Page 34 - 35Linear Algebra

34 What is Linear Algebra?

Are these matrices diagonal and why? Use the rule you found in prob-
0
0
lem 6 to compute the matrix products DD and D D. What do you
observe? Do you think the same property holds for arbitrary matrices?
What about products where only one of the matrices is diagonal?

(p.s. Diagonal matrices play a special role in in the study of matrices
in linear algebra. Keep an eye out for this special role.)
8. Find the linear operator that takes in vectors from n-space and gives
out vectors from n-space in such a way that

(a) whatever you put in, you get exactly the same thing out as what
you put in. Show that it is unique. Can you write this operator
as a matrix?
(b) whatever you put in, you get exactly the same thing out as when
you put something else in. Show that it is unique. Can you write
this operator as a matrix?

Hint: To show something is unique, it is usually best to begin by pre-
tending that it isn’t, and then showing that this leads to a nonsensical
conclusion. In mathspeak–proof by contradiction.

9. Consider the set S = {∗, ?, #}. It contains just 3 elements, and has
no ordering; {∗, ?, #} = {#, ?, ∗} etc. (In fact the same is true for
{1, 2, 3} = {2, 3, 1} etc, although we could make this an ordered set
using 3 > 2 > 1.)

(i) Invent a function with domain {∗, ?, #} and codomain R. (Re-
member that the domain of a function is the set of all its allowed
inputs and the codomain (or target space) is the set where the
outputs can live. A function is specified by assigning exactly one
codomain element to each element of the domain.)

(ii) Choose an ordering on {∗, ?, #}, and then use it to write your
function from part (i) as a triple of numbers.

(iii) Choose a new ordering on {∗, ?, #} and then write your function
from part (i) as a triple of numbers.

34