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Fityk is a program for nonlinear fitting of analytical functions (especially peak-shaped) to data (usually experimental data). There are also people using it to remove the baseline from data, or to display data only. Fityk is reportedly used in crystallography, chromatography, photoluminescence and photoelectron spectroscopy, infrared and Raman spectroscopy, to name but a few.

Fityk knows about common peak-shaped functions (Gaussian, Lorentzian, Voigt, Pearson VII, bifurcated Gaussian, EMG, Doniach-Sunjic, etc.) and polynomials. Fityk also supports user-defined functions. Fityk offers intuitive graphical interface (and also command line interface), variouse optimization methods (standard Marquardt least-square algorithm, Genetic Algorithms, Nelder-Mead simplex), equality constraints, automation of common tasks with scripts