Generalized dessins — Part 1: the definition

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Quick navigation: Part 1 (current) — Part 2Part 3 — to be continued…

Grothendieck’s initial wonder for the action of GQ=Gal(Q¯Q) on étale covers of the projective line minus three points comes from the “childlike” combinatorial description offered by the theory of dessins d’enfants. This theory, although elementary, is rich enough to be acted upon faithfully by GQ, as a consequence of Belyĭ’s theorem. In this blog post, I suggest a generalization of this elementary description for covers of the line branched at more than three points (dessins d’ados?).

We fix an integer n2, which serves as the number of colors used for the vertices (spoiler alert: this will be the number of branch points minus one, so n=2 is the case of dessins d’enfants). We use the following definition of a cyclical order (always on a finite set):

Definition (cyclical order): A cyclical order on a finite set X is a permutation of X consisting of a single cycle of length |X|. If xX, we call successor of x in the cyclical order the image of x by the corresponding permutation.

the category of dessins

Definition (n-dessin): An n-dessin consists of the following data:

  • For each color i{1,,n}, a finite set Vi, whose elements are called vertices of color i.
  • A finite set E, whose elements are called hyperedges, and n surjective maps si:EVi. If eE is a hyperedge, si(e) is called the vertex of e of color i.
  • For each color i{1,,n} and each vertex vVi, a cyclical order on the subset si1(v) of E (consisting of hyperedges whose vertex of color i is v).

The case n=2 corresponds to dessins d’enfants, for which hyperedges are simply edges.


We now give an example of a 3-dessin. This will be our go-to example to illustrate all later notions:

Example: We denote by D0 the 3-dessin defined by the following data:

  • V1={r,r}, V2={g,g}, V3={b,b}, E={1,2,3,4}.
  • The maps si are given by the following table:
  1 2 3 4
s1 r r r r
s2 g g g g
s3 b b b b
  • The cyclical order on hyperedges having g as their vertex of color 2 is (1,2,3). 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 D=(Vi,E,si) and D=(Vi,E,si) be two n-dessins. A morphism DD is a collection of maps Σi:ViVi and Φ:EE such that:

  • for every color i{1,,n}, we have Σisi=siΦ, i.e. the following diagram commutes:

    EsiViΦΣiEsiVi
  • for every vertex vVi and for every hyperedge e(si)1(x), the map Φ maps the successor of e in the cyclical order on (si)1(v) to the successor of Φ(e) in the cyclical order on si1(Σi(v)). This implies that (si)1(v) is obtained by “assembling” a certain (integer) number of copies of si1(Σi(v)), one after another.

When α is a morphism of dessins DD and v is a vertex (resp. e is a hyperedge) of D, we write α(v) (resp. α(e)) instead of Σi(v) (resp. Φ(e)).

These two notions define a category Dessinsn of n-dessins.


Definition (degree of a dessin): The degree of an n-dessin D is the number of hyperedges |E|.

Definition (degree of a vertex): The degree deg(v) of a vertex vVi is the number of hyperedges whose vertex of color i is v. In other words, deg(v)=|si1(v)|.

Proposition: Let α:DD be a morphism of dessins and v be a vertex of D. Then, deg(v) is a multiple of deg(α(v)).

Definition (index of a color): The index of the color i in an n-dessin D is the nonnegative integer:

indD(i)=|E||Vi|=vVi(deg(v)1).

drawing a dessin

To draw a dessin, we adopt the following conventions:

  • If n=2, vertices of V1 are represented by black disks, and vertices of V2 by white disks. If n3, the colors used for vertices are, in that order: red, green, blue, brown.
  • The interior of hyperedges is always colored yellow. If n=2, (hyper)edges are either represented as standard, one-dimensional edges, or as “digons”:

    Digon

    If n3, hyperedges are drawn as n-gons whose edges are not necessarily straight, or equivalently as embeddings of the disk with n marked points on its boundary, having a single marked point for each color i (given by the map si). Importantly, we require that the colors of the marked points appear in the cyclical order (1,2,,n) when the boundary of the n-gon/disk is travelled counterclockwise.

  • Let v be a vertex. The cyclical order on hyperedges having v as a vertex is the order in which these hyperedges appear when one rotates counterclockwise around the vertex v.

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 n-partite hypergraphs into compact oriented surfaces satisfying the conditions above, identified each time they are related by a direct homeomorphism of the surface.

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 D0 defined earlier:

An example of 3-dessin

As you can see, there is an (unavoidable) overlap between hyperedges 2 and 4. This illustrates the fact that this dessin can be embedded on the surface of a torus (you can imagine drawing this picture on the surface of a coffee cup: hyperedge 2 is drawn on the handle, hyperedge is drawn “below” the handle), but not on a genus 0 surface like a sheet of paper or a computer screen…

monodromy

Definition (monodromy elements): Let D=(Vi,E,si) be an n-dessin. For every integer i{1,,n}, the i-th monodromy element of D is the permutation σiSE of the hyperedges defined in the following way: σi is the product of as many cycles with disjoint supports as there are vertices in Vi, and for each vertex vVi, the corresponding cycle is the permutation of si1(v) associated to the cyclical order.

The degree of a vertex vVi is equal to the size of the cycle of σi associated to v. The order of σi is equal to the least common multiples of all degrees of vertices of color i.

Definition (monodromy group): Let D=(Vi,E,si) be an n-dessin. The monodromy group D of D is the subgroup of SE generated by the monodromy elements σ1,,σn.

We also define the (n+1)-th monodromy element σn+1 as the element (σ1σn)1D. This way, we have the useful identity:

σ1σnσn+1=1.

