We define the determinant of a square matrix in terms of cofactor expansion along the first row.

### DET-0010: Definition of the Determinant – Expansion Along the First Row

In this module we will define a function that assigns to each square matrix a scalar
output called the *determinant of *. For matrices with real number entries, the outputs
of the determinant function will be real numbers. We will denote the determinant of
by .

One important property of the determinant is its connection to matrix inverses. We will find that a matrix is singular if and only if . For nonsingular matrices, we will establish a formula that gives the inverse of a matrix exclusively in terms of determinants.

#### Determinants of Square Matrices ()

To start from the beginning, let us define the determinant of a matrix.

It is important to note that this definition is consistent with our goal of making a connection between determinants and invertibility. Observe that exists if and only if .Now we proceed to matrices. According to Formula form:detinverse of MAT-0050, the inverse of a
nonsingular matrix is given by
Observe that exists if and only if . We will call the number the *determinant* of
.

Our next goal is to define the determinant for a matrix. Let

Our definition will require three *minor* matrices associated with .

- is obtained from by deleting the first row and the first column of
.
- is obtained from by deleting the first row and the second column of
.
- is obtained from by deleting the first row and the third column of
.

We are now ready to define the determinant of a matrix.

We would like to point out several important features of this definition:

- The coefficients , and in front of determinants of minor matrices are the entries of the first row of matrix .
- To remember which minor matrix is associated with each first row
entry, cross out the row and column that the entry is in. For example,
minor matrix is found by crossing out the row and column that is
in.
- Coefficients follow an alternating sign pattern: , , .

#### Definition of the Determinant

Let be an matrix.
Define to be an matrix obtained from by deleting the first row and the column of .
We say that is the *-minor* of .

Recall that for a matrix we have We want to follow the same pattern to define the determinant of a larger matrix. A distinct feature of this expression is the alternating sign pattern. We want to preserve this feature as we increase matrix size. Before we present a formal definition, let’s take a look at an example.

Now we are ready to give the general formula. Pay close attention to how we will use subscripts to capture the alternating sign pattern.

Naturally, we would like to condense this formula. To accomplish this, let We will refer to as the*-cofactor of*. When we use the cofactor notation, the expression in Definition def:toprowexpansion turns into the following:

The process of computing the determinant given by Definition def:toprowexpansion is called the *cofactor
expansion along the first row*. We will later show that we can expand along
any row or column of a matrix and obtain the same value. This surprising
result, known as the Laplace Expansion Theorem, will be the subject of
DET-0050.

We conclude this section with a simple, but useful lemma.

- Proof
- Left to the reader.