SageMath is a free open-source mathematics software system licensed under the GPL. It builds on top of many existing open-source packages: NumPy, SciPy, matplotlib, Sympy, Maxima, GAP, FLINT, R and many more. Access their combined power through a common, Python-based language or directly via interfaces or wrappers.
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Pro Free open-source software. Compares favorably to Mathematica. INCLUDES Maxima in distro. Web-based CoCalc is excellent app based on Sagemath.
Jupyter Notebook supports Sagemath Kernel.
Though SageMath depends on other software packages for the heavy lifting, SageMath offers a centralized interface for working with many subfields of mathematics.
SageMath does not do much mathematics by itself. Instead, it depends on many CAS and non-CAS software packages, which it calls based on the desired operations. Installing SageMath pulls in all those packages as mandatory prerequisites. There is no lightweight GUI either.