Do matrix multiplication rules apply when multiplying matrices made up of smaller matrices?












3












$begingroup$


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.




  1. Is there a name for this kind of linear algebra property & are there more like it?


  2. Why does it work? (Just a reference would suffice)











share|cite|improve this question











$endgroup$








  • 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
















3












$begingroup$


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.




  1. Is there a name for this kind of linear algebra property & are there more like it?


  2. Why does it work? (Just a reference would suffice)











share|cite|improve this question











$endgroup$








  • 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














3












3








3


1



$begingroup$


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.




  1. Is there a name for this kind of linear algebra property & are there more like it?


  2. Why does it work? (Just a reference would suffice)











share|cite|improve this question











$endgroup$




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.




  1. Is there a name for this kind of linear algebra property & are there more like it?


  2. Why does it work? (Just a reference would suffice)








linear-algebra






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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














  • 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










1 Answer
1






active

oldest

votes


















6












$begingroup$

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).






share|cite|improve this answer











$endgroup$













    Your Answer





    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("mathjaxEditing", function () {
    StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
    StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
    });
    });
    }, "mathjax-editing");

    StackExchange.ready(function() {
    var channelOptions = {
    tags: "".split(" "),
    id: "69"
    };
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function() {
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled) {
    StackExchange.using("snippets", function() {
    createEditor();
    });
    }
    else {
    createEditor();
    }
    });

    function createEditor() {
    StackExchange.prepareEditor({
    heartbeatType: 'answer',
    autoActivateHeartbeat: false,
    convertImagesToLinks: true,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: 10,
    bindNavPrevention: true,
    postfix: "",
    imageUploader: {
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    },
    noCode: true, onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    });


    }
    });














    draft saved

    draft discarded


















    StackExchange.ready(
    function () {
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3043774%2fdo-matrix-multiplication-rules-apply-when-multiplying-matrices-made-up-of-smalle%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    1 Answer
    1






    active

    oldest

    votes








    1 Answer
    1






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    6












    $begingroup$

    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).






    share|cite|improve this answer











    $endgroup$


















      6












      $begingroup$

      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).






      share|cite|improve this answer











      $endgroup$
















        6












        6








        6





        $begingroup$

        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).






        share|cite|improve this answer











        $endgroup$



        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).







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 18 '18 at 22:12

























        answered Dec 17 '18 at 11:39









        littleOlittleO

        30.1k647110




        30.1k647110






























            draft saved

            draft discarded




















































            Thanks for contributing an answer to Mathematics Stack Exchange!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            Use MathJax to format equations. MathJax reference.


            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3043774%2fdo-matrix-multiplication-rules-apply-when-multiplying-matrices-made-up-of-smalle%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown







            Popular posts from this blog

            Plaza Victoria

            Puebla de Zaragoza

            Musa