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From: | Jim Heasley |
Subject: | [Bug-gsl] Bug(?) in reproducing gsl_multilinear_wfit test case |
Date: | Wed, 25 Apr 2012 07:33:56 -1000 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.28) Gecko/20120313 Thunderbird/3.1.20 |
I'm sorry to bother you folks with what should be simple, but I have been unable to get the sample code that executes the routine gsl_multilinear_wfit.c (which I literally cut and pasted from the on-line HTML documentation) to give an answer that is the same or even close to what is given in the documentation. Best as I can see this is not reported in the BUGS list or in the on-line bug reports, but I must admit to some difficulty navigating them. The input data for the test is created using the program generate.c which computes y = exp(x) + noise where 0.1 <= x <= 2.0. As the noise -> 0 one should recover the Taylor expansion of exp(x), namely exp(x) = 1.0 + x/1! + x^2/2! + ... which is in fact what is shown in the documentation for this case (allowing for the fact some considerable amount of noise has been added). My compiled code gives the result # best fit: Y = 1.18246 + 0.184573 X + 1.3031 X^2 If I cut the noise size down in generate by setting double sigma = 0.00001*y0 then I should get close to the analytic values, but instead get # best fit: Y = 1.08437 + 0.435157 X + 1.24313 X^2 I am running the code on a Ubuntu 10.03 LTS platform with an AMD Phenom processor with GSL version 1.15. The same behavior (and exactly the same answers are found on a 32 bit AMD XP2400 processor with GSL 1.9 or GSL 1.15) I have recompiled the library turning off all optimization in all modules to no avail, same results. So, having run out of ideas and having tested all the "standard" kind of stuff I'm submitting this report. (BTW, I tried the test example immediately preceding this one in the documentation and it reproduces the result in the manual perfectly.) I'm attaching to this report the following files: 1) generate.c - data generate (cut/pasted from the web) 2) exp.dat - output from above, input to fit 3) fit.c - data fitter (cut/pasted from the web) 4) fit.output - output from fit.c and exp.dat Please let me know if there is any other information I can provide to help explain this rather strange result. -- Jim Heasley Institute for Astronomy address@hidden University of Hawaii phone: 808-956-6826 2680 Woodlawn Drive fax: 808-956-4532 Honolulu, HI 96822
generate.c
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exp.dat
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fit.c
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fit.output
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