FAQ: What do the double vertical bars mean at the end of Section 4.1?
From FBSwiki
(Contributed by Richard Murray, 28 August 2010)
In the text at the end of Section 4.1, the following equation occurs
The expression 
represents the magnitude of the vector enclosed between the sets of double vertical bars. For example, if 
is a vector of length three with elements 
, 
, 
, then 
is given by
