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Message: [QUOTE="TJW, post: 2461097"] Hello, As others have mentioned, this is off-topic here. I have worked with the GNU Scientific Library (GSL) some so I will make a comment that might help, but in the future you might have better luck a GSL group. A quick look at lines 358-360 of file sinint.c reveals: if(x <= 0.0) { DOMAIN_ERROR(result); } This is in function int gsl_sf_Ci_e(const double x, gsl_sf_result * result); which means that you are calling Ci(x) with x < 0, which is a domain error. As I don't see a call to Ci(x) either explicitly or implicitly in the code you posted, it seems that your error is elsewhere. In case you deleted the GSL source or simply installed the packages, the source for this function can be found here: [URL]http://koders.com/c/fid5CD508768B20C663DA80E3ABF80E4373DD1F7E9F.aspx?s=Chebyshev[/URL] Good Luck, TJW [/QUOTE]