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Block matrix pseudoinverse : ウィキペディア英語版
Block matrix pseudoinverse

In mathematics, block matrix pseudoinverse is a formula of pseudoinverse of a partitioned matrix. This is useful for decomposing or approximating many algorithms updating parameters in signal processing, which are based on least squares method.
== Derivation ==
Consider a column-wise partitioned matrix:
: (A, \mathbf B ), \qquad \mathbf A \in \reals^, \qquad \mathbf B \in \reals^, \qquad m \geq n+p.
If the above matrix is full rank, the pseudoinverse matrices of it and its transpose are as follows.
:
\begin
\mathbf A, & \mathbf B
\end
^ = ((A, \mathbf B )^T (A, \mathbf B ))^ (A, \mathbf B )^T,

:
\begin
\mathbf A^T \\ \mathbf B^T
\end
^ = (A, \mathbf B ) ((A, \mathbf B )^T (A, \mathbf B ))^.

The pseudoinverse requires (''n'' + ''p'')-square matrix inversion.
To reduce complexity and introduce parallelism, we derive the following decomposed formula. From a block matrix inverse \mathbf ((A, \mathbf B )^T (A, \mathbf B ))^, we can have
:
\begin
\mathbf A, & \mathbf B
\end
^ = \left(P_B^\perp \mathbf A( \mathbf A^T \mathbf P_B^\perp \mathbf A)^, \quad \mathbf P_A^\perp \mathbf B(\mathbf B^T \mathbf P_A^\perp \mathbf B)^\right )^T,

:
\begin
\mathbf A^T \\ \mathbf B^T
\end
^ = \left(P_B^\perp \mathbf A( \mathbf A^T \mathbf P_B^\perp \mathbf A)^, \quad \mathbf P_A^\perp \mathbf B(\mathbf B^T \mathbf P_A^\perp \mathbf B)^\right ),

where orthogonal projection matrices are defined by
::
\begin
\mathbf P_A^\perp & = \mathbf I - \mathbf A (\mathbf A^T \mathbf A)^ \mathbf A^T, \\ \mathbf P_B^\perp & = \mathbf I - \mathbf B (\mathbf B^T \mathbf B)^ \mathbf B^T.
\end

Interestingly, from the idempotence of projection matrix, we can verify that the pseudoinverse of block matrix consists of pseudoinverse of projected matrices:
:
\begin
\mathbf A, & \mathbf B
\end
^
=
\begin
(\mathbf P_B^\mathbf A)^
\\
(\mathbf P_A^\mathbf B)^
\end,
:
\begin
\mathbf A^T \\ \mathbf B^T
\end
^
= (A^T \mathbf P_B^)^,
\quad (\mathbf B^T \mathbf P_A^)^
).
Thus, we decomposed the block matrix pseudoinverse into two submatrix pseudoinverses, which cost ''n''- and ''p''-square matrix inversions, respectively.
Note that the above formulae are not necessarily valid if (A, \mathbf B ) does not have full rank – for example, if \mathbf A \neq 0, then
:
\begin
\mathbf A, & \mathbf A
\end
^
= \frac
\begin
\mathbf A^ \\ \mathbf A^
\end
\neq
\begin
(\mathbf P_A^\mathbf A)^
\\
(\mathbf P_A^\mathbf A)^
\end
= 0


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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