# Mathematics Colloquia and Seminars

In joint work with J. Gubeladze we have explored the linear algebra'' properties of divisorial ideals over normal semigroup rings. In invariant theory these ideals appear as modules of semi-invariants of the actions of algebraic tori. The properties and invariants to be discussed are, among other things, the Cohen-Macaulayness and the number of generators. In particular we show that there exist, up to isomorphism, only finitely many Cohen-Macaulay (divisorial) ideals.