We give a model of restricted L_infinity-algebras in a nice preadditive symmetric monoidal infinity-category C as an algebra over the monad associated to an adjunction between C and the infinity-category of cocommutative bialgebras in C, where the left adjoint lifts the free associative algebra. If C is additive, we construct a canonical forgetful functor from restricted L_infinity-algebras in C to spectral Lie algebras in C and show that this functor is an equivalence if C is a Q-linear stable infinity-category. For every field K we construct a canonical forgetful functor from restricted L_infinity-algebras in connective K-module spectra to the infinity-category underlying a model structure on simplicial restricted Lie algebras over K.

In this dissertation we define an analogue, in the setting of infinity categories, of the classical notion of an enriched operad. We introduce six different models of enriched infinity operads. In particular, we generalize the operator category approach of Clark Barwick to the enriched setting as well as Moerdijk-Weiss' notion of dendroidal sets. The main part of the thesis consists of the comparison between different approaches to enriched operads.

We explore motivic homotopy theory over deeper bases than the spectrum of the integers: Starting from a commutative group object in a cartesian closed presentable infinity category, replacing the usual multiplicative group scheme in motivic spaces, we construct projective spaces, and show that infinite dimensional projective space is the classifying space of the group object. After passage to the stabilization, we construct a Snaith spectrum, calculate the cohomology represented by it for projective spaces and on its rationalization produce Adams operations and a splitting into summands of their eigenspaces.