Lax structures in 2-category theory

This thesis focuses on topics in 2-category theory: in particular on double categories, pseudomonads and codescent objects. In Chapter 2 we recall all the necessary notions. In Chapter 3 we show that factorization systems can be equivalently described as double categories satisfying certain properties. In Chapter 4 we focus on turning weak (colax) structures into strict ones in a universal way - this covers for instance colax monoidal categories or lax functors. In Chapter 5 we study the Kleisli 2-category for a lax-idempotent pseudomonad, the application of which is establishing weak cocompleteness of 2-categories such as the one of monoidal categories and lax monoidal functors. Finally, Chapter 6 focuses on the process of turning any 2-monad into a lax-idempotent one.