n,x] with respect to the first variable at {-2,x}. But, and this is weird, since Bell[n,x] is the n-th Bell polynomial so n must be a non-negative integer:

In[4]:== BellB[-1,2]
During evaluation of In[4]:== BellB::intnm: Non-negative machine-size integer expected at position 1 in Subscript[B, -1](2). >>

Really weird.

Andrzej Kozlowski

On 17 Oct 2010, at 12:06, Sam Takoy wrote:

> Hi,
>
> I am about to embark on a project that operates heavily in infinite
> series, so I started figuring out Mathematica's basis capabilities. I
> found them very impressive, but I came across this:
>
>
> f[x_] :== Sum[Log[n]/(n^2 Factorial[n]) x^n, {n, 1, Infinity}]
> Assuming[n > 0, SeriesCoefficient[f[x], {x, 0, 4}]]
>
>
>
> SeriesCoefficient[\!\(
> \*UnderoverscriptBox[\(\[Sum]\), \(n == 1\), \(\[Infinity]\)]
> \*FractionBox[\(
> \*SuperscriptBox[\(x\), \(n\)]\ Log[n]\), \(
> \*SuperscriptBox[\(n\), \(2\)]\ \(n!\)\)]\), {x, 0, 4}]
>
>
> Why doesn't Mathematica produce Log[n]/(n^2 Factorial[n]) as the answer?
>
> Thanks!
>
> Sam
>

```

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