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 Post subject: How to constrain parameters in a nonlinear equation?
PostPosted: Fri Jul 19, 2013 5:10 pm 
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Joined: Fri Jul 19, 2013 4:03 pm
Posts: 2
I am trying to solve the equation: a * ( x ^ c / b ^ c + 1 ) ^ (1/c) where a/b = constant

I can solve the equation but I don't know how to introduce the constraint a/b = constant (or alternatively ABS(constant - a/b) is minimized)

Would anyone be kind enough to set me on the right track here?

So far I have :

Code:
Public Sub function_ShrinkageFit_func(c As Double(), x As Double(), ByRef func As Double, obj As Object)
     func = c(0) * ((((x(0) ^ c(2)) / (c(1) ^ c(2))) + 1) ^ (1 / c(2)))
End Sub

Public Function ShrinkageFit()
    Dim x(,) As Double
    Dim y() As Double
    Dim c() As Double = New Double() {1, 1, 10}
    Dim bndl() As Double = New Double() {0.0, 0.0, 0.0}
    Dim bndu() As Double = New Double() {Double.PositiveInfinity, Double.PositiveInfinity, 50.0}
    Dim epsf As Double = 0
    Dim epsx As Double = 0.000001
    Dim maxits As Integer = 0
    Dim info As Integer
    Dim state As alglib.lsfitstate = New alglib.lsfitstate()
    Dim rep As alglib.lsfitreport = New alglib.lsfitreport()
    Dim diffstep As Double = 0.0001

    ... fill x and y

    alglib.lsfitcreatef(x, y, c, diffstep, state)
    alglib.lsfitsetbc(state, bndl, bndu)
    alglib.lsfitsetcond(state, epsf, epsx, maxits)
    alglib.lsfitfit(state, AddressOf function_ShrinkageFit_func, Nothing, Nothing)
    alglib.lsfitresults(state, info, c, rep)


Thanks,
Michael


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 Post subject: Re: How to constrain parameters in a nonlinear equation?
PostPosted: Mon Jul 22, 2013 8:44 pm 
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Joined: Fri Jul 19, 2013 4:03 pm
Posts: 2
Kindly ignore this. I was missing the obvious. Simply replace 'b' with 'a/constant' and get on with life.


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