function f = fnctnls(xtest);

% This is the function modeled on fitdemo:
% a nonlinear least squares problem

%  The model is
%
%    y_model =  c(1)*exp(-lam(1)*t) + c(2)*exp(-lam(2)*t)
%                + ... c(my_n/2) exp(-lam(my_n/2)*t)
%
%  The data is given as (t, y(t)) in the array my_Data,
%  and the vector xtest contains lam as the first my_n/2
%  components and c as the last my_n/2.
%  The residual is the vector my_Data(:,2)-y_model, and
%  the function value is the norm of this vector.

global my_Data my_n

f = norm(my_Data(:,2) -  ...
   exp(-my_Data(:,1)*xtest(1:my_n/2,1)')*xtest(my_n/2+1:my_n,1), 2)^2;