ZHANG, Ruda August 07, 2020:  Correct math: use 1 with set subscript for characteristic function / indicator function of the set; use "[ x ]" instead of "[x]" to avoid GFM checkbox syntax. [1ac98ba]
ZHANG, Ruda February 05, 2020:  More on dynamical systems. [72482d6]
ZHANG, Ruda January 23, 2020:  More on Hilbert space. [a81f4f5]
ZHANG, Ruda January 18, 2020:  Convert math-text words to text only when the math symbols are ASCII only. [e4c945c]
ZHANG, Ruda December 05, 2019:  More on manifolds, from John Marshall Lee's textbooks. [6ba9029]