RE: Expansion Coefficients in Multivariate Series Expansions?

• To: mathgroup at smc.vnet.net
• Subject: [mg26607] RE: [mg26579] Expansion Coefficients in Multivariate Series Expansions?
• From: "David Park" <djmp at earthlink.net>
• Date: Thu, 11 Jan 2001 10:39:16 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```extractCoeff[term_][expr_] := Select[f, NumberQ[#/term] &]/term

f = 50 + 2*x + 3*y + 4*z + 5*x*y + 6*x^2*y*z;

(extractCoeff[#1][f] & ) /@ {1, x, y, z, x*y, x^2*y*z,
x^3}
{50, 2, 3, 4, 5, 6, 0}

David Park

> From: A. E. Siegman [mailto:siegman at stanford.edu]
To: mathgroup at smc.vnet.net
>
> Given an expression that is the sum of products of integer powers of
> three variables (say x,y,z) multiplied by numerical coefficients:
>
>    f = c000 + c100 x + c010 y + c001 z + c110 x y + c211 x^2 y z + . . .
>
> where the  cijk's are all real numbers, and f may or may not be in
> Normal (or other) form, how do I find the coefficient of a particular
> term, e.g. c100 for the x term (*not*  c1000 + c110 y), or  ckmn for the
> x^k y^m z^n term?
>
> Neither Coefficient nor SeriesCoefficient seems to do this -- and the
> SeriesCoefficient[expr, {n1,n2,n3,..}] syntax seems to be mentioned but
> neither explained or illustrated in the on-line Help.
>
> Thanks, siegman at stanford.edu
>

```

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