In a category with interval object one has for every object a notion of paths in . (Indeed, the very definition of category with interval object is meant to guarantee that for every object there is its fundamental category in the Trimblean sense).
The idea is that if for all these paths in there is a reverse path, then the object is undirected or groupoidal. Otherwise it is strictly directed (not to be confused with a directed object).
An object in is called undirected or an undirected object with respect to , – or -undirected – if this morphism is a weak equivalence:
otherwise it is called strictly directed or a strictly directed object with respect to , or strictly -directed.
Once we have a closed monoidal homotopical structure on the category of directed topological spaces, it should be true that in with the standard directed interval, an object is undirected precisely if it contains no nontrivial directed path.
Whether or not an object is undirected depends on the choice of interval object.
For instance the terminal object itself is always a an interval object, , but in a trivial way. Every object is -undirected.
A bit more generally, the interval object may be not equal to the terminal object, but still be weakly equivalent to it. For instance the standard (undirected) topological interval is not equal to but is weakly equivalent to the point in the standard model structure on topological spaces. This implies in particular that with respect to the standard undirected topological interval even a directed topological space should undirected. (Well, we still need to specify the closed structure on to make this true…) It’s only the standard directed topological interval which can detect the directedness of a strictly directed topological space.
Analogous statements are true in the example of strict omega-categories, where the analogue of the standard directed topological interval is the 1-globe , while the example of the standard undirected topological interval is the groupoid version of this, , where the morphism from to is an isomorphism. As opposed to this is weakly equivalent to the terminal category (the 0-globe) (with respect to the folk model structure). It should be true that every strict -category is -undirected, even if it is strictly -directed.