A system of linear equations in unknowns with coefficients and is a list of simultaneous equations
As the notation suggests, we can turn a system of linear equations into a matrix equation and study it using matrix methods.
Every system of linear equations can be written in matrix form: the above system is equivalent to saying that , where , , and .
The system of linear equations
(3.9) |
has matrix form
This connection means that we can use systems of linear equations to learn about matrices, and use matrices to learn about systems of linear equations. For example, if is invertible and we want to solve the matrix equation
we could multiply both sides by to see that there is a unique solution .
We are going to make two more observations about solving linear systems based on what we know about matrix multiplication. The first is that by Proposition 3.2.1, the vectors which can be written as for some are exactly the ones which are linear combinations of the columns of , that is, vectors of the form
where is the th column of . So the matrix equation has a solution if and only if can be written as a linear combination of the columns of . This set of linear combinations is therefore important enough to have a name.
The column space of a matrix , written , is the set of all linear combinations of the columns of .
A homogeneous matrix equation is one of the form . These are particularly important because the solutions to any matrix equation can be expressed in terms of the solutions to the corresponding homogeneous equation .
Let be a solution of the matrix equation . Then any solution of can be written as for some vector such that .
Suppose is a solution of . Then , so . Letting we get as claimed. ∎
The theorem tells you that if you can solve the homogeneous equation and you can somehow find a particular solution of , you know all the solutions of the inhomogeneous equation .
What does it mean for to be true? Using Proposition 3.2.1 again, it says that
(3.10) |
where the are the entries of and the are the columns of . An equation of the form (3.10) is called a linear dependence relation, or just a linear dependence, on . We’ve seen that solutions of the matrix equation correspond to linear dependences on the columns of .
The solutions of the matrix equation are so important that they get their own name.
The nullspace of an matrix , written , is .
The homogeneous equation has the property that the zero vector is a solution, if and are solutions then so is , and if is a number then is also a solution. This is what it means to say that is a subspace of , something we will cover in the final chapter of MATH0005.
The augmented matrix of a system of linear equations whose matrix form is is the matrix which you get by adding as an extra column on the right of . We write this as or just .
For example, the augmented matrix for the system of linear equations (3.9) above would be
A solution to a matrix equation is a vector (of numbers this time, not unknowns) such that .
A system of linear equations may have a unique solution, many different solutions, or no solutions at all. In future lectures we will see how to find out how many solutions, if any, a system has.