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Compute matrix products column by column and entry by entry. Interpret matrix multiplication in terms of the underlying linear transformations. Use the rules of matrix algebra. Multiply block matrices. If possible, compute the matrix products using paper and pencil. $$\left[\begin{array}{lll} 1 & 2 & 3 \end{array}\right]\left[\begin{array}{l} 3 \\ 2 \\ 1 \end{array}\right]$$

Short Answer

Expert verified
\(\begin{bmatrix}1 & 2 & 3\end{bmatrix}\begin{bmatrix}3 \ 2 \ 1\end{bmatrix} = \begin{bmatrix}10\end{bmatrix}\).

Step by step solution

01

Identify the Matrices

Recognize that you are multiplying a 1x3 matrix by a 3x1 matrix. The given matrices are: \(\begin{bmatrix}1 & 2 & 3\end{bmatrix}\) (Matrix A) and \(\begin{bmatrix}3 \ 2 \ 1\end{bmatrix}\) (Matrix B).
02

Set up the Multiplication

The product of a 1x3 matrix and a 3x1 matrix will result in a 1x1 matrix (a scalar). To compute this, multiply corresponding elements and add up the products.
03

Compute the Product

Follow the matrix multiplication rule: sum the products of the corresponding entries from each row of A with each column of B. Thus, \(1*3 + 2*2 + 3*1 = 3 + 4 + 3 = 10\).
04

Write Down the Result

The result is a 1x1 matrix (scalar): \(\begin{bmatrix}10\end{bmatrix}\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Linear Transformations
Understanding linear transformations is crucial when dealing with matrix multiplication. Essentially, a linear transformation is a function between two vector spaces that respects the operations of vector addition and scalar multiplication. In simple terms, if you take any vectors and perform a linear transformation, their addition and multiplication by a scalar will behave predictably and consistently.

In the context of matrix multiplication, each matrix represents a linear transformation. For example, when we multiply a matrix by a vector, the resulting vector is a transformation of the original vector's position. This can be seen as a transformation of space, often visualized in geometric applications such as rotations, scalings, or shearing of shapes.

Each entry in the resulting matrix from a matrix product can be interpreted as the effect of one vector's components on another through linear transformation. This is evident in the exercise where the matrix product results in a scalar, representing a dimension reduction from a vector space to a single point value.
Matrix Algebra
Matrix algebra is a branch of mathematics concerning the study and manipulation of matrices. In matrix algebra, there are well-defined rules for operations such as addition, subtraction, multiplication, and inversion of matrices.

Specifically, with matrix multiplication, the rules dictate how to combine matrices to produce a new matrix. However, not all matrices can be multiplied together; for instance, the number of columns in the first matrix must match the number of rows in the second matrix for multiplication to be possible.

Matrix algebra also includes the distributive, associative, and, for square matrices, the commutative properties. When dealing with exercises like the one provided, understanding these rules can vastly simplify the computation and help avoid common pitfalls, such as assuming that matrix multiplication is always commutative—it isn't!
Block Matrices
Block matrices are essentially matrices that are partitioned into smaller 'blocks' or submatrices. This approach is commonly used in various areas of linear algebra for simplifying complex matrix operations, managing large matrices more efficiently, or working with piecewise functions.

When multiplying block matrices, one can often use the same principles that apply to individual matrix elements on these larger submatrices. However, the blocks must be conformable—that is, the row and column dimensions must align properly for multiplication to take place.

For instance, if we had larger matrices partitioned into blocks, we would multiply corresponding blocks as though they were elements of the matrices. While the exercise provided does not include block matrices, it's important to grasp their utility and the potential for computational simplification they offer in more complex matrix multiplication scenarios.
Matrix Products Computation
Computation of matrix products is a fundamental task in matrix algebra and involves rules specific to dimensional conformity and entry-by-entry multiplication and summation. As seen in the exercise, you multiply each entry of a row in the first matrix by each entry of a column in the second matrix and then add up these products to attain a single entry in the product matrix.

The process is repeated for each entry in the resulting matrix's rows and columns. The final result of the exercise is a 1x1 matrix, which is essentially a scalar value, signifying that the larger vector space has been 'collapsed' onto a single value through the multiplication process.

This computation reflects not just the inner products between vectors but also provides a clear linkage between algebraic operations and geometric interpretations in higher dimensions within the framework of linear algebra.

