Do matrix multiplication rules apply when multiplying matrices made up of smaller matrices?
$begingroup$
I came across a proof that did the following.
$a$, $b$, $c$ are $3 times 1$ vectors. $A$ is a $3 times 3$ matrix and $d$ is a $3 times 1$ vector
$begin{bmatrix}A & dend{bmatrix} begin{bmatrix} a & b & c \ 0 & 0 & 1 end{bmatrix} = begin{bmatrix} Aa & Ab & Ac +dend{bmatrix}$
So we are left multiplying a $4 times 3$ matrix by a $3 times 4$ matrix, but the elements of both matrices are themselves vectors and matrices.
I know this is correct because the rest of the proof in the paper follows. But, I don't really know how to justify using regular matrix multiplication properties to multiply two matrices made up of matrices. For example, $Aa$ is a matrix-vector product, whereas in regular matrix multiplication it would be some scalar product.
Is there a name for this kind of linear algebra property & are there more like it?
Why does it work? (Just a reference would suffice)
linear-algebra
$endgroup$
add a comment |
$begingroup$
I came across a proof that did the following.
$a$, $b$, $c$ are $3 times 1$ vectors. $A$ is a $3 times 3$ matrix and $d$ is a $3 times 1$ vector
$begin{bmatrix}A & dend{bmatrix} begin{bmatrix} a & b & c \ 0 & 0 & 1 end{bmatrix} = begin{bmatrix} Aa & Ab & Ac +dend{bmatrix}$
So we are left multiplying a $4 times 3$ matrix by a $3 times 4$ matrix, but the elements of both matrices are themselves vectors and matrices.
I know this is correct because the rest of the proof in the paper follows. But, I don't really know how to justify using regular matrix multiplication properties to multiply two matrices made up of matrices. For example, $Aa$ is a matrix-vector product, whereas in regular matrix multiplication it would be some scalar product.
Is there a name for this kind of linear algebra property & are there more like it?
Why does it work? (Just a reference would suffice)
linear-algebra
$endgroup$
1
$begingroup$
You can try reading about block matrices and their arithmetic operations.
$endgroup$
– Shivani Goel
Dec 17 '18 at 10:31
4
$begingroup$
en.wikipedia.org/wiki/Block_matrix
$endgroup$
– Rahul
Dec 17 '18 at 10:33
$begingroup$
Thank you both! I didn't know there was something called Block Matrix.
$endgroup$
– Carpetfizz
Dec 17 '18 at 10:34
1
$begingroup$
I'm fairly sure that matrix multiplication is thoroughly recursive in that respect. Don't rely on what I've said though ... I just have a recollection of learning that it is.
$endgroup$
– AmbretteOrrisey
Dec 17 '18 at 11:00
add a comment |
$begingroup$
I came across a proof that did the following.
$a$, $b$, $c$ are $3 times 1$ vectors. $A$ is a $3 times 3$ matrix and $d$ is a $3 times 1$ vector
$begin{bmatrix}A & dend{bmatrix} begin{bmatrix} a & b & c \ 0 & 0 & 1 end{bmatrix} = begin{bmatrix} Aa & Ab & Ac +dend{bmatrix}$
So we are left multiplying a $4 times 3$ matrix by a $3 times 4$ matrix, but the elements of both matrices are themselves vectors and matrices.
I know this is correct because the rest of the proof in the paper follows. But, I don't really know how to justify using regular matrix multiplication properties to multiply two matrices made up of matrices. For example, $Aa$ is a matrix-vector product, whereas in regular matrix multiplication it would be some scalar product.
Is there a name for this kind of linear algebra property & are there more like it?
Why does it work? (Just a reference would suffice)
linear-algebra
$endgroup$
I came across a proof that did the following.
$a$, $b$, $c$ are $3 times 1$ vectors. $A$ is a $3 times 3$ matrix and $d$ is a $3 times 1$ vector
$begin{bmatrix}A & dend{bmatrix} begin{bmatrix} a & b & c \ 0 & 0 & 1 end{bmatrix} = begin{bmatrix} Aa & Ab & Ac +dend{bmatrix}$
So we are left multiplying a $4 times 3$ matrix by a $3 times 4$ matrix, but the elements of both matrices are themselves vectors and matrices.
I know this is correct because the rest of the proof in the paper follows. But, I don't really know how to justify using regular matrix multiplication properties to multiply two matrices made up of matrices. For example, $Aa$ is a matrix-vector product, whereas in regular matrix multiplication it would be some scalar product.
Is there a name for this kind of linear algebra property & are there more like it?
Why does it work? (Just a reference would suffice)
linear-algebra
linear-algebra
edited Dec 17 '18 at 10:56
The Pointer
2,64421638
2,64421638
asked Dec 17 '18 at 10:24
CarpetfizzCarpetfizz
491413
491413
1
$begingroup$
You can try reading about block matrices and their arithmetic operations.
