Hello, Thanks! I was not sure it was appropriate to open a bug in the bug tracker until I was convinced it was indeed a bug in GSL.
-- Phyks Le 17/02/2017 à 23:17, Patrick Alken a écrit : > Hello, > > I'm not familiar with the mathieu functions, but I entered your report > into the bug tracker so hopefully someone with more knowledge than I can > take a look and fix it. > > On 02/17/2017 08:53 AM, Phyks wrote: >> Hi, >> >> I have some code that I prototyped in Mathematica and am now writing >> in C using GSL, that makes use of Mathieu functions. I have different >> results between the two of them, and I cannot figure out whether this >> is a bug in GSL, Mathematica or simply some misunderstanding from my >> part. >> >> I am using `MathieuC` function in latest Mathematica >> (http://reference.wolfram.com/language/ref/MathieuC.html) which should >> be the same function as `gsl_sf_mathieu_ce` >> (https://www.gnu.org/software/gsl/manual/html_node/Angular-Mathieu-Functions.html#Angular-Mathieu-Functions) >> except that the former one takes a single `a` argument being the >> characteristic value whereas the GSL 2.3 implementation takes the >> order `n` and the `q` parameter directly. >> >> So, I guess, >> ``` >>> N[MathieuC[MathieuCharacteristicA[0, -1], -1, 2*Pi/180]] >> 1.41071 >> ``` >> >> should be equivalent to >> ``` >> gsl_sf_mathieu_ce(0, -1.0, 2.0 * M_PI / 180.0) >> ``` >> which gives a totally different value: 0.99751942347886335. >> >> >> I tried to debug with different values, and the discrepancies between >> Mathematica and GSL seems to appear only when the `q` parameter (-1.0 >> here) is negative. If I take 1.0 instead, I get values in agreement. I >> tried to find yet another implementation to debug it, and found Scipy >> (https://docs.scipy.org/doc/scipy/reference/generated/scipy.special.mathieu_cem.html#scipy.special.mathieu_cem) >> which relies on Fortran SPECFUN library apparently, and is in >> agreement with GSL. >> >> I am missing something? Thanks! > > >
