$G$-Categories

A [italic]G-category is a category on which a group [italic]G acts. This work studies the 2-category [italic]G-cat of [italic]G-categories, [italic]G-functors (functors which commute with the action of [italic]G) and [italic]G-natural transformations (natural transformations which commute with the [italic]G-action). There is a particular emphasis on the relationship between a [italic]G-category and its stable subcategory, the largest sub-[italic]G-category on which [italic]G operates trivially. Also contained here are some very general applications of the theory to various additive [italic]G-categories and to [italic]G-topoi.