count
number of degrees of freedom
Orlaith Burke
Alejandra Gonzalez-Beltran
Philippe Rocca-Serra
The number degree of freedom is a count evaluating the number of values in a calculation that can vary. In statistics, the number of degrees of freedom ν is equal to N-1 in the case of the direct measurement of a quantity estimated by the arithmetic mean of N independent observations.
http://stats.stackexchange.com/questions/16921/how-to-understand-degrees-of-freedom
http://www.optique-ingenieur.org/en/courses/OPI_ang_M07_C01/co/Contenu_07.html
the rank of the quadratic form (mathematical definition)
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