1. The

-modules are the (additive) abelian groups, so the simple ones are the simple abelian groups. These are the prime cycles

(in additive notation) where
p is a prime. Hence the semisimple

-modules are the the direct sums of copies of these

for various primes
p. The homogeneous semisimple

-modules are the direct sums of copies of

for a fixed prime
p.
2. a. Since
RR is complemented by Theorem 1, we have
R =
L ⊕
M for some left ideal
M. Write 1 =
e +
f where
e
L and
f
M. Hence
Re ⊆
L, and we claim this is equality. If
x
L then
x −
xe =
xf L ∩
M = 0 because ...