[Numpy-discussion] warning or error for non-physical multivariate_normal covariance matrices?

[Michael Gilbert]

hi,

when using numpy.random.multivariate_normal, would it make sense to warn
the user that they have entered a non-physical covariance matrix? i was
recently working on a problem and getting very strange results until i
finally realized that i had actually entered a bogus covariance matrix.

its easy to determine when this is the case -- its when the
determinant of the covariance matrix is negative.  i.e. the
multivariate normal distribution has det(C)^1/2 as part of the
normalization factor, so when det(C)<0, you end up with an imaginary
probability distribution.

a warning might be better than an error since there may be cases where
the user would intentionally want this type of configuration.

mike