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MathGroup Archive 2005

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Getting crude approximation to a function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg58832] Getting crude approximation to a function
  • From: "Mukhtar Bekkali" <mbekkali at gmail.com>
  • Date: Wed, 20 Jul 2005 00:29:29 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Assume I have a function f[x], x is some variable, given below (my real
function is much more complex). I would like to obtain its crude
approximation. I used command Series, first order expansion. The
resulting function has coefficients that have high precision. I do not
need that since my expansion is very crude anyway. I need coefficients
that are rational number approximations to these coefficients. How do I
obtain this? It seems to me that command Chop takes care of
coefficients that are not product with variable x but cannot handle
coefficients that are not standalone. For instance, in this example

\!\(\(\(Normal[
        Series[0.71  p\
            x + \(1\/3\)
            x\^2 - 4, {x, 1, 1}]] // Expand\) // Chop\) //
Rationalize\)

I would like to obtain output of the form (-13/10)+(29/10)x.
Mathematica gives me (-13/10)+2.8972x instead, where it keeps
2.8971974507154195` in the memory. I need this because I use
InequalitySolve package and it refuses to function unless all numbers
are rational.  

Mukhtar Bekkali


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