c=cov(a,b)
c=cov(a)
Inputs  
a 
Data Matrix 
b 
Optional input . Data matrix. If prescribed, it should have the same number of elements as a .

Outputs  
c 
The covariance matrix of the inputs. 
a
is returned,
otherwise the 2X2
crosscovariance matrix of a[:]
and b[:]
is returned.
(Note that a[:]
is the column vector of the elements of a
.)