Derivations in Codifferential Categories

Abstract: Derivations provide a way of transporting ideas from the calculus of manifolds to algebraic settings where there is no sensible notion of limit. In this paper, we consider derivations in certain monoidal categories, called codifferential categories. Differential categories were introduced as the categorical framework for modelling differential linear logic. The deriving transform of a differential category, which models the differentiation inference rule, is a derivation in the dual category. We here explore that derivation's universality. One of the key structures associated to a codifferential category is an algebra modality. This is a monad $T$ such that each object of the form $TC$ is canonically an associative, commutative algebra. Consequently, every $T$-algebra has a canonical commutative algebra structure, and we show that universal derivations for these algebras can be constructed quite generally. It is a standard result that there is a bijection between derivations from an associative algebra $A$ to an $A$-module $M$ and algebra homomorphisms over $A$ from $A$ to $A\oplus M$, with $A\oplus M$ being considered as an infinitesimal extension of $A$. We lift this correspondence to our setting by showing that in a codifferential category there is a canonical $T$-algebra structure on $A\oplus M$. We call $T$-algebra morphisms from $TA$ to this $T$-algebra structure Beck $T$-derivations. This yields a novel, generalized notion of derivation. The remainder of the paper is devoted to exploring consequences of that definition. Along the way, we prove that the symmetric algebra construction in any suitable symmetric monoidal category provides an example of codifferential structure, and using this, we give an alternative definition for differential and codifferential categories.