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You recall that a linear differential equation

was called homogeneous if , and non-homogeneous or inhomogeneous
otherwise. We use the same terminology for systems of linear equations and for
matrix equations:
A matrix equation

is called **homogeneous** if
is the zero vector (all entries are zero). A
system of linear equations is called **homogeneous** if the equivalent matrix
equation is homogeneous.
Homogeneous matrix equations have some special properties:

- 1.
- The matrix equation
always has at least one solution, the zero solution
(Here 0 stands for a column vector all of whose entries are zero.)

- 2.
- If the column vectors
and
are two solutions to the matrix
equation
then so is any linear combination of them,
.

- 3.
- The complete solution to a matrix equation
is always given in the form
where , , ...
are solutions and , , ...,
are
parameters. The number of parameters depends on the dimension of the ``solution
space.''

You can see why property (1) holds; a system of linear equations like

will always be satisfied by setting all the variables equal to zero. (This is the same reason a
homogeneous linear differential equation can always be satisfied by setting .)
Property (2) depends on the linearity of multiplication by . If

then we have that
Property (3) also really comes from the linearity, since if we have

then we have that
This is the same reason that the general solution to a homogeneous linear differential equation
is a linear combination of particular solutions, such as
In the case of differential equations,
the number of different particular solutions, or the number of constants in the general solution,
depends on the order of the differential equation; one solution for a first order equation, two
different solutions for a second order equation, etc. In the case of matrix equations, the
number of particular solutions is the number of paramters in the general or complete solution,
the dimension of the solution space.
We can also see property (3) in action by solving a matrix equation. Here's the equation:

The augmented matrix of this equation has the row echelon form
so we can write down the complete general solution
We can rewrite this as
The particular solutions from which we can put together this complete solution are
The really nice thing we get out of this is a method for finding solutions to
*non-homogeneous* systems of linear equations (or non-homogeneous matrix equations.) It
works exactly the same way as solutions for linear differential equations:

If the matrix equation

has one particular solution , and the associated homogeneous equation
has the complete solution , then the complete solution to the original
non-homogeneous equation is
**Example:**

has the complete solution (which we computed earlier)
which we can rewrite as
This is the sum of the solution to the associated homogeneous system, which we wrote down in the
previous example,
and a particular solution to this inhomogeneous system
**Example:** The homogeneous system of linear equations

has the complete solution
The non-homongeous system
has one particular solution
To get the complete solution to the non-homongeneous system
we add these together:

**Exercise 1**
Rewrite the systems of linear equations of
Exercise 4 (in the last handout) as matrix equations.

**Exercise 2**
Rewrite the following matrix equations as systems of linear
equations.

**Exercise 3**
Carry out the following matrix multiplications, or explain why they
cannot be carried out.

**Exercise 4**
Solve the following matrix equations using row-reduction.

**Exercise 5**
What does it say about the set of solutions to a system of linear
equations if its augmented matrix, when put into row-reduced form:

(a.) Has a row whose leading entry is in the last column (the column corresponding to the
constant terms)?

(b.) Has all zeroes in the last column?

(c.) Has a column, other than the last column, in which no row has a leading entry?

(d.) Has rows with leading entries in every column except the last one?

**Exercise 6**
Write down the associated homogeneous matrix equations for the matrix equations in
exercise

4. Now write down the complete solution to each of these homogeneous
matrix equations.

**Exercise 7**
The matrix equation

has one solution given by

Give the complete solution to this matrix equation.

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** Up:** Solutions To Matrix Equations
** Previous:** Linear functions
*Peter Kostelec*

*2000-05-05*