Question

Suppose we represent vectors \(v=\left(v_{i}\right)\) as sequences of numbers, and matrices \(m=\left(m_{i j}\right)\) as sequences of vectors (the rows of the matrix). For example, the matrix $$ \left[\begin{array}{llll} 1 & 2 & 3 & 4 \\ 4 & 5 & 6 & 6 \\ 6 & 7 & 8 & 9 \end{array}\right] $$ is represented as the sequence \(\left(\begin{array}{lllllllllll}1 & 2 & 3 & 4\end{array}\right)\left(\begin{array}{lllllll}4 & 5 & 6 & 6\end{array}\right)\left(\begin{array}{llll}6 & 7 & 8 & 9\end{array}\right)\). With this representation, we can use sequence operations to concisely express the basic matrix and vector operations. These operations (which are described in any book on matrix algebra) are the following: $$ \begin{array}{ll} \text { (dot-product } v w \text { ) } & \text { returns the sum } \sum_{i} v_{i} w_{i} ; \\ \text { (matrix-*-vector } m \text { v) } & \text { returns the vector } t, \text { where } t_{i}=\sum_{j} m_{i j} v_{j} ; \\ \text { (matrix-*-matrix } m \text { n ) } & \text { returns the matrix } p, \text { where } p_{i j}=\sum_{k} m_{i k} n_{k j} ; \\ \text { (transpose } m \text { ) } & \text { returns the matrix } n, \text { where } n_{i j}=m_{j i} . \end{array} $$

Short answer

Matrix and vector operations simplify calculations using sequences.

Step-by-step solution

Understanding Dot Product

The dot product between two vectors \( v \) and \( w \) is computed by multiplying corresponding elements and summing the results: \( \sum_{i} v_{i} w_{i} \). For example, given vectors \( v = (1, 2, 3) \) and \( w = (4, 5, 6) \), the dot product is \( 1\times4 + 2\times5 + 3\times6 = 32 \).

Calculating Matrix-Vector Product

To calculate the product of a matrix \( m \) and a vector \( v \), compute each element of the resulting vector as \( t_i = \sum_{j} m_{ij} v_{j} \). For example, with matrix \( m = \left(\begin{array}{ll}1 & 2\3 & 4\end{array}\right) \) and vector \( v = (1, 2) \), the resulting vector \( t \) is \((1\times1 + 2\times2, 3\times1 + 4\times2) = (5, 11) \).

Calculating Matrix-Matrix Product

To multiply two matrices \( m \) and \( n \), the element at position \( (i, j) \) in the resulting matrix \( p \) is calculated by \( p_{ij} = \sum_{k} m_{ik} n_{kj} \). For example, if \( m = \left(\begin{array}{ll}1 & 2\3 & 4\end{array}\right) \) and \( n = \left(\begin{array}{ll}5 & 6\7 & 8\end{array}\right) \), then \( p = \left(\begin{array}{ll}19 & 22\43 & 50\end{array}\right) \).

Computing Transpose of a Matrix

The transpose of a matrix \( m \) is a new matrix \( n \) where the rows and columns are swapped; that is, \( n_{ij} = m_{ji} \). For a matrix \( m = \left(\begin{array}{ll}1 & 2\ 3 & 4\end{array}\right) \), the transpose \( n \) is \( \left(\begin{array}{ll}1 & 3\ 2 & 4\end{array}\right) \).

Key concepts

Dot Product

The dot product is a key operation in matrix algebra, especially when dealing with vectors. It's a mathematical operation that combines two equal-sized sequences of numbers into a single numerical result. The formula to compute the dot product of two vectors, say \( v \) and \( w \), is \( \sum_{i} v_{i} w_{i} \). This simply means multiplying each pair of corresponding elements from the vectors and then summing these products.

For example, given two vectors \( v = (1, 2, 3) \) and \( w = (4, 5, 6) \), the dot product is found by calculating \( 1 \times 4 + 2 \times 5 + 3 \times 6 = 32 \). Dot products are not only central in basic matrix operations but also in various applications, including projections and determining orthogonal vectors.

• Helps determine similarity between two vectors
• Used in finding projections in linear algebra
• Key in geometry for finding angles between vectors

Matrix-Vector Multiplication

Matrix-vector multiplication is an essential operation used frequently in various fields such as computer graphics, engineering, and scientific computations. It allows us to transform a vector using a matrix, which can represent various linear transformations like rotations and translations.

To perform a matrix-vector multiplication, compute each element of the resulting vector as \( t_i = \sum_{j} m_{ij} v_{j} \). Each entry \( t_i \) is essentially the dot product of the \( i \)-th row of the matrix with the vector. For instance, if we have a matrix \( m = \left( \begin{array}{ll}1 & 2\3 & 4\end{array}\right) \) and a vector \( v = (1, 2) \), then the resulting vector \( t \) would be \( (1 \times 1 + 2 \times 2, 3 \times 1 + 4 \times 2) = (5, 11) \).

• Enables transformations of vectors in various applications
• Widely used in computer graphics for rendering
• Foundation for understanding more complex matrix operations

Matrix-Matrix Multiplication

Matrix-matrix multiplication is a fundamental operation through which two matrices are combined into a third matrix. The product matrix's element at position \((i, j)\) is obtained by computing the dot products of the appropriate row of the first matrix \(m\) and column of the second matrix \(n\). The formula for this is \( p_{ij} = \sum_{k} m_{ik} n_{kj} \).

For example, consider matrices \( m = \left( \begin{array}{ll}1 & 2\3 & 4\end{array}\right) \) and \( n = \left( \begin{array}{ll}5 & 6\7 & 8\end{array}\right) \). Their product matrix \( p \) has elements calculated as such: for \( p_{11} \), compute \( 1 \times 5 + 2 \times 7 = 19 \); for \( p_{12} \), \( 1 \times 6 + 2 \times 8 = 22 \), and so forth, resulting in \( p = \left( \begin{array}{ll}19 & 22\43 & 50\end{array}\right) \).

• Extensively used in computer science, graphics, and data science
• Involved in solving systems of equations
• Foundational to concepts in machine learning algorithms

Transpose of a Matrix

The transpose of a matrix is an operation that flips it over its diagonal, effectively swapping its rows with columns. This operation is denoted by \( n_{ij} = m_{ji} \), meaning the element in the \( i \)-th row and \( j \)-th column of the new matrix is taken from the \( j \)-th row and \( i \)-th column of the original matrix.

To illustrate, assume \( m = \left( \begin{array}{ll}1 & 2\3 & 4\end{array}\right) \). The transpose of this matrix \( n \) would be \( \left( \begin{array}{ll}1 & 3\2 & 4\end{array}\right) \).

• Used to simplify the multiplication process
• Ensures compatibility in matrix operations
• Vital in deriving symmetric matrices

Transpose is widely utilized in solving linear equations and is also key in optimizing certain algorithms in computational mathematics.