How do I definde interdependent parameters using fminsearch?

Hello,

currently I am using fminsearch to fit a measurement's curve. The (mathematical) function I use depends on 4 parameters (in the following I will call them a, b, c and d). So, far it works fine. I also applied fminserachbnd to add lower and upper boundaries for the parameters.

The problem I have is the following: I want to include further constraints so that a+b>0 and c+d>0 (these constraints have physical reasons).

Is it possible to include this constraints using fminsearch? Which other function can you recommend to solve this problem?

In my oprinion, the problem - and the large difference compared to the boundary conditions or constraints defined in fminsearchbnd or fminsearchcon - is that the values change every loop. How can I make the constraints changing with every new set of parameters calculated during the fitting?

This is the code I use.

   model = @TwoDef;
   fitParam = fminsearchbnd(model, guess,lb, ub, options);
   sse=TwoDef(fitParam);

   function [sse, FittedCurve] = TwoDef(params)
        a = params(1);
        b = params(2);
        c = params(3);
        d = params(4);

        FittedCurve = 1./((1./(a.*X+b))+ (1./(c.*X+d)));
        ErrorVector = FittedCurve-Y;
        sse = sum(ErrorVector .^ 2);
   end

[Torsten]

Use lb_a for a and lb_b for b such that lb_a + lb_b >= 0.

This ensures a+b >= lb_a + lb_b >= 0. (Same for c and d).

Best wishes

Torsten.