Generalized dessins — Part 1: the definition
Grothendieck’s initial wonder for the action of
We fix an integer
Definition (cyclical order):
A cyclical order on a finite set
the category of dessins
Definition (
- For each color
, a finite set , whose elements are called vertices of color . - A finite set
, whose elements are called hyperedges, and surjective maps . If is a hyperedge, is called the vertex of of color . - For each color
and each vertex , a cyclical order on the subset of (consisting of hyperedges whose vertex of color is ).
The case
We now give an example of a
Example:
We denote by
, , , .- The maps
are given by the following table:
- The cyclical order on hyperedges having
as their vertex of color is . All other cyclical orders are forced because they are on sets of size two or one.
We now define morphisms between dessins:
Definition (morphism of dessins):
Let
-
for every color
, we have , i.e. the following diagram commutes: -
for every vertex
and for every hyperedge , the map maps the successor of in the cyclical order on to the successor of in the cyclical order on . This implies that is obtained by “assembling” a certain (integer) number of copies of , one after another.
When
These two notions define a category
Definition (degree of a dessin):
The degree of an
Definition (degree of a vertex):
The degree
Proposition:
Let
Definition (index of a color):
The index of the color
drawing a dessin
To draw a dessin, we adopt the following conventions:
- If
, vertices of are represented by black disks, and vertices of by white disks. If , the colors used for vertices are, in that order: red, green, blue, brown. -
The interior of hyperedges is always colored yellow. If
, (hyper)edges are either represented as standard, one-dimensional edges, or as “digons”:If
, hyperedges are drawn as -gons whose edges are not necessarily straight, or equivalently as embeddings of the disk with marked points on its boundary, having a single marked point for each color (given by the map ). Importantly, we require that the colors of the marked points appear in the cyclical order when the boundary of the -gon/disk is travelled counterclockwise. - Let
be a vertex. The cyclical order on hyperedges having as a vertex is the order in which these hyperedges appear when one rotates counterclockwise around the vertex .
These constraints define a notion of “good embeddings” of a dessin on a compact oriented surface.
If one moreover requires that the complement of the image of this embedding be a disjoint union of disks (to avoid “unnecessary genus”), these constraints rigidify the surface and the embedding if we identify two embeddings which are related by a direct homeomorphism between the surfaces.
In other words, dessins (which we have defined abstractly) also admit an equivalent (slightly more geometrical) definition as embeddings of
Because of these conventions, dessins can not always be drawn on paper (or, in this case, on your computer screen): because of this, we will have to draw some overlaps (an hyperedge passes “above” another), in which case we use the same conventions as when drawings e.g. knots or braids.
Example:
We draw the dessin
As you can see, there is an (unavoidable) overlap between hyperedges
monodromy
Definition (monodromy elements):
Let
The degree of a vertex
Definition (monodromy group):
Let
We also define the
Example:
The
The definition of morphisms of dessins directly implies:
Lemma:
Let
dessins and branched covers of the line
configurations and bouquets
Denote by
Definition (bouquet):
A bouquet of
- for each
, is homotopic to the positively oriented generator of when the points are put back in, - for each
, is homotopic to the trivial loop when the point is put back in, - the trajectories of the loops
and do not intersect (except at the point at the beginning and end) when , - the cyclical order in which one meets these loops when one rotates positively around the point
is .
Under these conditions, the loops
dessins and covers
Fix a bouquet
Now, if
- the set
of vertices of color is the set of cycles (including fixed points) in the image in of the -th letter (generator of ), - the cyclical order on hyperedges (elements of
) whose vertex of color are a given is given by the cycle itself.
In other words, an
These two inverse constructions define an equivalence of categories between
Theorem:
The category
By Riemann’s existence theorem, the latter category is itself equivalent to the category of dominant and generically étale finite maps from a smooth curve to
The degree of the cover corresponding to a given dessin is the number of hyperedges of that dessin.
graphical interpretation
The relation between
Consider a finite cover of
an off-topic question
The following fact was told to me by Olivier Benoist:
Theorem:
A curve that has a finite morphism to
This is a generalization of the “easy part” of Belyĭ’s theorem.
For the proof, we denote by \overline{\Q(T)}
for some reason)
Proof:
Using a homography, one can assume that the four branch points are
-
If
is algebraic, extension of scalars induces an isomorphism:So our cover, and hence the curve, is defined over a number field.
-
If
is transcendental, choose an embedding of into which maps to . As before, extension of scalars (from to ) induces an isomorphism:Hence our cover comes from a cover of
via extension of scalars, and hence the curve is defined over . ■
More generally, the transcendance degree (over
Question: Is the theorem above an equivalence? i.e., can we generalize Belyĭ’s theorem to more general situations in this way?
My guess is that the answer is no.
In the next blog post, we will discuss the notions of connectedness and regularity of a dessin. Our ultimate goal is to describe “as combinatorially as possible” the action(s) of the absolute Galois group on these generalized dessins.