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July  2022, 42(7): 3465-3538. doi: 10.3934/dcds.2022022

## A combinatorial approach to Rauzy-type dynamics II: The labelling method and a second proof of the KZB classification theorem

 Université Paris-Saclay, France

Received  February 2018 Published  July 2022 Early access  March 2022

Rauzy-type dynamics are group actions on a collection of combinatorial objects. The first and best known example (the Rauzy dynamics) concerns an action on permutations, associated to interval exchange transformations (IET) for the Poincaré map on compact orientable translation surfaces. The equivalence classes on the objects induced by the group action have been classified by Kontsevich and Zorich, and by Boissy through methods involving both combinatorics algebraic geometry, topology and dynamical systems. Our precedent paper [5] as well as the one of Fickenscher [8] proposed an ad hoc combinatorial proof of this classification.

However, unlike those two previous combinatorial proofs, we develop in this paper a general method, called the labelling method, which allows one to classify Rauzy-type dynamics in a much more systematic way. We apply the method to the Rauzy dynamics and obtain a third combinatorial proof of the classification. The method is versatile and will be used to classify three other Rauzy-type dynamics in follow-up works.

Another feature of this paper is to introduce an algorithmic method to work with the sign invariant of the Rauzy dynamics. With this method, we can prove most of the identities appearing in the literature so far ([10], [6], [2], [5]...) in an automatic way.

Citation: Quentin De Mourgues. A combinatorial approach to Rauzy-type dynamics II: The labelling method and a second proof of the KZB classification theorem. Discrete and Continuous Dynamical Systems, 2022, 42 (7) : 3465-3538. doi: 10.3934/dcds.2022022
##### References:
 [1] C. Boissy, Classification of Rauzy classes in the moduli space of abelian and quadratic differentials, Discrete Contin. Dyn. Syst., 32 (2012), 3433-3457.  doi: 10.3934/dcds.2012.32.3433. [2] C. Boissy, Labeled Rauzy classes and framed translation surfaces, Annales de l'Institut Fourier, 63 (2013), 547–572. doi: 10.5802/aif.2769. [3] C. Boissy and E. Lanneau, Pseudo-Anosov homeomorphisms on translation surfaces in hyperelliptic components have large entropy, Geom. Funct. Anal., 22 (2012), 74-106.  doi: 10.1007/s00039-012-0152-0. [4] Q. de Mourgues, A Combinatorial Approach to Rauzy-Type Dynamics, Chapter 6, Ph.D thesis, Université Paris 13, 2017, https://hal.archives-ouvertes.fr/tel-02285651. [5] Q. de Mourgues and A. Sportiello, A combinatorial approach to Rauzy-type dynamics I: Permutations and the Kontsevich–Zorich–Boissy classification theorem,, https://arXiv.org/abs/1705.01641. [6] V. Delecroix, Cardinalités des classes de Rauzy, Ann. Inst. Fourier, 63 (2013), 1651-1715.  doi: 10.5802/aif.2811. [7] A. Eskin, H. Masur and A. Zorich, Moduli spaces of abelian differentials: The principal boundary, counting problems, and the Siegel–Veech constants, Publications Mathématiques de l'IHÉS, 97 (2003), 61-179.  doi: 10.1007/s10240-003-0015-1. [8] J. Fickenscher, A combinatorial proof of the Kontsevich–Zorich–Boissy classification of {R}auzy classes, Discrete Contin. Dyn. Syst., 36 (2016), 1983-2025.  doi: 10.3934/dcds.2016.36.1983. [9] R. Gutiérrez-Romo, Classification of Rauzy-Veech groups: Proof of the Zorich conjecture, Invent. Math., 215 (2019), 741–778. eprint, arXiv: 1706.04923 doi: 10.1007/s00222-018-0836-7. [10] M. Kontsevich and A. Zorich, Connected components of the moduli spaces of abelian differentials with prescribed singularities, Inventiones Mathematicæ, 153 (2003), 631-678.  doi: 10.1007/s00222-003-0303-x. [11] E. Lanneau, Connected components of the strata of the moduli spaces of quadratic differentials, Annales Scientifiques de l'École Normale Supérieure, 41 (2008), 1-56.  doi: 10.24033/asens.2062. [12] G. Rauzy, Échanges d'intervalles et transformations induites, Acta Arithmetica, 34 (1979), 315-328.  doi: 10.4064/aa-34-4-315-328. [13] W. A. Veech, Gauss measures for transformations on the space of interval exchange maps, Ann. Math. (2), 115 (1982), 201-242.  doi: 10.2307/1971391.

