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oyr
1 Posts |
 Posted - 03/09/2012 : 03:33:49 AM
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hi
i have one problem
that is non-linear curve fit from ogden function
function : y = A*(x^B-x^(-0.5*B))/x+C*(x^D-x^(-0.5*D))/x+E*(x^F-x^(-0.5*F))/x
and parameter : A, B, C, D, E, F
but parameter relation is A*B>0, C*D>0, E*F>0
so, i try to parameter init code using
i write parameter init to A*B>0, C*D>0, E*F>0
but this solution don't select fitting
help me |
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greg
USA
1378 Posts |
Posted - 03/09/2012 : 12:53:33 PM
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First, "A*B>0, C*D>0, E*F>0" is not initializing anything so I think you mean you want that as a constraint. We do NOT support non-linear contraints; we only support linear.
If all parameters are positive, then you could get by with:
A>0;B>0;C>0;D>0;E>0;F>0;
All negative would be:
A<0;B<0;C<0;D<0;E<0;F<0;
Since the product of two positives or two negatives is always positive, the the A*B>0 condition would be met without resorting to the unsupported non-linear constraints.
(Note use of ';' to separate conditions.)
That would only begin to solve the problem of fitting with this over parameterized function which consists of the sum of three identical
expressions. Such a function would be doomed to forever wandering through parameter space in search of some better fit which could never occur unless you put some Bounds on each parameter. You can, of course, place Bounds on each parameter in a fit. |
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ogden non-linear curve fit problem