Tangent Categories and their Applications

One of the most fundamental notions when studying functions of a real variable is the rate of change of a function, as measured by its derivative. Geometrically, the derivative is the slope of the tangent line. Both the notion of the derivative and the tangent line (or more generally, the tangent bundle of a smooth manifold) can be defined purely axiomatically because of the underlying structure of the category of smooth functions. The derivative, for example, is determined by its properties (such as the sum and product formulae for differentiation, the chain rule, etc.). This structural approach to the derivative leads to the notion of a differential category. When working with models of differential categories such as smooth functions, the listed properties seem like natural consequence of the model. On the contrary, the differential category structure shows that these properties determine the structure of the derivative, rather than the other way around.