I have a 2 parameter model that includes an integral function and I´m quite lost how to solve this.

My data consists of a set of given molecule sizes, r_m, and a calculated response, K.

I want to fit this K with a theoretical model, where each K_i comes from integrating a particle size distribiution function, f(r), for instance a Gaussian distribution. The equations then look like this:

as you can see the integral function has the parameteres that I want to fit, but also the lower limit of the integral in the numerator involves a variable, r_m. I´m not sure if I can implement this situation to one of Matlab´s built-in fitting procedures, or do I have to code my own fitting algorithm? Here´s something I tried but I know I´m just playing Frankenstein here:

` % Data = ...`

[0.5 1 1.1 0.83 1.6 0.74 2.2 0.55 2.5 0.28 3.5 0];r = Data(:,1);K_exp = Data(:,2);F = @(x,xdata) quad( (exp(-1/2.*((xdata - x(1))/x(2)).^2).*(1 - (xdata(1)/xdata).^2)),xdata(1),120)./quad(exp(-1/2.*((xdata - x(1))/x(2)).^2,0,120) ; ;x0 = [6 0.5] ;[x,resnorm,~,exitflag,output] = lsqcurvefit(F,x0,r,K_exp)

Any help on how to approach this problem is greatly appreciated!

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