The multinormal distribution with mean
and covariance
has the density
 |
(4.47) |
We write
.
How is this multinormal distribution with mean
and covariance
related to the multivariate standard normal
?
Through a linear transformation
using the results of Section 4.3, as shown in the next theorem.
THEOREM 4.5
Let

and

(Mahalanobis transformation
). Then
i.e., the elements

are independent, one-dimensional

variables.
PROOF:
Note that
.
Application of (4.45) gives
, hence
 |
(4.48) |
which is by (4.47) the pdf of a
.
Note that
the above Mahalanobis transformation yields
in fact a random variable
composed of independent
one-dimensional
since
Here each
is a standard normal density
.
From this it is clear that
and
.
How can we create
variables on the basis of
variables? We use the inverse linear transformation
 |
(4.49) |
Using (4.11) and (4.23) we can also check that
and
.
The following theorem is useful because it presents the distribution
of a variable after it has been linearly transformed.
The proof is left as an exercise.
THEOREM 4.6
Let

and

,
where

is nonsingular.
Then
is again a
-variate Normal, i.e.,
 |
(4.50) |
From (4.47) we see that the density of the
distribution
is constant on ellipsoids of the form
 |
(4.51) |
EXAMPLE 4.14
Figure
4.3 shows the contour ellipses of a
two-dimensional normal distribution. Note that these contour ellipses
are the iso-distance curves (
2.34) from the mean of
this normal distribution corresponding to the metric

.
Figure 4.3:
Scatterplot
of a normal sample and contour ellipses for
and
.
MVAcontnorm.xpl
|
According to Theorem 2.7 in Section 2.6 the
half-lengths of the axes in the contour
ellipsoid are
where
are
the eigenvalues of
and
are the eigenvalues of
.
The rectangle inscribing an ellipse has sides with length
and is thus
naturally proportional to the standard deviations of
.
The distribution of the quadratic form in (4.51) is given in
the next theorem.
THEOREM 4.8
The characteristic function (cf) of a multinormal

is given by
 |
(4.52) |
We can check Theorem 4.8 by transforming the cf back:
since
Note that if
(e.g., the Mahalanobis-transform), then
which is consistent with (4.33).
Singular Normal Distribution
Suppose that we have
, where
is
the dimension of
.
We define the (singular) density of
with the aid of the
-Inverse
of
,
 |
(4.53) |
where
- (1)
lies on the hyperplane
with
and
.
- (2)
is the
-Inverse of
, and
are the nonzero eigenvalues of
.
What is the connection to a multinormal with
-dimensions? If
 |
(4.54) |
then there exists an orthogonal matrix
with
so that
where
has a singular pdf of the form (4.53).
The second important copula that we want to present is the
Gaussian or normal copula,
 |
(4.55) |
see Embrechts et al. (1999). In (4.55),
denotes the bivariate normal density
function with correlation
for
. The functions
and
in (4.55) refer to the corresponding
one-dimensional standard normal cdfs of the margins.
In the case of vanishing correlation,
, the Gaussian copula becomes
Summary

- The pdf of a
-dimensional multinormal
is
The contour curves of a multinormal are ellipsoids with half-lengths
proportional to
, where
denotes the eigenvalues of
(
).

-
The Mahalanobis transformation transforms
to
.
Going the other direction,
one can create a
from
via
.

-
If the covariance matrix
is singular (i.e.,
),
then it defines a singular normal distribution.

-
The density of a singular normal distribution is given by