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 December 30, 2018:  Use dollar style math markup. [1b7036c]
ZHANG, Ruda March 23, 2018:  Global remove trailing whitespaces [2b7ff90]
Ruda Zhang June 26, 2016:  one sentence per line [e15360a]
Ruda Zhang June 26, 2016:  add category tag to wiki pages [a9b356c]
Ruda Zhang June 26, 2016:  use canonical filename and no internal H1 [eeca8ff]