Extrema can be downloaded from SourceForge at https://sourceforge.net/projects/extrema

Ubuntu Linux users can install Extrema via the command: sudo apt-get install extrema

Gentoo Linux users can install Extrema via the command:   emerge extrema

Fedora Linux users can install Extrema via the command:    yum install extrema

Slackware Linux users can install Extrema via a SlackBuild script:   http://slackbuilds.org/academic/extrema

Note for international Linux users:
Extrema is built with Unicode support. Unfortunately, the expression parser and evaluator are my own creations,
and work on a character by character analysis, so the decimal point, '.', in a number is hard coded in the sources. This means that if your locale uses a comma for real numbers and you try to enter a real number into Extrema using a comma (for example X=0,1) Extrema will say it is an invalid use of a comma, but using the decimal point (for example X=0.1) also fails as an invalid real number. The solution for now seems to be to run Extrema from a script which sets the locale to the standard "C" before running Extrema and then resets it to your locale afterwards, i.e.,
export LC_ALL="C"
extrema &
export LC_ALL="your_locale"

where, for example, "your_locale" might be "de_DE.UTF-8" for Germany or "fr_FR.UTF-8" for France.