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RE: dropping higher order terms

• To: mathgroup at smc.vnet.net
• Subject: [mg32639] RE: [mg32596] dropping higher order terms
• From: "David Park" <djmp at earthlink.net>
• Date: Fri, 1 Feb 2002 02:02:51 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

Chad,

Let's assume that you don't want to disturb any higher order terms that
might be contained in g[x]. Then something like the following would work.

f[x_] = Array[a, 10, 0] . Array[x^#1 & , 10, 0]*g[x]
(a[0] + x*a[1] + x^2*a[2] + x^3*a[3] + x^4*a[4] +
   x^5*a[5] + x^6*a[6] + x^7*a[7] + x^8*a[8] + x^9*a[9])*
  g[x]

MapAt[Take[#, 3] &, f[x], 1]
(a[0] + x*a[1] + x^2*a[2])*g[x]

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/

> From: Chad Junkermeier [mailto:cej38 at email.byu.edu]
To: mathgroup at smc.vnet.net
>
> I have a problem where I have a function of the form
>
> f(x) = (a0 x^0 + a1 x^1 +a2 x^2 + ...+ aN x^N) * g(x)
>
> I would like to be able to get rid of the higher order terms without
> altering the form of g(x).  Resulting in:
>
> f(x) = (a0 x^0 + a1 x^1 +a2 x^2) * g(x)
>
> I tried the following:
>
> Series[f(x)/g(x),{x,0,2}] * g(x)
>
> But this gives the wrong answer.   How can I do this?
>
> Chad Junkermeier
>
> Department of Physics and Astronomy
>
> Brigham Young University
>
> email: junkermeier at byu.edu
>
> phone: 801-368-3529
>
>
>
>