Example: The 3-dessin D0 described earlier has monodromy elements σ1=(12)(34), σ2=(132), σ3=(14)(23), σ4=(134), and its monodromy group D0 is the alternating group A4.


The definition of morphisms of dessins directly implies:

Lemma: Let α:DD be a morphism of n-dessins, i{1,,n} be a color, and eE be a hyperedge of D. Then α(σi.e)=σi.α(e). In particular, if D is a dessin, the action of the monodromy group D on hyperedges commutes with the action of the automorphism group of D on hyperedges.

dessins and branched covers of the line

configurations and bouquets

Denote by PConfn+2(C) the set of ordered lists of n+2 distinct points of the Riemann sphere P1(C), which we call configurations. Fix a configuration (t0,t1,,tn,tn+1)PConfn+2(C). Let X denote the pointed topological space (P1(C){t1,,tn+1},t0).

Definition (bouquet): A bouquet of X is a set of loops γ1,,γn+1 based at t0 satisfying the following conditions:

  • for each i{1,,n+1}, γi is homotopic to the positively oriented generator of π1(P1(C){ti},t0)Z when the points tj,ji are put back in,
  • for each i{1,,n+1}, γi is homotopic to the trivial loop 1π1(P1(C){tj,ji},t0) when the point ti is put back in,
  • the trajectories of the loops γi and γj do not intersect (except at the point t0 at the beginning and end) when ij,
  • the cyclical order in which one meets these loops when one rotates positively around the point t0 is (γ1,,γn+1).

Under these conditions, the loops γ1,,γn generate π1(X) freely, and these loops satisfy the relation γ1γn+1=1. We denote the set of bouquets of X by Isom+(π1(X),Fn).

dessins and covers

Fix a bouquet γ1,,γn+1, which we see as a (very particular) isomorphism of groups γ:π1(X)Fn. This isomorphism induces an equivalence of categories between the category of finite étale covers of X, classically equivalent to the category of finite π1(X)-sets, and that of finite Fn-sets (finite sets equipped with an action of the free group on n-letters).

Now, if D is a dessin, the morphism φ:FnSE which maps the i-th letter to the i-th monodromy element of D defines a π1(X)-set. This construction actually defines a functor DessinsnFn-sets (exercise: define the image of morphisms of dessins). In the other direction, if one has an Fn-sets incarnated by a group morphism FnSE for some finite set E, we can define an n-dessin in the following way:

  • the set Vi of vertices of color i is the set of cycles (including fixed points) in the image in SE of the i-th letter (generator of Fn),
  • the cyclical order on hyperedges (elements of E) whose vertex of color i are a given vVi is given by the cycle v itself.

In other words, an n-dessin whose set of hyperedges is E is characterized by the n-tuple (σ1,,σn) of elements of SE (image of the letters under the morphism φ), or equivalently by the (n+1)-tuple (σ1,,σn,σn+1) of elements satisfying the additional property that σ1σn+1=1.

These two inverse constructions define an equivalence of categories between n-dessins and Fn-sets. This means that for each choice of configuration and bouquet, we have:

Theorem: The category Dessinsn of n-dessins is equivalent to the category of finite unramified covers of the Riemann sphere punctured at n+1 given points.

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 PC1, unramified outside the points t1,,tn+1, or to the category of field extensions of C(T) which are unramified outside n+1 places.

The degree of the cover corresponding to a given dessin is the number of hyperedges of that dessin.

graphical interpretation

The relation between n-dessins and covers of the n+1-punctured Riemann sphere can be interpreted graphically. To this end, one draws a yellow disk on the Riemann sphere, not containing the point tn+1, on the boundary of which one marks n points t1,,tn, appearing in this order (or a circular permutation thereof) when one travels counterclockwise along the boundary.

Disk with n marked points

Consider a finite cover of P1(C), unramified outside t1,,tn+1. The hyperedges of the corresponding n-dessin are the inverse images of the yellow disk. The red vertices are the inverse images of the point t1, etc. The cyclical order on the hyperedges with a given vertex is seen in the following way: when one keep rotating counterclockwise around the green point t2 (following the loop γ2 multiple times), one crosses the yellow disk at each new loop. Look at the trajectory of a lift of these rotations through the covering map: you see a rotating path around a green vertex v, which intersects different yellow disks (having v as their green vertex) until it eventually loops. The order of appearances of these yellow disks during the full loop is the cyclical order on the set of hyperedges whose green vertex is v.

an off-topic question

The following fact was told to me by Olivier Benoist:

Theorem: A curve that has a finite morphism to PC1, generically étale and unramified outside four points, is necessarily defined over a finite extension of Q(T).

This is a generalization of the “easy part” of Belyĭ’s theorem. For the proof, we denote by Q¯T the algebraic closure of Q(T) (MathJax messes the formatting of \overline{\Q(T)} for some reason)

Proof: Using a homography, one can assume that the four branch points are 0,1,,λ for some λ (deduced from the cross-ratio of the original four branch points).

  • If λ is algebraic, extension of scalars induces an isomorphism:

    π1ét(PC1{0,1,,λ})π1ét(PQ¯1{0,1,,λ}).

    So our cover, and hence the curve, is defined over a number field.

  • If λ is transcendental, choose an embedding of Q¯T into C which maps T to λ. As before, extension of scalars (from Q¯T to C) induces an isomorphism:

    π1ét(PC1{0,1,,λ})π1ét(PQ¯T1{0,1,,T}).

    Hence our cover comes from a cover of PQ¯T1 via extension of scalars, and hence the curve is defined over Q¯T. ■

More generally, the transcendance degree (over Q) of the field of definition of a cover of PC1 is at most equal to the transcendance degree of the Q-algebra generated by all cross-ratios between the branch points.

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.