$endgroup$
– Shivani Goel
Dec 17 '18 at 10:31
4
$begingroup$
en.wikipedia.org/wiki/Block_matrix
$endgroup$
– Rahul
Dec 17 '18 at 10:33
$begingroup$
Thank you both! I didn't know there was something called Block Matrix.
$endgroup$
– Carpetfizz
Dec 17 '18 at 10:34
1
$begingroup$
I'm fairly sure that matrix multiplication is thoroughly recursive in that respect. Don't rely on what I've said though ... I just have a recollection of learning that it is.
$endgroup$
– AmbretteOrrisey
Dec 17 '18 at 11:00
add a comment |
1
$begingroup$
You can try reading about block matrices and their arithmetic operations.
$endgroup$
– Shivani Goel
Dec 17 '18 at 10:31
4
$begingroup$
en.wikipedia.org/wiki/Block_matrix
$endgroup$
– Rahul
Dec 17 '18 at 10:33
$begingroup$
Thank you both! I didn't know there was something called Block Matrix.
$endgroup$
– Carpetfizz
Dec 17 '18 at 10:34
1
$begingroup$
I'm fairly sure that matrix multiplication is thoroughly recursive in that respect. Don't rely on what I've said though ... I just have a recollection of learning that it is.
$endgroup$
– AmbretteOrrisey
Dec 17 '18 at 11:00
1
1
$begingroup$
You can try reading about block matrices and their arithmetic operations.
$endgroup$
– Shivani Goel
Dec 17 '18 at 10:31
$begingroup$
You can try reading about block matrices and their arithmetic operations.
$endgroup$
– Shivani Goel
Dec 17 '18 at 10:31
4
4
$begingroup$
en.wikipedia.org/wiki/Block_matrix
$endgroup$
– Rahul
Dec 17 '18 at 10:33
$begingroup$
en.wikipedia.org/wiki/Block_matrix
$endgroup$
– Rahul
Dec 17 '18 at 10:33
$begingroup$
Thank you both! I didn't know there was something called Block Matrix.
$endgroup$
– Carpetfizz
Dec 17 '18 at 10:34
$begingroup$
Thank you both! I didn't know there was something called Block Matrix.
$endgroup$
– Carpetfizz
Dec 17 '18 at 10:34
1
1
$begingroup$
I'm fairly sure that matrix multiplication is thoroughly recursive in that respect. Don't rely on what I've said though ... I just have a recollection of learning that it is.
$endgroup$
– AmbretteOrrisey
Dec 17 '18 at 11:00
$begingroup$
I'm fairly sure that matrix multiplication is thoroughly recursive in that respect. Don't rely on what I've said though ... I just have a recollection of learning that it is.
$endgroup$
– AmbretteOrrisey
Dec 17 '18 at 11:00
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
This block matrix notation is very useful. The meaning of the notation is clear: for example, if
$$
A = begin{bmatrix} a_{11} & a_{12} \ a_{21} & a_{22} end{bmatrix}, quad
B = begin{bmatrix} b_{11} & b_{12} \ b_{21} & b_{22} end{bmatrix}, quad
C = begin{bmatrix} c_{11} & c_{12} \ c_{21} & c_{22} end{bmatrix}, quad
D = begin{bmatrix} d_{11} & d_{12} \ d_{21} & d_{22} end{bmatrix}
$$
then $begin{bmatrix} A & B \ C & D end{bmatrix}$ denotes the $4 times 4$ matrix
$$
begin{bmatrix} A & B \ C & D end{bmatrix} =
begin{bmatrix} a_{11} & a_{12} & b_{11} & b_{12} \
a_{21} & a_{22} & b_{21} & b_{22} \
c_{11} & c_{12} & d_{11} & d_{12} \
c_{21} & c_{22} & d_{21} & d_{22}
end{bmatrix}.
$$
It is straightforward to prove the following basic rules
for matrix multiplication using block notation:
$A begin{bmatrix} B & C end{bmatrix} =
begin{bmatrix} AB & AC end{bmatrix}$.- $begin{bmatrix}
A \
B
end{bmatrix} C =
begin{bmatrix}
AC \
BC
end{bmatrix}.
$ - $
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} C \ D end{bmatrix}
= AC + BD.
$
(For each rule, we must assume that
the matrices $A, B, C$, and $D$
have compatible shapes.)
Using these basic rules, we can easily derive any more complicated block matrix multiplication rule that we need. For example,
begin{align}
begin{bmatrix} A & B \ C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
&=
begin{bmatrix}
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} X \ Y end{bmatrix} \
begin{bmatrix} C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
end{bmatrix} \
&=
begin{bmatrix}
AX + BY \
CX + DY
end{bmatrix}
end{align}
(assuming that the matrices $A, B, C, D, X$, and $Y$ have compatible shapes).
