We called this number the determinant of A. It is
clear from this that we would like to have similar
result for bigger matrices (meaning higher orders).
There is similar notion of determinant for any
square matrix, which determines whether square
matrix is invertible or not. In order to generalize
such notion to higher orders, it is required to study
the determinant and see what kind of properties it
satisfies.
Notation
The following notation is used for the determinant:
ad cb
c d
a b
- = =
=
c d
a b
det
c d
a b
determinant of
1.2 Properties of the Determinant
(1) Any matrix A and its transpose have the
same determinant, meaning
det (A) = det(AT )
(2) The determinant of a triangular matrix is
the product of the entries on the diagonal,
that is
ad
c d
a
d
a b
= =
0
0
(3) If we interchange two rows, the
determinant of the new matrix is the
opposite of the old one, that is
a b
c d
c d
a b
= -
(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
kc kd
a b
c d
a b
k
c d
ka kb
= =
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
c ka d kb
a b
c d
a b
c d
a kc b kd
+ +
= =
+ +
(6) We have
det (AB) = det(A) . det(B)
If A and B are similar, then
det (A) = det(B)
2. FORMING SINGULAR MATRIX
2.1 Special Properties
Let us consider following special properties
a11 + a22 – a21 = a12
a21 + a32 – a31 = a22
a12 + a23 – a22 = a13
a22 + a33 – a32 = a33
The determinant of any matrix constructed using
above properties is zero except matrix of order
(2 X 2) and (1 X 1) and the sum of elements in the
two diagonals is also equal.
2.2 Theorem
Theorem Any Matrix formed using the above
special properties is singular
Proof: Let us consider the 3 X 3matrix.
11 12 13
21 22 23
31 32 33
a a a
A a a a
a a a
=
From special properties (see Section. 2.1), we
derive
a11 – a12 = a21 – a22 = a31 – a32 (1)
a12 – a13 = a22 – a23 = a32 – a33 (2)
Subtracting second column from the first column
We get
11 12 12 13
21 22 22 23
31 32 32 33
_
_
_
A
a a a a
a a a a
a a a a
=