       Problem with using /.

• To: mathgroup at smc.vnet.net
• Subject: [mg69207] Problem with using /.
• From: "Nag" <Naga1010 at gmail.com>
• Date: Fri, 1 Sep 2006 18:41:09 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Hello:

I am trying to compute the moments of a distribution using the moment
generating function, MGF[t,k]. Here is the relevant function for
computing the j'th moment.

M[j_, t_, k_] :=    D[MGF[t, k], {t, j}] /. {u -> 0, Sum[Subscript[p,
i],{i, 0, k}] = 1};

After taking the j'th derivative, need to set u to 0 and the sum of
p(i)'s to 1.

When I execute M[3, u, 5], get the following error.(converted output to
input form)

\!\(\*
StyleBox[
ErrorBox[\(Set::
"\<write\>" :
"\<Tag \!\(Sum\) in \!\(\(â??\+\(i = 0\)\%5\) p\_i\) is
Protected. \!\(\*ButtonBox[\"Moreâ?¦\",
\", ButtonFrame->None, ButtonData:>\"General::write\"]\)\>"\)],
ShowStringCharacters->True]\)

Of course I can explicitly set p0+p1+..+p5 = 1 after executing M[3,u,5]
but involves more effort as k increases.
Is it possible to make the substitution work for general k in the
function definition M[....]?

Best
Nag

```

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