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Topic: complex results using ODE45
Replies: 3   Last Post: Oct 22, 2013 2:25 PM

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Jeffrey

Posts: 3
Registered: 10/15/13
Re: complex results using ODE45
Posted: Oct 20, 2013 11:44 PM
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"Nasser M. Abbasi" wrote in message <l3k0lp$fad$2@speranza.aioe.org>...
> On 10/15/2013 12:53 PM, Jeffrey wrote:
> > I have the two m files given below. "a" and "T" are returned as complex with
> >
> > Why are values becoming complex?
> >
> > Please help.
> >
> > Jeff
> > _______________________________
> > %this is in file alpha.m
> > function dadT = alpha(T,a)
> > b = 20
> > l= 50.1
> > k=exp(l)
> > ner = -36108.73
> > n=1.269
> > dadT = (k/b)*(exp(ner/T)) *((1-a)^n
> > ________________________________________
> >

>
> You have missing ")" above btw. Last line.
>
> But besides this, which I assume a cut/paste issue only,
> look at the expression itself.
>
> What happens for (1-a)^n, and "a" happens to be
> larger than one?
>
> EDU>> (1-1.5)^(1.269)
>
> ans =
>
> -0.2754 - 0.3104i
>

Nasser,

yes ")" was missing and thanks for the explanation.

On inspection, all coefficients of "i" for "T" were zero. Coefficients of "i" for "a" only became nonzero near the end, as the real value "a" approaches 1.

Physically "a" must be < or = to 1.

The real part of "a" gives an accurate solution.

I think that the "a's" that become >1 are the intermediate values of "a" are being computed to in order to evaluate the "a" at "T+h" using the Runge-Kutta formulation.

Anyway to limit the value of these intermediate "a's" to a maximum value of 1?



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