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Subject: Re: Curve fitting - How to get polynomial of form ax^2 + 0x + 0?
Date: Wed, 9 Jul 2008 04:21:02 +0000 (UTC)
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"David Migl" <migl.spam@gmail.com> wrote in message
<g51dfa$q8h$1@fred.mathworks.com>...
> Hi,
> 
> I am using MATLAB to fit a curve to data. I have a physics
> formula of the form y=ax^2 and I am trying to determine the
> value of the constant _a_ using the data. When I fit a
> second degree polynomial to the data (using polyfit), MATLAB
> gives me the constants a b and c of the polynomial in the
> form of ax^2 + bx + c. Of course, that doesn't help me find
> _a_ for my formula. The extra terms of power<2 throw me off.
> I need a polynomial of the form ax^2 + 0x + 0 that fits the
> data. How can I accomplish this?

Hi
Very simply. Your problem is a special form of polynomial.
The coefficient a may be found by the statement
     
     a = x(:).^2\y(:);

It is a reduced form of the polynomial fit, coefficient of
which can be found from the formula (should x and y be
column vectors)

     c = [ones(size(x)), x, x.^2, x.^3,...]\y

Have a nice day.
Mira