The notion of regular directed complex, introduced by Hadzihasanovic, is a notion of pasting diagram shape. There, a pasting diagram is encoded as a kind of “directed cell complex”, that is, a cell complex together with an orientation on each cell, which divides its boundary into two halves: an input half and an output half.
In this framework, a pasting diagram will be encoded as an oriented graded poset (Def. ), which is a poset, whose covering diagram is graded, and each edge possess an orientation. To see this consider the following pasting diagram (referred in this page as the running pasting diagram) and the covering diagram associated to it
Of course, not all oriented graded posets will lead to a well-formed pasting diagrams: before defining the one that will count, we introduce some terminology.
(oriented graded poset)
A graded poset is a poset together with a rank function such that
An oriented graded poset is a graded poset together with a sign for each edge in its covering diagram.
The grading function of an oriented graded poset defines a notion of dimension of an element. For instance, in the running pasting diagram, the dimensions of are repectively . If is an oriented graded poset, we write for its elements of dimension . We speak also of the dimension of an oriented graded poset to mean the maximal of the dimension of its elements.
(faces and cofaces)
Let be an oriented graded poset. For all in , and (resp. ), we define the input (resp. output) faces
Dually, we define the input (reps. output) cofaces
(
With that, we can define a morphism of oriented graded posets to be a function of the underlying sets such that, for all and , induces a bijection between and .
Oriented graded posets and morphisms form the category .
Morphisms of oriented graded posets are order-preserving, closed and dimension-preserving.
(maximal element)
An element of an oriented graded poset is maximal if for , , that is, is maximal if it is covered by no element.
(boundaries)
Let be a closed subset of an oriented graded poset, let (resp. ), and let , let
The input (reps. output) -boundary is the closed subset
In the running pasting diagram, calling it , we have
and
We write
etc… The general convention is that if the superscript is not provided to an operator, it is the union of the operator with , and with .
Similarly, if the subscript is not provided, it means “the dimension of the oriented graded poset minus 1”, that is, we write:
We can also combine both notations, for instance
Last, if the index is negative, the result is the empty set, for instance,
etc.
For any , , , etc.. are subsets of , and the inclusion they defines
are all morphisms of oriented graded posets.
The last bit of terminology we introduce is roundness. Consider a topological -ball , and its boundary the -sphere . The latter is made of the gluing of two -balls and along an -sphere , in particular:
Similarly, is an -sphere, which is made of a copie of two -balls which intersect at an -sphere, etc.. We will say that an oriented graded poset is round if it satisfies similar intersection conditions.
(round)
Let be an oriented graded poset of dimension . We say that is round if for all , we have
We can now construct inductively a well-formed class of oriented graded posets, called the molecules, which model composable pasting diagrams. There are 3 constructors:
Then is a molecule.
Then, we add a top element to with orientation such that and . This defines , which is a molecule.
If we take two copies of (Point) and apply (Atom) to them, we obtain the arrow : Then, we can use (Paste) with and on the 0-boundary to obtain :
Now we use (Atom) to construct : (which is also the second oriental).
(atom)
An atom is a molecule with a maximal element.
If a molecule is an atom, then was produced by either (Point) or (Atom).
(regular directed complex)
A regular directed complex is an oriented graded poset such that for all in , is an atom.
Of course, every molecule is a regular directed complex (see Lemma ), but for instance, is a regular directed complex but not a molecule.
It may seem that the above construction of molecules depends on the choice of an isomorphism during the construction. However we prove already:
Isomorphisms of molecules are unique.
See (Corollary 3.4.12)
We will write to mean that there exists a unique isomorphism between and .
The underlying poset of any regular directed complex is oriented thin. This means that any interval in with contains exactly 4 elements , with orientation satisfying
The above Proposition is still still true when considering the augmentation of (see Def. ). This additional result translates into the fact that any 1-dimensional element in has exactly one negative face and one positive face.
Common properties of molecules include:
Let be a molecule. Then
The category of molecule, with pastings and boundaries satisfies all the axioms of strict -categories. More precisely, we have the following results.
(globularity)
Any molecule is globular, that is, for all and all , we have
(boundaries)
Let be molecules such that is defined, let and , then
(unitality)
Let be a molecule of dimension , then for all , we have
Notice that in the constructor (Paste), we require that for to be defined. However, as long as , the pushout in (Paste) makes sense, so we slightly overload the notation.
(associativity)
Let be molecules such that or is defined, then
(exchange)
Let be molecules, let such that is defined, then
The set of all molecules is a strict -category, with
In fact, we can do better, for an oriented graded poset, we let to be the category whose objects are (the isomorphism classes of) morphisms , for a molecule. Precomposing by defines , while if is in , such that , then the universal property of the pushout defines a unique element in . Hence:
For all oriented graded poset , is a strict -category. Furthermore, any morphism induces a strict -functor
sending to . This defines a functor .
