The aim of this section is to present a duality relationship
between the two approaches shown in Sections 8.2 and 8.3.
Consider the eigenvector equations in
 |
(8.6) |
for
, where
.
Multiplying by
, we have
so that each eigenvector
of
corresponds to an
eigenvector
of
associated with the same eigenvalue
.
This means that every non-zero eigenvalue of
is an eigenvalue
of
. The corresponding eigenvectors are related by
where
is some constant.
Now consider the eigenvector equations in
:
 |
(8.9) |
for
Multiplying by
, we have
 |
(8.10) |
i.e., each eigenvector
of
corresponds to an
eigenvector
of
associated with the
same eigenvalue
.
Therefore, every non-zero eigenvalue of
is an
eigenvalue of
. The corresponding eigenvectors are related by
where
is some constant.
Now, since
we have
. This lead to the following result:
THEOREM 8.4 (Duality Relations
)
Let

be the rank of

. For

, the eigenvalues

of

and

are the same
and the eigenvectors
(

and

, respectively) are related by
 |
|
|
(8.11) |
 |
|
|
(8.12) |
Note that the projection of the
variables
on the factorial axis
is given by
 |
(8.13) |
Therefore, the eigenvectors
do not have to be explicitly
recomputed to get
.
Note that
and
provide the SVD of
(see Theorem
2.2). Letting
and
we have
so that
 |
(8.14) |
In the following section
this method is applied in analysing consumption behavior
across different household types.
Summary

-
The matrices
and
have the
same non-zero eigenvalues
, where
.

-
The eigenvectors of
can be calculated from the
eigenvectors of
and vice versa:

-
The coordinates representing the variables (columns) of
in a
-dimensional subspace can be easily calculated by
.