Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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**Block Matrix Multiplication**

*Introduction:*

In linear algebra, matrices can be partitioned into smaller submatrices, which allows for simplification of complex matrix operations. This section illustrates how to multiply two 4x4 matrices, A and B, using their 2x2 block submatrices.

*Block Partitioning of Matrices:*

Consider matrices A and B, both of size 4x4. They are partitioned into 2x2 submatrices or "blocks" as follows:

Matrix \( A \):

\[
A = \begin{bmatrix} 
A_{11} & A_{12} \\ 
A_{21} & A_{22} 
\end{bmatrix}
\]

Matrix \( B \):

\[
B = \begin{bmatrix} 
B_{11} & B_{12} \\ 
B_{21} & B_{22} 
\end{bmatrix}
\]

*Blockwise Matrix Multiplication:*

The product \( AB \) can be computed using blockwise multiplication. The resulting matrix is:

\[
AB = \begin{bmatrix} 
A_{11}B_{11} + A_{12}B_{21} & A_{11}B_{12} + A_{12}B_{22} \\ 
A_{21}B_{11} + A_{22}B_{21} & A_{21}B_{12} + A_{22}B_{22} 
\end{bmatrix}
\]

*Explanation:*

- The element at position (1,1) in the product matrix \( AB \) is obtained by multiplying block \( A_{11} \) with \( B_{11} \) and adding the product of \( A_{12} \) with \( B_{21} \).
- Similarly, for the element at position (1,2), multiply \( A_{11} \) with \( B_{12} \) and add it to the product of \( A_{12} \) with \( B_{22} \).
- For the element at position (2,1), calculate the sum of the products of \( A_{21} \) with \( B_{11} \) and \( A_{22} \) with \( B_{21} \).
- Finally, the element at position (2,2) is the sum of \( A_{21}B_{12
Transcribed Image Text:**Block Matrix Multiplication** *Introduction:* In linear algebra, matrices can be partitioned into smaller submatrices, which allows for simplification of complex matrix operations. This section illustrates how to multiply two 4x4 matrices, A and B, using their 2x2 block submatrices. *Block Partitioning of Matrices:* Consider matrices A and B, both of size 4x4. They are partitioned into 2x2 submatrices or "blocks" as follows: Matrix \( A \): \[ A = \begin{bmatrix} A_{11} & A_{12} \\ A_{21} & A_{22} \end{bmatrix} \] Matrix \( B \): \[ B = \begin{bmatrix} B_{11} & B_{12} \\ B_{21} & B_{22} \end{bmatrix} \] *Blockwise Matrix Multiplication:* The product \( AB \) can be computed using blockwise multiplication. The resulting matrix is: \[ AB = \begin{bmatrix} A_{11}B_{11} + A_{12}B_{21} & A_{11}B_{12} + A_{12}B_{22} \\ A_{21}B_{11} + A_{22}B_{21} & A_{21}B_{12} + A_{22}B_{22} \end{bmatrix} \] *Explanation:* - The element at position (1,1) in the product matrix \( AB \) is obtained by multiplying block \( A_{11} \) with \( B_{11} \) and adding the product of \( A_{12} \) with \( B_{21} \). - Similarly, for the element at position (1,2), multiply \( A_{11} \) with \( B_{12} \) and add it to the product of \( A_{12} \) with \( B_{22} \). - For the element at position (2,1), calculate the sum of the products of \( A_{21} \) with \( B_{11} \) and \( A_{22} \) with \( B_{21} \). - Finally, the element at position (2,2) is the sum of \( A_{21}B_{12
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