
A rectangular array of numbers, say n by m, is called a matrix. The ijth element of the matrix A is the element in the ith row and jth column, and is denoted as A_{ij}.
Here are examples of matrices one two by two and the other two by three
If matrix A has the same number of columns as B has rows, we define the product matrix, AB to be the matrix whose elements are dot products between the rows of A and the columns of B. The element obtained by taking the dot product of the ith row of A and the jth column of B is described as (AB)_{ij}. See also Section 32.2 for a fuller discussion of matrices and their properties.
Exercises:
3.7 Find the product of the two matrices above.
3.8 Build a spreadsheet that multiplies 4 by 4 matrices. Solution
3.9 In exercise 3.8:
1. Where is the matrix product AB?
2. What appears in columns p, q, r and s in the first four rows?
If you change any of the entries in A or B the product will change automatically, so you have built an 4 by 4 matrix automatic product finder.
3. Can you use this to find the product of a 2 by 3 matrix and a 3 by 4 one? How?
4. Find the tenth power of a matrix A using your product finder. (Hint: use it for A and for B and look in the right place and you have it.)
A vector v can be written either as a matrix consisting of a single row, or of a single column. When writing it as a column we will write v> ; as a row, <v. The square of the length of v can then be written as the matrix product <vv>.
A vector v is an eigenvector of a matrix M when Mv is a multiple of v. The multiple is called the eigenvalue of M having eigenvector v. If the eigenvalue is s, then we have Mv = sv.
The applet here allows you to enter any 2 by 2 matrix, and move the vector v around. When Mv lines up with v, v is an eigenvector of M with real eigenvalue which is given by the ratio of the length of Mv (called v' in the applet) to that of v, with a sign that is positive when they point in the same direction.
Exercise 3.10 Choose a symmetric matrix and use the applet to determine the two eigenvectors, approximately. Draw them on a piece of paper. Can you notice something about them? What?
