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Fityk is a flexible and portable program for nonlinear fitting of analytical functions (especially peak-shaped) to data (usually experimental data). In other words, for nonlinear peak separation and analysis.
It was developed for analyzing diffraction patterns, but can be also used in other fields, since concepts and operations specific for crystallography are separated from the rest of the program.
Fityk offers various nonlinear fitting methods, subtracting background, calibrating data, easy placement of peaks and changing peak parameters, automation of common tasks with scripts, and much more. The main advantage of the program is a flexibility - parameters of peaks can be arbitrarily binded with each other, eg. width of peak can be an independent variable, can be the same as width of other peak or can be given by complicated - common for all peaks - formula.
The GUI was written with portable wxWidgets library, so fityk works in both Unix/Linux and MS Windows environments.
As far as I know, the only program which also is specialized in fitting peaks is PeakFit. Some of useful features found in PeakFit, that are not present in fityk yet, will be implemented in future.
Fityk is free software; you can redistribute and modify it under the terms of GPL, version 2. See Appendix E, License for details. You can download the latest version of fityk from http://www.unipress.waw.pl/~wojdyr/fityk . or http://fityk.sf.net . To contact author, visit the same page.