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We study the connection between two combinatorial notions associated to a quiver: the quiver algebra and the path coalgebra. We show that the quiver coalgebra can be recovered from the quiver algebra as a certain type of finite dual, and we show precisely when the path coalgebra is the classical finite dual of the quiver algebra, and when all finite dimensional quiver representations arise as comodules over the path coalgebra. We discuss when the quiver algebra can be recovered as the rational part of the dual of the path coalgebra. Similar results are obtained for incidence (co)algebras. We also study connections to the notion of coreflexive (co)algebras, and give a partial answer to an open problem concerning tensor products of coreflexive coalgebras., Comment: 23 pages, to appear, Pacific Journal of Mathematics

Let $(R, S,_R\negthinspace M_S,_S\negthinspace N_R, f, g)$ be a general Morita context, and let $T=[{cc} R &_RM_S_SN_R & S]$ be the ring associated with this context. Similarly, let $T'=[{cc} R' & M' N' & S']$ be another Morita context ring. We study the set ${Iso}(T,T')$ of ring isomorphisms from $T$ to $T'$. Our interest in this problem is motivated by: (i) the problem to determine the automorphism group of the ring $T$, and (ii) the recovery of the non-diagonal tiles problem for this type of generalized matrix rings. We introduce two classes of isomorphisms from $T$ to $T'$, the disjoint union of which is denoted by ${Iso}_0(T,T')$. We describe ${Iso}_0(T,T')$ by using the $\Z$-graded ring structure of $T$ and $T'$. Our main result characterizes ${Iso}_0(T,T')$ as the set consisting of all semigraded isomorphisms and all anti-semigraded isomorphisms from $T$ to $T'$, provided that the rings $R'$ and $S'$ are indecomposable and at least one of $M'$ and $N'$ is nonzero; in particular ${Iso}_0(T,T')$ contains all graded isomorphisms and all anti-graded isomorphisms from $T$ to $T'$. We also present a situation where ${Iso}_0(T,T')={Iso}(T,T')$. This is in the case where $R,S,R'$ and $S'$ are rings having only trivial idempotents and all the Morita maps are zero. In particular, this shows that the group of automorphisms of $T$ is completely determined.

We study the injective envelopes of the simple right $C$-comodules, and their duals, where $C$ is a coalgebra. This is used to give a short proof and to extend a result of Iovanov on the dimension of the space of integrals on coalgebras. We show that if $C$ is right co-Frobenius, then the dimension of the space of left $M$-integrals on $C$ is $\leq {\rm dim}M$ for any left $C$-comodule $M$ of finite support, and the dimension of the space of right $N$-integrals on $C$ is $\geq {\rm dim}N$ for any right $C$-comodule $N$ of finite support. If $C$ is a coalgebra, it is discussed how far is the dual algebra $C^*$ from being semiperfect. Some examples of integrals are computed for incidence coalgebras.

We define the concept of a regular object with respect to another object in an arbitrary category. We present basic properties of regular objects and we study this concept in the special cases of abelian categories and locally finitely generated Grothendieck categories. Applications are given for categories of comodules over a coalgebra and for categories of graded modules, and a link to the theory of generalized inverses of matrices is presented. Some of the techniques we use are new, since dealing with arbitrary categories allows us to pass to the dual category.

We extend Morita theory to abelian categories by using wide Morita contexts. Several equivalence results are given for wide Morita contexts between abelian categories, widely extending equivalence theorems for categories of modules and comodules due to Kato, Muller and Berbec. In the case of Grothendieck categories we derive equivalence results by using quotient categories. We apply the general equivalence results to rings with identity, rings with local units, graded rings, Doi-Hopf modules and coalgebras., Comment: 35 pages

We construct a structure of a ring with local units on a co-Frobenius coalgebra. We study a special class of co-Frobenius coalgebras whose objects we call symmetric coalgebras. We prove that any semiperfect coalgebra can be embedded in a symmetric coalgebra. A dual version of Brauer's equivalence theorem is presented, allowing a characterization of symmetric coalgebras by comparing certain functors. We define an automorphism of the ring with local units constructed from a co-Frobenius coalgebra, which we call the Nakayama automorphism. This is used to give a new characterization to symmetric coalgebras and to describe Hopf algebras that are symmetric as coalgebras. As a corollary we obtain as a consequence the known characterization of Hopf algebras that are symmetric as algebras., Comment: 23 pages

Let H be a finite dimensional non-semisimple Hopf algebra over an algebraically closed field k of characteristic 0. If H has no nontrivial skew-primitive elements, we find some bounds for the dimension of H_1, the second term in the coradical filtration of H. Using these results, we are able to show that every Hopf algebra of dimension 14 is semisimple and thus isomorphic to a group algebra or the dual of a group algebra. Also a Hopf algebra of dimension pq where p and q are odd primes with p

We classify gradings on matrix algebras by a finite abelian group. A grading is called good if all elementary matrices are homogeneous. For cyclic groups, all gradings on a matrix algebra over an algebraically closed field are good. We can count the number of good gradings by a cyclic group. Using descent theory, we classify non-good gradings on a matrix algebra that become good after a base extension., Comment: 13 pages