The determinant of a matrix is a number associated with a square (nxn) matrix. The determinant can tell us if columns are linearly correlated, if a system has any nonzero solutions, and if a matrix is invertible. See the wikipedia entry for more details on this.

Computing a determinant is key to a lot of linear algebra, and by extension, to a lot of machine learning. It is easy to calculate the determinant for a 2x2 matrix:

{%math%} \begin{align} A = \begin{bmatrix} a & b

c & d \end{bmatrix}

det A = \begin{vmatrix} a & b

c & d \end{vmatrix}

det A = ad - bc \end{align} {%endmath%}

Calculating the determinant for a bigger matrix is a bit more complicated, as we will see. All the code for this is available from the algorithms repository.

## Laplace Expansion

Determinants for larger matrices can be recursively obtained by the Laplace Expansion. This computes the matrix determinant by making it equal to a sum of the scaled minors of the matrix. A minor is the determinant of a matrix after deleting one row and one column (so a 3x3 matrix would turn into a 2x2 matrix).

So, let’s start with this matrix:

{%math%} A = \begin{bmatrix} 1 & 1 & 2

2 & 3 & 4

3 & 4 & 5 \end{bmatrix} {%endmath%}

To find the determinant of this matrix, we will first consult the formula for laplace expansion.

{%math%} \sum_{j=1}^{n}(-1)^{1+j}a_{1j}M_{1j} {%endmath%}

- {%m%}\sum_{j=1}^{n}{%em%} - this means the sum from j=1 to n, in this case from the first column to the last one.
- {%m%}(-1)^{1+j}{%em%} - this ensures that alternating entries will be added and subtracted
- {%m%}a_{1j}{%em%} - the element in the matrix A at position 1,j. So, A[1][j].
- {%m%}M_{1j}{%em%} - the minor of matrix A after removing row 1 and column j.

So, we are taking the sum from j=1 to n of -1 to the power (1+j) times the element a at index (1,j) in the original matrix times the minor of the matrix after removing row 1 and column j.

Let’s expand this out for our matrix:

{%math%} \begin{align} 1 * 1 * \begin{vmatrix} 3 & 4

4 & 5 \end{vmatrix} + -1 * 1 * \begin{vmatrix} 2 & 4

3 & 5 \end{vmatrix} + 1 * 2 * \begin{vmatrix} 2 & 3

3 & 4 \end{vmatrix}

(3*5 - 4*4) -(2*5 - 4*3) + 2 * (2*4 - 3*3)

-1 +2 -2

-1 \end{align} {%endmath%}

So, our final determinant for this matrix is -1.

## Implementation

Now that we know the formula, we can formalize it in pseudocode:

```
Suppose that we have an nxn matrix A, with number of columns j.
if the number of columns is 2, compute the determinant using ad-bc and return.
Iterate through all of the columns.
Calculate the multiplier by taking -1 to the power (1+j) times the element at A[1][j].
Delete row 1 and column j from A, and create a new matrix X.
Find the determinant of X through recursion (start again at the top with A=X).
Multiply the determinant by the multiplier.
Sum all of the values and return.
```

This will work by recursing through our matrix to eventually reduce it to a series of 2x2 matrices, where the minors can be calculated.

In order to implement this, we will use the Matrix class that we already developed, with one addition:

```
def del_column(self, key):
"""
Delete a specified column
"""
for i in xrange(0,self.rows):
del self.X[i][key]
```

We can then implement a function that takes in a matrix object and computes its determinant:

```
def recursive_determinant(X):
"""
Find the determinant in a recursive fashion. Very inefficient
X - Matrix object
"""
#Must be a square matrix
assert X.rows == X.cols
#Must be at least 2x2
assert X.rows > 1
term_list = []
#If more than 2 rows, reduce and solve in a piecewise fashion
if X.cols>2:
for j in xrange(0,X.cols):
#Remove i and j columns
new_x = deepcopy(X)
del new_x[0]
new_x.del_column(j)
#Find the multiplier
multiplier = X[0][j] * math.pow(-1,(2+j))
#Recurse to find the determinant
det = recursive_determinant(new_x)
term_list.append(multiplier*det)
return sum(term_list)
else:
return(X[0][0]*X[1][1] - X[0][1]*X[1][0])
```

We can verify if it works by testing out the matrix that we used above:

```
X = Matrix([[1,1,2],[2,3,4],[3,4,5]])
recursive_determinant(X)
```

```
-1.0
```

## Applications

We can now test matrices to see if their columns are linearly dependent:

```
X = Matrix([[1,1,2],[2,2,4],[4,4,8]])
recursive_determinant(X)
```

```
0.0
```

The zero indicates that the columns are dependent on each other.

This will also tell us if the matrix can be inverted.

```
X = Matrix([[1,1,2],[2,2,4],[4,4,8]])
X.invert()
```

The above should cause an error.

## Performance improvements

There are certainly other, higher-performing, solutions to finding a matrix determinant, like LU Decomposition but I like this because it makes it easy to figure out what is happening under the hood.

Possible performance improvements to this algorithm could be:

- Creating a global “minor cache” to avoid recomputing the same minors over and over for large matrices.
- Use the 3x3 formula instead of the 2x2 formula, which will avoid some recursion steps.