# Introduction to Determinants

For any square matrix of order 2, we have found a necessary and sufficient condition for invertibility. Indeed, consider the matrix

The matrix A is invertible if and only if . We called this number the determinant of A. It is clear from this, that we would like to have a similar result for bigger matrices (meaning higher orders). So is there a similar notion of determinant for any square matrix, which determines whether a square matrix is invertible or not?

In order to generalize such notion to higher orders, we will need to study the determinant and see what kind of properties it satisfies. First let us use the following notation for the determinant

Properties of the Determinant

1.
Any matrix A and its transpose have the same determinant, meaning

This is interesting since it implies that whenever we use rows, a similar behavior will result if we use columns. In particular we will see how row elementary operations are helpful in finding the determinant. Therefore, we have similar conclusions for elementary column operations.
2.
The determinant of a triangular matrix is the product of the entries on the diagonal, that is

3.
If we interchange two rows, the determinant of the new matrix is the opposite of the old one, that is

4.
If we multiply one row with a constant, the determinant of the new matrix is the determinant of the old one multiplied by the constant, that is

In particular, if all the entries in one row are zero, then the determinant is zero.
5.
If we add one row to another one multiplied by a constant, the determinant of the new matrix is the same as the old one, that is

Note that whenever you want to replace a row by something (through elementary operations), do not multiply the row itself by a constant. Otherwise, you will easily make errors (due to Property 4).
6.
We have

In particular, if A is invertible (which happens if and only if ), then

If A and B are similar, then .

Let us look at an example, to see how these properties work.

Example. Evaluate

Let us transform this matrix into a triangular one through elementary operations. We will keep the first row and add to the second one the first multiplied by . We get

Using the Property 2, we get

Therefore, we have

which one may check easily.

The determinant of matrices of higher order will be dealt with on the next page.

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Author: M.A. Khamsi