[leasqr] simple fit does not converge

[Martijn]

Hi,
I am fitting a dispersion function (the Sellmeier equation) to
tablulated refractive index data but leasqr.m does not converge. I am
able to fit e.g. a sine to a data set, so leasqr does work.

After reduction to the bare essentials my code looks like:


E=0.5:.05:6;
E=E(:);
Ntab = sqrt(GetSopraDielFunc(E, 'bk7'));
f = @(E, p) sqrt(1+p(1)./(1-p(2)*E.^2/1.54));
p0 = [1.25, .011];
[Nslm, p, convg, niter] = leasqr(E, Ntab, p0, f);
plot(E, [Ntab, Nslm], 'linewidth', 2)

The strange thing is that not only the covergence is not achieved, but p
does not differ from p0, indicating something is going really wrong.

The initial estimate is pretty good as can be seen by
plot(E, Ntab, E, f(E, [1.25, 0.011])

What's wrong here? Is it possible to show the intermediate values of p?
Where is leasqrdemo gone? If I remember correctly, it did show the
intermediate parameter values.

Thanks,

Martijn