This thesis provides general constructions in the context of decomposition spaces, generalising classical results from combinatorics to the homotopical setting. This requires developing general tools in the theory of decomposition spaces and new viewpoints, which are of general interest, independently of the applications to combinatorics. In the first chapter, we summarise the homotopy theory and combinatorics of the 2-category of groupoids. We continue with a review of needed notions from the theory of ∞-categories. We then summarise the theory of decomposition spaces. In the second chapter, we identify the structures that have incidence bi(co)modules: they are certain augmented double Segal spaces subject to some exactness conditions. We establish a Möbius inversion principle for (co)modules, and a Rota formula for certain more involved structures called Möbius bicomodule configurations. The most important instance of the latter notion arises as mapping cylinders of infinity adjunctions, or more generally of adjunctions between Möbius decomposition spaces, in the spirit of Rota's original formula. In the third chapter, we present some tools for providing situations where the generalised Rota formula applies. As an example of this, we compute the Möbius function of the decomposition space of finite posets, and exploit this to derive also a formula for the incidence algebra of any directed restriction species, free operad, or more generally free monad on a finitary polynomial monad. In the fourth chapter, we show that Schmitt's hereditary species induce monoidal decomposition spaces, and exhibit Schmitt's bialgebra construction as an instance of the general bialgebra construction on a monoidal decomposition space. We show furthermore that this bialgebra structure coacts on the underlying restriction-species bialgebra structure so as to form a comodule bialgebra. Finally, we show that hereditary species induce a new family of examples of operadic categories in the sense of Batanin and Markl. In the fifth chapter, representing joint work with Joachim Kock, we introduce a notion of antipode for monoidal (complete) decomposition spaces, inducing a notion of weak antipode for their incidence bialgebras. In the connected case, this recovers the usual notion of antipode in Hopf algebras. In the non-connected case it expresses an inversion principle of more limited scope, but still sufficient to compute the Möbius function as μ = ζ ◦ S, just as in Hopf algebras. At the level of decomposition spaces, the weak antipode takes the form of a formal difference of linear endofunctors S_even - S_odd, and it is a refinement of the general Möbius inversion construction of Gálvez--Kock--Tonks, but exploiting the monoidal structure.
- Espai de descomposició; Espacio de descomposición; Decomposition space; Combinatòria objectiva; Combinatoria objetiva; Objective combinatorics; Categories d'ordre infinit; Infinito categorias; Higher category theory