The example given in nonlinearfit.html (paragraph 8.1) does not work, at least as far as version 1.14.3 is concerned
(I didn't have a chance to take a look at the .5 version).

Attached is a version that does.

Date: 2011-04-05 16:52
Sender: Yoshiki Tsunesada

It looks running perfect. What's wrong with the script?

yoshiki@hastings[23] ruby test_gsl_leastsquares.rb
iter: 0 x =      1.00000000      0.00000000      0.00000000 |f(x)|
= 117.749
iter: 1 x =      1.52223970      0.01204385      0.52223970 |f(x)|
= 84.6007
iter: 2 x =      2.00812827      0.04430986      1.48879929 |f(x)|
= 73.2149
iter: 3 x =      3.52117634      0.06629985      0.94957045 |f(x)|
= 30.702
iter: 4 x =      4.80654030      0.10317119      1.07774335 |f(x)|
= 11.817
iter: 5 x =      5.03712230      0.10408189      1.02410822 |f(x)|
= 10.3668
iter: 6 x =      5.03731855      0.10405357      1.02411625 |f(x)|
= 10.3668
A      = 5.03732 +/- 0.06147
lambda = 0.10405 +/- 0.00216
b      = 1.02412 +/- 0.01342