The formal theory of tangentads

Abstract: Tangent categories offer a categorical context for differential geometry, by categorifying geometric notions like the tangent bundle functor, vector fields, Euclidean spaces, vector bundles, connections, etc. In the last decade, the theory has been extended in new directions, providing concepts such as tangent monads, tangent fibrations, tangent restriction categories, reverse tangent categories and many more. It is natural to wonder how these new flavours interact with the geometric constructions offered by the theory. How does a tangent monad or a tangent fibration lift to the tangent category of vector fields of a tangent category? What is the correct notion of vector bundles for a tangent restriction category? In this paper, we answer these questions by adopting the formal approach of tangentads. Introduced in a previous paper, tangentads proved to be a powerful tool for capturing the different flavours of the theory and for extending constructions like the Grothendieck construction or the equivalence between split restriction categories and $\mathscr{M}$-categories, to the tangent-categorical context. We construct formal notions of vector fields, differential objects, differential bundles, and connections for tangentads by isolating the correct universal properties enjoyed by these constructions in ordinary tangent categories. We show that vector fields form a Lie algebra and a $2$-monad; we extend the result that differential bundles over the terminal object are equivalent to differential objects; we recapture the notions of covariant derivative, curvature and torsion for connections. We also show how to construct vector fields and connections using PIE limits, and we compute the formal constructions for some examples of tangentads.