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Most popular questions from this chapter

In this exercise we will verify part (b) of Theorem 2.3 .11 in the special case when \(A\) is the transition matrix \(\left[\begin{array}{ll}0.4 & 0.3 \\\ 0.6 & 0.7\end{array}\right]\) and \(\vec{x}\) is the distribution vector \(\left[\begin{array}{l}1 \\ 0\end{array}\right]\) [We will not be using parts (a) and (c) of Theorem \(2.3 .11 .\) ] The general proof of Theorem 2.3 .11 runs along similar lines, as we will see in Chapter 7. a. Compute \(A\left[\begin{array}{l}1 \\ 2\end{array}\right]\) and \(A\left[\begin{array}{c}1 \\ -1\end{array}\right] .\) Write \(A\left[\begin{array}{c}1 \\ -1\end{array}\right]\) as a scalar multiple of the vector \(\left[\begin{array}{c}1 \\ -1\end{array}\right]\). b. Write the distribution vector \(\vec{x}=\left[\begin{array}{l}1 \\\ 0\end{array}\right]\) as a linear combination of the vectors \(\left[\begin{array}{l}1 \\ 2\end{array}\right]\) and \(\left[\begin{array}{c}1 \\\ -1\end{array}\right]\). c. Use your answers in parts (a) and (b) to write \(A \vec{x}\) as a linear combination of the vectors \(\left[\begin{array}{l}1 \\ 2\end{array}\right]\) and \(\left[\begin{array}{c}1 \\ -1\end{array}\right]\) More generally, write \(A^{m} \vec{x}\) as a linear combination of the vectors \(\left[\begin{array}{l}1 \\ 2\end{array}\right]\) and \(\left[\begin{array}{c}1 \\\ -1\end{array}\right],\) for any positive integer \(m\). See Exercise 81.

For the matrices \(A\) compute \(A^{2}=A A, A^{3}=A A A,\) and \(A^{4} .\) Describe the pattern that emerges, and use this pattern to find \(A^{1001}\). Interpret your answers geometrically, in terms of rotations, reflections, shears, and orthogonal projections. $$\frac{1}{2}\left[\begin{array}{rr} -1 & -\sqrt{3} \\ \sqrt{3} & -1 \end{array}\right]$$

Is the product of two lower triangular matrices a lower triangular matrix as well? Explain your answer.Consider the matrix \\[A=\left[\begin{array}{lll} 1 & 2 & 3 \\ 2 & 6 & 7 \\ 2 & 2 & 4 \end{array}\right].\\] a. Find lower triangular elementary matrices \(E_{1}\) \(E_{2}, \ldots, E_{m}\), such that the product $$E_{m} \cdots E_{2} E_{1} A$$ is an upper triangular matrix \(U\). Hint: Use elementary row operations to eliminate the entries below the diagonal of \(A\). b. Find lower triangular elementary matrices \(M_{1}\) \(M_{2}, \ldots, M_{m}\) and an upper triangular matrix \(U\) such that \\[A=M_{1} M_{2} \cdots M_{m} U.\\] c. Find a lower triangular matrix \(L\) and an upper triangular matrix \(U\) such that \\[A=L U.\\] Such a representation of an invertible matrix is called an \(L U\) -factorization. The method outlined in this exercise to find an \(L U\) -factorization can be streamlined somewhat, but we have seen the major ideas. An \(L U\) -factorization (as introduced here) does not always exist. See Exercise 92 d. Find a lower triangular matrix \(L\) with 1 's on the diagonal, an upper triangular matrix \(U\) with 1 's on the diagonal, and a diagonal matrix \(D\) such that \(A=L D U .\) Such a representation of an invertible matrix is called an \(L D U\) -factorization.

Consider a square matrix that differs from the identity matrix at just one entry, off the diagonal, for example, \\[\left[\begin{array}{rrr} 1 & 0 & 0 \\ 0 & 1 & 0 \\ -\frac{1}{2} & 0 & 1 \end{array}\right].\\] In general, is a matrix \(M\) of this form invertible? If so, what is the \(M^{-1} ?\)

Find all matrices \(X\) that satisfy the given matrix equation. $$\left[\begin{array}{ll} 1 & 2 \\ 2 & 4 \end{array}\right] X=\left[\begin{array}{ll} 0 & 0 \\ 0 & 0 \end{array}\right]$$

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