690 Appendix E
8. A unit matrix (I) is a diagonal matrix where a
ii
= 1; consequently, |(I)| =
1.
9. A zero matrix (0) is the matrix where a
ij
= 0, for all i, j.
E.2 Matrix Algebra
E.2.1 Definitions
1. Addition. The sum of two matrices exists only if the two matrices have
the same size, i.e., the same number of rows and columns,
(A) + (B) = (a
ij
)
mn
+ (b
ij
)
mn
= (a
ij
+ b
ij
)
mn
(E.2)
2. Multiplication. If α is a scalar
α(A) = (αa
ij
)
mn
(E.3)
3. Product of matrices. The product of two matrices exists only if the
number of columns in (A) equals the number of rows in (B), i.e.,
(A) = (a
ij
)
mp
, (B) = (b
ij
)
pn
,
(A)(B) = (c
ij
)
mn
(E.4)
where i = 1, 2 ···m, j = 1, 2 ···n, and
c
ij
=
p
X
k=1
a
ik
b
kj
= a
i1
b
1j
+ a
i2
b
2j
+ ··· + a
ip
b
pj
E.2.2 Matrix Algebraic Formulas
In the following expressions, (A) = (a
ij
)
mn
, (B) = (b
ij
)
mn
, (C) = (c
ij
)
mn
are matrices and α and β are scalars.
(A) + (B) = (B) + (A) (E.5)
[(A) + (B)] + (C) = (A) + [(B) + (C)] (E.6)
α[(A) + (B)] = α(A) + α(B) (E.7)
(α + β)(A) = α(A) + β(A) (E.8)
(A)(B) 6= (B)(A) (E.9)
α[(A)(B)] = [α(A)](B) = (A)[α(B)] (E.10)
[(A)(B)](C) = (A)[(B)(C)] (E.11)
[(A) + (B)](C) = (A)(C) + (B)(C) (E.12)