www.mathomatic.org
Mathomaticâ„˘ is a portable, commandline, educational CAS and calculator software, written entirely in the C programming language. It is Free and Open Source Software (FOSS), published under the GNU Lesser General Public License (LGPL version 2.1), and has been under continual development since 1986. The software can symbolically solve, simplify, combine, and compare algebraic equations, simultaneously performing generalized standard, complex number, modular, and polynomial arithmetic, as needed. It does some calculus and is very easy to compile/install, learn, and use.
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http://www.mathomatic.org/