Mathomatic is a free, portable, general-purpose Computer Algebra System (CAS) that can automatically solve, differentiate, simplify, combine, and compare algebraic equations, perform standard, complex number, modular, and polynomial arithmetic, etc. It does some calculus and is very easy to compile/install, learn, and use. Plotting expressions with gnuplot is also supported.
Donations are needed for the Mathomatic project's continued well-being.
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I am a free software developer and mathematics enthusiast. I maintain the Mathomatic computer algebra system.