function[f,g,GN]=expfun(b,xvec,yvec)


% Residual sum of squares.
r = yvec - b(1)*(1-exp(-b(2)*xvec));
f = .5*r'*r;
J = [ - ( 1-exp(-b(2)*xvec) ), - b(1)*xvec.*exp(-b(2)*xvec)];
g = J'*r;
GN = J'*J;