Comparing the non-unital and unital settings for directed homotopy

TL;DR

This paper compares unital and non-unital frameworks in directed homotopy, analyzing model structures of flows and enriched categories, revealing limitations of certain adjunctions and characterizing minimal models.

Contribution

It establishes relationships between different model structures for flows and categories, highlighting the non-liftability of the Ilias structure and constructing a minimal model with specific properties.

Findings

The Ilias model structure cannot be left-lifted along the identity-adding functor.

A minimal model structure on flows has a homotopy category of a 3-element chain.

The q-model structure of flows can be right-lifted to enriched categories with weak equivalences inducing fundamental category equivalences.

Abstract

This note explores the link between the q-model structure of flows and the Ilias model structure of topologically enriched small categories. Both have weak equivalences which induce equivalences of fundamental (semi)categories. The Ilias model structure cannot be left-lifted along the left adjoint adding identity maps. The minimal model structure on flows having as cofibrations the left-lifting of the cofibrations of the Ilias model structure has a homotopy category equal to the $3$-element totally ordered set. The q-model structure of flows can be right-lifted to a q-model structure of topologically enriched small categories which is minimal and such that the weak equivalences induce equivalences of fundamental categories. The identity functor of topologically enriched small categories is neither a left Quillen adjoint nor a right Quillen adjoint between the q-model structure and the Ilias model structure.