$endgroup$
add a comment |
Your Answer
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
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active
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$begingroup$
This block matrix notation is very useful. The meaning of the notation is clear: for example, if
$$
A = begin{bmatrix} a_{11} & a_{12} \ a_{21} & a_{22} end{bmatrix}, quad
B = begin{bmatrix} b_{11} & b_{12} \ b_{21} & b_{22} end{bmatrix}, quad
C = begin{bmatrix} c_{11} & c_{12} \ c_{21} & c_{22} end{bmatrix}, quad
D = begin{bmatrix} d_{11} & d_{12} \ d_{21} & d_{22} end{bmatrix}
$$
then $begin{bmatrix} A & B \ C & D end{bmatrix}$ denotes the $4 times 4$ matrix
$$
begin{bmatrix} A & B \ C & D end{bmatrix} =
begin{bmatrix} a_{11} & a_{12} & b_{11} & b_{12} \
a_{21} & a_{22} & b_{21} & b_{22} \
c_{11} & c_{12} & d_{11} & d_{12} \
c_{21} & c_{22} & d_{21} & d_{22}
end{bmatrix}.
$$
It is straightforward to prove the following basic rules
for matrix multiplication using block notation:
$A begin{bmatrix} B & C end{bmatrix} =
begin{bmatrix} AB & AC end{bmatrix}$.- $begin{bmatrix}
A \
B
end{bmatrix} C =
begin{bmatrix}
AC \
BC
end{bmatrix}.
$ - $
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} C \ D end{bmatrix}
= AC + BD.
$
(For each rule, we must assume that
the matrices $A, B, C$, and $D$
have compatible shapes.)
Using these basic rules, we can easily derive any more complicated block matrix multiplication rule that we need. For example,
begin{align}
begin{bmatrix} A & B \ C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
&=
begin{bmatrix}
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} X \ Y end{bmatrix} \
begin{bmatrix} C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
end{bmatrix} \
&=
begin{bmatrix}
AX + BY \
CX + DY
end{bmatrix}
end{align}
(assuming that the matrices $A, B, C, D, X$, and $Y$ have compatible shapes).
$endgroup$
add a comment |
$begingroup$
This block matrix notation is very useful. The meaning of the notation is clear: for example, if
$$
A = begin{bmatrix} a_{11} & a_{12} \ a_{21} & a_{22} end{bmatrix}, quad
B = begin{bmatrix} b_{11} & b_{12} \ b_{21} & b_{22} end{bmatrix}, quad
C = begin{bmatrix} c_{11} & c_{12} \ c_{21} & c_{22} end{bmatrix}, quad
D = begin{bmatrix} d_{11} & d_{12} \ d_{21} & d_{22} end{bmatrix}
$$
then $begin{bmatrix} A & B \ C & D end{bmatrix}$ denotes the $4 times 4$ matrix
$$
begin{bmatrix} A & B \ C & D end{bmatrix} =
begin{bmatrix} a_{11} & a_{12} & b_{11} & b_{12} \
a_{21} & a_{22} & b_{21} & b_{22} \
c_{11} & c_{12} & d_{11} & d_{12} \
c_{21} & c_{22} & d_{21} & d_{22}
end{bmatrix}.
$$
It is straightforward to prove the following basic rules
for matrix multiplication using block notation:
$A begin{bmatrix} B & C end{bmatrix} =
begin{bmatrix} AB & AC end{bmatrix}$.- $begin{bmatrix}
A \
B
end{bmatrix} C =
begin{bmatrix}
AC \
BC
end{bmatrix}.
$ - $
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} C \ D end{bmatrix}
= AC + BD.
$
(For each rule, we must assume that
the matrices $A, B, C$, and $D$
have compatible shapes.)
Using these basic rules, we can easily derive any more complicated block matrix multiplication rule that we need. For example,
begin{align}
begin{bmatrix} A & B \ C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
&=
begin{bmatrix}
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} X \ Y end{bmatrix} \
begin{bmatrix} C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
end{bmatrix} \
&=
begin{bmatrix}
AX + BY \
CX + DY
end{bmatrix}
end{align}
(assuming that the matrices $A, B, C, D, X$, and $Y$ have compatible shapes).
