SageMath
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.
http://www.sagemath.org/
Offifial documents
Symbolic Computation
http://doc.sagemath.org/html/en/reference/calculus/sage/calculus/calculus.html
Symbolic Equations and Inequalities
http://doc.sagemath.org/html/en/reference/calculus/sage/symbolic/relation.html
Questions - ASKSAGE: Sage Q&A Forum
https://ask.sagemath.org/questions/
Online environment
CoCalc
Examples
Ocean Science Hack: Sageで方程式を解く
http://oceansciencehack.blogspot.com/2010/09/sage.html
Ocean Science Hack: Sageで方程式
http://oceansciencehack.blogspot.com/2012/09/sage.html
Ocean Science Hack: Sageで数学　素数
http://oceansciencehack.blogspot.com/2010/01/sage.html
級数
code:python
sum(1/x^2,x,1,oo)
1/6*pi^2
A First Course in Linear Algebra (A Free Textbook)
http://linear.ups.edu/sage-fcla.html
fourier series - using sage - YouTube
https://www.youtube.com/watch?v=yK_-4nRZFjw
preCalculus 1200 Intro to SAGE! - YouTube
https://www.youtube.com/watch?v=vP9iAIb2XWM&list=PLL956Pn2cKSj_D7PliLz1_VdQ1rRpgaE1
Algorithmic Graph Theory and Sage
https://www.freetechbooks.com/algorithmic-graph-theory-and-sage-t974.html
Tips
simplify
exponentialize
https://share.cocalc.com/share/719b6cb2-caaa-47b2-a6d0-cef7ed7214d4/Simplify.sagews?viewer=share
substitute
convert function to variable - ASKSAGE: Sage Q&A Forum
https://ask.sagemath.org/question/23984/convert-function-to-variable/
Defining constants after solving ODE/PDE - ASKSAGE: Sage Q&A Forum
https://ask.sagemath.org/question/25373/defining-constants-after-solving-odepde/
substitute expression instead of formal function symbol - ASKSAGE: Sage Q&A Forum
https://ask.sagemath.org/question/8110/substitute-expression-instead-of-formal-function-symbol/
Use a symbolic solved variable in another later equation - ASKSAGE: Sage Q&A Forum
https://ask.sagemath.org/question/23933/use-a-symbolic-solved-variable-in-another-later-equation/
What does assume(x, "real")? - ASKSAGE: Sage Q&A Forum
https://ask.sagemath.org/question/25892/what-does-assumex-real/
Un peu de math...: How to handle float error for plots near discontinuities in Sage
http://drvinceknight.blogspot.com/2013/12/how-to-handle-float-error-for-plots.html
Substitute variable in differential equation - ASKSAGE: Sage Q&A Forum
https://ask.sagemath.org/question/50632/substitute-variable-in-differential-equation/
Tutorials
Sage for undergraduates (2014)
http://www.people.vcu.edu/~clarson/bard-sage-for-undergraduates-2014.pdf
Sage by Hiroshi TAKEMOTO
http://sage.math.canterbury.ac.nz/home/pub/89/
Introduction to Sage - YouTube
https://www.youtube.com/watch?v=BqB0YzIMzTc
Dr. Michael O'Sullivan
https://mosullivan.sdsu.edu/teaching.html
Links
ウィキペディア SageMath
https://ja.wikipedia.org/wiki/SageMath