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Let A be a finite-dimensional algebra given by quiver and monomial relations. In [18] we see that the Ext-algebra of A is finitely generated only if all the Ext-algebras of certain cycle algebras overlying A are finitely generated. Here a cycle algebra Lambda is a finite-dimensional algebra given by quiver and monomial relations where the quiver is an oriented cycle. The main result of this thesis gives necessary and sufficient conditions for the Ext-algebra of such a Lambda to be finitely generated; this is achieved by defining a computable invariant of Lambda, the smo-tube. We also give necessary and sufficient conditions for the Ext-algebra of Lambda to be Noetherian.;Let Lambda be a triangular matrix algebra, defined by algebras T and U and a T-U-bimodule M. Under certain conditions we show that if the Ext-algebras of T and U are right (respectively left) Noetherian rings, then the Ext-algebra of Lambda is a right (respectively left) Noetherian ring. An example shows the hypotheses used cannot be improved. We also specialise to the case where Lambda is a one-point extension: we give a specific presentation of a result that parallels a similar theorem for the more general case above.

We develop theory, and construct examples, of graded rings that have properties similar to the conjectured form of the stable homotopy group of the p-completion of the sphere spectrum π*(Sp) in the p-local stable homotopy category. In particular, these rings are non-Noetherian self injective connected graded rings of finite type that contain at least one non-nilpotent element. Moreover, we also construct examples of non-Noetherian graded rings that have properties similar to the conjectured form of the ring π* (Sp), yet fail to be self injective as graded rings. These examples have an extremely complicated structure, and they also provide counter examples to potential converses to the main theory of self injective graded rings.

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We discuss Andre-Quillen homology for simplicial algebras and algebras over simplicial algebras, extending the classical notion for rings. This extension is also discussed by Goerss and Hopkins, however our statements are proven in a more explicit way. We are then further able to construct spectral sequences for Andre-Quillen homology like the spectral sequence for the indecomposables or the Fundamental spectral sequence according to Quillen. The Andre-Quillen homology for algebras constructed as cellular complexes is calculated and we apply this homology theory to obtain notions of atomic and nuclear algebras, thus extending results from Baker and May. We define the notion of i-stable algebras and are able to give a comparison theorem between Andre-Quillen homology, stabilisation and Gamma-homology for i-stable algebras up to degree i. In the second part of the thesis we discuss topological Andre-Quillen homology and extend certain results by Gilmour about cellular complexes in this setting.

The three generator Artin groups A(m,n.2) are known to be have CAT(O) dimension strictly greater than two if both m and n are odd [BC]. In Chapter 1 we introduce the notions of CAT(O) dimension and three generator Artin groups. In Chapter 2 we show that if one of m or n is even, then the three generator Artin group has CAT(O) dimension two. In Chapter 3 we extend work by Noel Brady and John Crisp [BC] to enlarge the subclass of groups A(m.n.2) known to have CAT(O) dimension three. In Chapter 4 we classify the structure of a canonical cell complex which the group A(m,n,2) acts on for the case where m is even, greater or equal to six and not divisible by four and n is prime, greater or equal to five. Finally, in Chapter 5 we use the results of Chapter 4 to exhibit classes of rank four Artin groups with CAT(O) dimension two. and a class of rank six Artin groups with CAT(O) dimension two.

This thesis consists of two parts, both of which involve the study of algebraic structures constructed via the multiplication of modules. In the first part we look at Hall algebras. We consider the Hall algebra of a cyclic quiver algebra with relations of length two and present a multiplication formula for the explicit calculation of products in this algebra. We then look at the case of a cyclic quiver with two vertices and describe the corresponding composition algebra as a quotient of the positive part of a quantised enveloping algebra of type Ã₁ We then look at quotients of Hall algebras of self-injective algebras. We give an abstract result describing the quotient of such a Hall algebra by the ideal generated by isomorphism classes of projective modules, and also a more explicit result describing quotients of Hall algebras of group algebras for cyclic 2-groups and some related polynomial algebras. The second part of the thesis deals with Green rings. We compare the Green rings of a group algebra and the corresponding Jennings algebra for certain p-groups. It is shown that these two Green rings are isomorphic in the case of a cyclic p-group. In the case of the Klein four group it is shown that the two Green rings are not isomorphic, but that there exist quotients of these rings which are isomorphic. It is conjectured that the corresponding quotients will also be isomorphic in the case of a dihedral 2-group. The properties of these quotients are studied, with the aim of producing evidence to support this conjecture. The work on Green rings also includes some results on the realisation of quotients of Green rings as group rings over ℤ.