Categorical Quotients

Factorial Algebraic Group Actions and Categorical Quotients

Given an action of an affine algebraic group with only trivial characters on a factorial variety, we ask for categorical quotients. We characterize existence in the category of algebraic varieties. Moreover, allowing constructible sets as quotients, we obtain a more general existence result, which, for example, settles the case of a finitely generated algebra of invariants. As an application, w...

Categorical Abstract Algebraic Logic: Tarski Congruence Systems, Logical Morphisms and Logical Quotients

A general notion of a congruence system is introduced for π-institutions. Congruence systems in this sense are collections of equivalence relations on the sets of sentences of the π-institution that are preserved both by signature morphisms and by fixed collections of natural transformations from finite tuples of sentences to sentences. Based on this notion of a congruence system, the notion of...

Categorical Logic Categorical Logic

Quotients Homophones des Groupes Libres Homophonic Quotients of Free Groups

Ah, la recherche! Du temps perdu. Soit G le quotient du groupe libre a 26 g en era-teurs a; b; c; : : : ; z par les relations A = B, pour tout couple de mots (A; B) pouvant avoir la m^ eme prononciation en anglais. (Un groupe d eeni d'une mani ere analogue a et e consid er e dans Landsburg 1986].) Notre but est de d eterminer la structure du groupe G. Théorème. G est trivial. Démonstration. (No...

6 Toric Degenerations of Git Quotients , Chow Quotients

The moduli spaceM0,n plays important roles in algebraic geometry and theoretical physics. Yet, some basic properties of M 0,n still remain open. For example, M0,n is rational and nearly toric (that is, it contains a toric variety as a Zariski open subset), but it is not a toric variety itself starting from dimension 2 (n ≥ 5). So, a basic question is: Can it be degenerated flatly to a projectiv...