ZHANG, Ruda December 30, 2018:  Use dollar style math markup. [1b7036c]
ZHANG, Ruda September 30, 2018:  CSS redesign; more edits. [dd44d96]
ZHANG, Ruda July 31, 2018:  More on probability density estimation and manifold learning. [0fe7ce8]
Ruda Zhang September 20, 2016:  PCA vs SVD. [69be998]
Ruda Zhang July 05, 2016:  Add leaflet, QGIS; notes on decision trees. [9f2e957]
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]