$endgroup$
add a comment |
$begingroup$
This block matrix notation is very useful. The meaning of the notation is clear: for example, if
$$
A = begin{bmatrix} a_{11} & a_{12} \ a_{21} & a_{22} end{bmatrix}, quad
B = begin{bmatrix} b_{11} & b_{12} \ b_{21} & b_{22} end{bmatrix}, quad
C = begin{bmatrix} c_{11} & c_{12} \ c_{21} & c_{22} end{bmatrix}, quad
D = begin{bmatrix} d_{11} & d_{12} \ d_{21} & d_{22} end{bmatrix}
$$
then $begin{bmatrix} A & B \ C & D end{bmatrix}$ denotes the $4 times 4$ matrix
$$
begin{bmatrix} A & B \ C & D end{bmatrix} =
begin{bmatrix} a_{11} & a_{12} & b_{11} & b_{12} \
a_{21} & a_{22} & b_{21} & b_{22} \
c_{11} & c_{12} & d_{11} & d_{12} \
c_{21} & c_{22} & d_{21} & d_{22}
end{bmatrix}.
$$
It is straightforward to prove the following basic rules
for matrix multiplication using block notation:
$A begin{bmatrix} B & C end{bmatrix} =
begin{bmatrix} AB & AC end{bmatrix}$.- $begin{bmatrix}
A \
B
end{bmatrix} C =
begin{bmatrix}
AC \
BC
end{bmatrix}.
$ - $
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} C \ D end{bmatrix}
= AC + BD.
$
(For each rule, we must assume that
the matrices $A, B, C$, and $D$
have compatible shapes.)
Using these basic rules, we can easily derive any more complicated block matrix multiplication rule that we need. For example,
begin{align}
begin{bmatrix} A & B \ C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
&=
begin{bmatrix}
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} X \ Y end{bmatrix} \
begin{bmatrix} C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
end{bmatrix} \
&=
begin{bmatrix}
AX + BY \
CX + DY
end{bmatrix}
end{align}
(assuming that the matrices $A, B, C, D, X$, and $Y$ have compatible shapes).
$endgroup$
This block matrix notation is very useful. The meaning of the notation is clear: for example, if
$$
A = begin{bmatrix} a_{11} & a_{12} \ a_{21} & a_{22} end{bmatrix}, quad
B = begin{bmatrix} b_{11} & b_{12} \ b_{21} & b_{22} end{bmatrix}, quad
C = begin{bmatrix} c_{11} & c_{12} \ c_{21} & c_{22} end{bmatrix}, quad
D = begin{bmatrix} d_{11} & d_{12} \ d_{21} & d_{22} end{bmatrix}
$$
then $begin{bmatrix} A & B \ C & D end{bmatrix}$ denotes the $4 times 4$ matrix
$$
begin{bmatrix} A & B \ C & D end{bmatrix} =
begin{bmatrix} a_{11} & a_{12} & b_{11} & b_{12} \
a_{21} & a_{22} & b_{21} & b_{22} \
c_{11} & c_{12} & d_{11} & d_{12} \
c_{21} & c_{22} & d_{21} & d_{22}
end{bmatrix}.
$$
It is straightforward to prove the following basic rules
for matrix multiplication using block notation:
$A begin{bmatrix} B & C end{bmatrix} =
begin{bmatrix} AB & AC end{bmatrix}$.- $begin{bmatrix}
A \
B
end{bmatrix} C =
begin{bmatrix}
AC \
BC
end{bmatrix}.
$ - $
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} C \ D end{bmatrix}
= AC + BD.
$
(For each rule, we must assume that
the matrices $A, B, C$, and $D$
have compatible shapes.)
Using these basic rules, we can easily derive any more complicated block matrix multiplication rule that we need. For example,
begin{align}
begin{bmatrix} A & B \ C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
&=
begin{bmatrix}
begin{bmatrix} A & B end{bmatrix} begin{bmatrix} X \ Y end{bmatrix} \
begin{bmatrix} C & D end{bmatrix} begin{bmatrix} X \ Y end{bmatrix}
end{bmatrix} \
&=
begin{bmatrix}
AX + BY \
CX + DY
end{bmatrix}
end{align}
(assuming that the matrices $A, B, C, D, X$, and $Y$ have compatible shapes).
edited Dec 18 '18 at 22:12
answered Dec 17 '18 at 11:39
littleOlittleO
30.1k647110
30.1k647110
add a comment |
add a comment |
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1
$begingroup$
You can try reading about block matrices and their arithmetic operations.
$endgroup$
– Shivani Goel
Dec 17 '18 at 10:31
4
$begingroup$
en.wikipedia.org/wiki/Block_matrix
$endgroup$
– Rahul
Dec 17 '18 at 10:33
$begingroup$
Thank you both! I didn't know there was something called Block Matrix.
$endgroup$
– Carpetfizz
Dec 17 '18 at 10:34
1
$begingroup$
I'm fairly sure that matrix multiplication is thoroughly recursive in that respect. Don't rely on what I've said though ... I just have a recollection of learning that it is.
$endgroup$
– AmbretteOrrisey
Dec 17 '18 at 11:00