(submolecule inclusion) The class of submolecule inclusions is the smallest subclass of inclusions of molecules containing all isomorphisms, closed under composition, and for all molecules such that is defined, both and are submolecule inclusions.
Not all inclusions of molecule are submolecule inclusion, but notably, we have the following:
Let be two molecules such that . We write to signify that the canonical inclusion is a submolecule inclusion. For instance, by the above Proposition, for all , .
(rewritable submolecule)
Let be a molecule. A submolecule is rewritable if is round (Def. ) and .
Rewritable submolecule can be substituted: if is rewritable and if is another molecule such that is defined, then we can “replace” by inside , whose result we write . We can construct this formally in two steps: first we take the following pushout where is the submolecule inclusion . Second, can be interpreted as a higher-dimensional rewrite rule that matches in and transforms it into , so we define the substition to be the “result” of this rewrite rule, i.e.:
The choice of terminology “substitution” and “rewriting”, is more than a mere analogy: rewritable submolecules and their interpretations for rewriting system are discussed in there.
(layering)
Let be a molecule, let , and
A -layering of is a sequence such that
and .
For instance let be (i.e. ) and be , then the molecule has no -1-layering, one 0-layering
and two 1-layerings: calling and , then the molecule is both equal to and .
The running pasting diagram has only on 0-layering.
(layering dimension)
Let be a molecule, the layering dimension of is the integer
The main theorem about layerings is as follows:
Let be a molecule, . Then admits a -layering.
Other results include:
Let be a molecule, and let be a -layering of . Then
Furthermore, if is another molecule such that is defined, then
Regular directed complexes are closed under many relevant operations used for higher category theory.
(Gray product)
Let be oriented graded posets. The Gray product of and is the oriented graded poset whose
The Gray product is
Let be regular directed complexes. Then is a regular directed complex.
The proof is extremely combinatorial, and can be found in Section 7.2.
The Gray product is associtive and has unit the point . It is not symmetrical.
Similarly, we can take the join of regular directed complexes. If the underlying poset of a join is easy to describe, describing the precise orientation can be quite cumbersome; since we have the Gray product, we can take the following shortcut.
(augmentation and diminution)
Let be an oriented graded poset.
(join)
Let be an oriented graded poset. The join of and is the oriented graded poset
The underlying poset of has elements
Let be regular directed complexes. Then is a regular directed complex.
The join is associtive and has unit , the empty regular directed complex. It is not symmetrical.
The -th iterated join of the point is the -th oriental.
(suspension)
Let be an oriented graded poset. The suspension of is the oriented graded poset whose
for all and .
Let be a regular directed complex. Then is a regular directed complex.
Let be molecules.
The -th iterated suspension is the -th globe.
Since every regular directed complex is in particular a poset, we can take its nerve, which is a simplicial set , whose -simplices are chains in . Via the classical geometric realization of simplicial sets, we define a functor from regular directed complexes to (a convenient category of) topological spaces.
As expected, round molecules are round:
Furthermore, Gray products, joins, and suspensions are sent to their topological counterpart:
Let be regular directed complexes, then
Since molecules are defined inductively, we can use structural induction to prove statement about them. To prove that a statement is true for all molecules, we can do as follows:
For instance:
Any molecule is a regular directed complex.
Let be a molecule, we need to prove that for all , is an atom. We proceed by induction on the construction of . If was produced by (Point), then must be the unique element of whose closure is itself. If was produced by (Paste), then , and or ; the inductive hypothesis applies. If was produced by (Atom), it is equal to , and either or , in which case the inductive hypothesis applies, or , and is an atom by definition.
The proof by induction on submolecules allows us to prove a statement for all molecules as follows: assume that a molecule satisfies , then
This induction principle is justified since every proper submolecule of has stricly fewer elements than , and the set of minimal elements for is presicely .
The proof that molecules are stable under Gray product uses induction on submolecules.
Another proof technique is the induction on the layering dimension: to prove that a property holds for all molecules U, it suffices to
Indeed, by Theorem , any molecule has a layering, and Proposition gives the well-foundedness.
For instance, let us prove the following:
Every molecule is made of pasting of atoms.
Let be a molecule, we proceed by induction on . If , then this follows by definition since is an atom. Let , and take a -layering of
with . Then by induction, each is a composition of atoms, thus so is .
The proof works equally well by structural induction.
The main reference on regular directed complexes:
For a computational perspective on regular directed complexes:
Last revised on August 8, 2024 at 11:09:35. See the history of this page for a list of all contributions to it.