(a) We prove it with the help of principle of mathematical induction. For
, it is true by [
3]. Suppose that it holds for all
We will prove that it is also true for
We take the case for
. If
is invertible, then for any
, we have
. Therefore, we have
and
Since,
and
is additive. From the above two equations we obtain
By replacing
with
in the above equation, we get
Now, if
is not invertible in
, we can find a sufficiently large number say
r such that
is invertible in
. Then
. Since
is invertible in
, therefore from the above equation we get
For
sinse
We have
Since
is additive. Therefore from above equation we get
Which follows that
i.e,
for all
and