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# Introduction

## What is the program for?

Fityk is a program for nonlinear fitting of analytical functions (especially peak-shaped) to data (usually experimental data). The most concise description: peak fitting software. There are also people using it to remove the baseline from data, or to display data only.

It is reportedly used in crystallography, chromatography, photoluminescence and photoelectron spectroscopy, infrared and Raman spectroscopy, to name but a few. Although the author has a general understanding only of experimental methods other than powder diffraction, he would like to make it useful to as many people as possible.

Fityk offers various nonlinear fitting methods, simple background subtraction and other manipulations to the dataset, easy placement of peaks and changing of peak parameters, support for analysis of series of datasets, automation of common tasks with scripts, and much more. The main advantage of the program is flexibility - parameters of peaks can be arbitrarily bound to each other, e.g. the width of a peak can be an independent variable, the same as the width of another peak, or can be given by complex (and general for all peaks) formula.

Fityk is free software; you can redistribute and modify it under the terms of the GPL, version 2 or (at your option) any later version. See Appendix C. License for details. You can download the latest version of fityk from http://www.unipress.waw.pl/fityk (or http://fityk.sf.net). To contact the author, visit the same page.

## How to read this manual

After this introduction, you may read the Getting started, look for tutorials in wiki and postpone reading the manual until you need to write a script, put constraints on variables, add user-defined function or understand better how the program works.

In case you are not familiar with the term weighted sum of squared residuals or you are not sure how it is weighted, have a look at Nonlinear optimization. Remember that you must set correctly standard deviations of y’s of points, otherwise you will get wrong results.

## GUI vs CLI

The program comes in two versions: the GUI (Graphical User Interface) version - more comfortable for most users, and the CLI (Command Line Interface) version (named cfityk to differentiate).

If the CLI version was compiled with the GNU Readline Library, command line editing and command history as per bash will be available. Especially useful is `TAB`-expanding. Data and curves fitted to data are visualized with gnuplot (if it is installed).

The GUI version is written using the wxWidgets library and can be run on Unix species with GTK+ and on MS Windows. There are also people using it on MacOS X (see details in the wiki).

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