We characterize Frobenius algebras A as algebras having a comultiplication which is a map of A-modules. This characterization allows a simple demonstration of the compatibility of Frobenius algebra structure with direct sums. We then classify the indecomposable Frobenius algebras as being either “annihilator algebras” — algebras whose socle is a principal ideal — or field extensions. The relationship between two-dimensional topological quantum field theories and Frobenius algebras is then formulated as an equivalence of categories. The proof hinges on our new characterization of Frobenius algebras. These results together provide a classification of the indecomposable two-dimensional topological quantum field theories.

We characterize noncommutative Frobenius algebras A in terms of the existence of a coproduct which is a map of left A e -modules. We show that the category Ž . of right left comodules over A, relative to this coproduct, is isomorphic to the Ž . category of right left modules. This isomorphism enables a reformulation of the cotensor product of Eilenberg and Moore as a functor of modules rather than comodules.We prove that the cotensor product M I N of a right A-module M and a left A-module N is isomorphic to the vector space of homomorphisms from a particular left A e -module D to N m M, viewed as a left A e -module. Some properties of D are described. Finally, we show that when A is a symmetric algebra, the cotensor product M I N and its derived functors are given by the Hochschild cohomology over A of N m M. ᮊ 1999 Academic Press

The Poincaré duality of classical cohomology and the extension of this duality to quantum cohomology endows these rings with the structure of a Frobenius algebra. Any such algebra possesses a canonical "characteristic element;" in the classical case this is the Euler class, and in the quantum case this is a deformation of the classical Euler class which we call the "quantum Euler class." We prove that the characteristic element of a Frobenius algebra A is a unit if and only if A is semisimple, and then apply this result to the cases of the quantum cohomology of the finite complex Grassmannians, and to the quantum cohomology of hypersurfaces. In addition we show that, in the case of the Grassmannians, the [quantum] Euler class equals, as [quantum] cohomology element and up to sign, the determinant of the Hessian of the [quantum] Landau-Ginzbug potential.

Abstract-The human cerebral cortex is topologically equivalent to a sphere when it is viewed as closed at the brain stem. Due to noise and/or resolution issues, magnetic resonance imaging may see "handles" that need to be eliminated to reflect the true spherical topology. Shattuck and Leahy [2] present an algorithm to correct such an image. The basis for their correction strategy is a conjecture, which they call the spherical homeomorphism conjecture, stating that the boundary between the foreground region and the background region is topologically spherical if certain associated foreground and background multigraphs are both graph-theoretic trees. In this paper, we prove the conjecture, and its converse, under the assumption that the foreground/background boundary is a surface.

We catalog up to a type of reducibility all cellular automorphisms of the sphere, projective plane, torus, Klein bottle, and three-crosscaps (Dyck's) surface. We also show how one can obtain all self-dual embeddings in a surface S given a catalog of all irreducible cellular automorphisms in S.