 
Summary: Rings and Algebras Problem set #11. Dec. 1, 2011.
1. Suppose e # R is a basic idempotent in a semiperfect ring R and suppose that M is a generator of RMod, i. e.
for each Rmodule N there is an epimorphism f : # I
M # N . Then Re is isomorphic to a direct summand of M .
2. Let R be the ring of all N×N matrices over R which can be written as the sum of a scalar matrix and a strictly
lower triangular matrix with only finitely many nonzero entries. Show that R is left perfect but not right perfect.
3. Let A = K#/I be a finite dimensional path algebra defined by relations. Let e 1 , . . . , e n denote the idempotents
corresponding to vertices.
a) Show that the module Ae i is indecomposable projetive and it is the projective cover of the simple module
Ae i /J(A)e i .
b) Show that D(e i A) = HomK (e i A, K) is indecomposable injective and it is the injective envelope of
Ae i /J(A)e i .
4. a) Show that the following are equivalent for a module RM :
(i) M is faithful;
(ii) M cogenerates RR;
(iii) M cogenerates every finitely generated projective module.
5. Show that if F : RMod # SMod is a categorical equivalence then a module RM is faithful if and only if S F (M)
is faithful. Derive from this that R is (semi)primitive if and only if S is semiprimitive.
6. Let RPS and S QR be bimodules satisfying the conditions in the the theorem characterizing Moritaequivalence.
Prove the natural isomorphisms:
