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Expansions of matrix-vector products 




I Expansions of the matrix-vector product AX. Consider the two matrices:

 



     ole.gif



and


               ole1.gif





There are two important expansions of the matrix-vector product



   ole2.gif    ole3.gif





The first important expansion is:


ole4.gif   ole5.gif    ole6.gif



                                      ole7.gif



 

 where Ri is the i-th row of matrix A and the product Ri∙X is the m-dot product. This expansion follows directly from the definition of the product of two matrices.





The second important expansion is:


 

ole8.gif ole9.gif   



where Ci represents the i-th column of matrix A.


It can be seen that this second expansion gives the same result as the first:





                          ole10.gif




Expansion (2) can be used to rewrite expressions of the type



             ole11.gif   




It says they can be written in the condensed matrix form


          ole12.gif ole13.gif   .








II Expansions of the vector-matrix product XA.



There are two important expansions for the vector-matrix product:



   ole14.gif   .






The first expansion is:


     ole15.gif    



 where Ci is the i-th column of matrix A and X∙Ci is the m-dot product. This expansion follows directly from the definition of the product of two matrices.



The second expansion is:


ole16.gif

            

 


  where Ri represents the i-th row of matrix A.                                   



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