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MathGroup Archive 1997

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Re: Q: Collect, Coefficient, and non-integer exponents

  • To: mathgroup at smc.vnet.net
  • Subject: [mg8759] Re: [mg8733] Q: Collect, Coefficient, and non-integer exponents
  • From: Allan Hayes <hay at haystack.demon.co.uk>
  • Date: Sun, 21 Sep 1997 20:51:04 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

sergio at scisun.sci.ccny.cuny.edu (Sergio Rojas)
[mg8733] Q: Collect, Coefficient, and non-integer exponents
wants to use Collect and Coefficient on

expr = Expand[Sum[(a*n +b*n -c)*x^(n+0.12/n),{n,1,4}]];

He writes:
> One knows in advance that a power of x is, for example, 1.12 and
> we want to know its coefficient. Apparently Coefficient and Collect
> work only with integers. Is there a way to deal with this
> situation?

Sergio,

Coefficient seems to work with inexact exponents in 3.0:

Coefficient[expr, x^1.12]

	a+b-c
	
For Collect, you might try

Collect[expr/.x^n_->x^(Round[n/.01]), x]/.x^n_->x^(.01 n)

(a + b - c)*x^1.12 + (2*a + 2*b - c)*x^2.06 +
  (3*a + 3*b - c)*x^3.04 + (4*a + 4*b - c)*x^4.03

Allan

Allan Hayes
hay at haystack.demon.co.uk
http://www.haystack.demon.co.uk/training.html
voice:+44 (0)116 2714198
fax: +44 (0)116 2718642
Leicester,  UK


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