Normalizing the Taylor expansion of non-deterministic {\lambda}-terms, via parallel reduction of resource vectors

It has been known since Ehrhard and Regnier’s seminal work on the Taylor expansion of λ-terms that this operation commutes with normalization: the expansion of a λ-term is always normalizable and its normal form is the expansion of the Böhm tree of the term. We generalize this result to the non-uniform setting of the algebraic λ-calculus, i.e., λ-calculus extended with linear combinations of terms. This requires us to tackle two difficulties: foremost is the fact that Ehrhard and Regnier’s techniques rely heavily on the uniform, deterministic nature of the ordinary λ-calculus, and thus cannot be adapted; second is the absence of any satisfactory generic extension of the notion of Böhm tree in presence of quantitative non-determinism, which is reflected by the fact that the Taylor expansion of an algebraic λ-term is not always normalizable. Our solution is to provide a fine grained study of the dynamics of β-reduction under Taylor expansion, by introducing a notion of reduction on resource vectors, i.e. infinite linear combinations of resource λ-terms. The latter form the multilinear fragment of the differential λ-calculus, and resource vectors are the target of the Taylor expansion of λterms. We show the reduction of resource vectors contains the image of any β-reduction step, from which we deduce that Taylor expansion and normalization commute on the nose. We moreover identify a class of algebraic λ-terms, encompassing both normalizable algebraic λ-terms and arbitrary ordinary λ-terms: the expansion of these is always normalizable, which guides the definition of a generalization of Böhm trees to this setting.