show all references

##### References:
 [1] C. Boissy, Classification of Rauzy classes in the moduli space of abelian and quadratic differentials, Discrete Contin. Dyn. Syst., 32 (2012), 3433-3457.  doi: 10.3934/dcds.2012.32.3433. [2] C. Boissy, Labeled Rauzy classes and framed translation surfaces, Annales de l'Institut Fourier, 63 (2013), 547–572. doi: 10.5802/aif.2769. [3] C. Boissy and E. Lanneau, Pseudo-Anosov homeomorphisms on translation surfaces in hyperelliptic components have large entropy, Geom. Funct. Anal., 22 (2012), 74-106.  doi: 10.1007/s00039-012-0152-0. [4] Q. de Mourgues, A Combinatorial Approach to Rauzy-Type Dynamics, Chapter 6, Ph.D thesis, Université Paris 13, 2017, https://hal.archives-ouvertes.fr/tel-02285651. [5] Q. de Mourgues and A. Sportiello, A combinatorial approach to Rauzy-type dynamics I: Permutations and the Kontsevich–Zorich–Boissy classification theorem,, https://arXiv.org/abs/1705.01641. [6] V. Delecroix, Cardinalités des classes de Rauzy, Ann. Inst. Fourier, 63 (2013), 1651-1715.  doi: 10.5802/aif.2811. [7] A. Eskin, H. Masur and A. Zorich, Moduli spaces of abelian differentials: The principal boundary, counting problems, and the Siegel–Veech constants, Publications Mathématiques de l'IHÉS, 97 (2003), 61-179.  doi: 10.1007/s10240-003-0015-1. [8] J. Fickenscher, A combinatorial proof of the Kontsevich–Zorich–Boissy classification of {R}auzy classes, Discrete Contin. Dyn. Syst., 36 (2016), 1983-2025.  doi: 10.3934/dcds.2016.36.1983. [9] R. Gutiérrez-Romo, Classification of Rauzy-Veech groups: Proof of the Zorich conjecture, Invent. Math., 215 (2019), 741–778. eprint, arXiv: 1706.04923 doi: 10.1007/s00222-018-0836-7. [10] M. Kontsevich and A. Zorich, Connected components of the moduli spaces of abelian differentials with prescribed singularities, Inventiones Mathematicæ, 153 (2003), 631-678.  doi: 10.1007/s00222-003-0303-x. [11] E. Lanneau, Connected components of the strata of the moduli spaces of quadratic differentials, Annales Scientifiques de l'École Normale Supérieure, 41 (2008), 1-56.  doi: 10.24033/asens.2062. [12] G. Rauzy, Échanges d'intervalles et transformations induites, Acta Arithmetica, 34 (1979), 315-328.  doi: 10.4064/aa-34-4-315-328. [13] W. A. Veech, Gauss measures for transformations on the space of interval exchange maps, Ann. Math. (2), 115 (1982), 201-242.  doi: 10.2307/1971391.
Let $x\in X_n$ be a combinatorial structure on $n$ vertices. $A_x$ choose a permutation $\sigma$ depending on $x$ and permutes the vertices of $x$ according to it. We represent this symmetric-group action as a diagrammatic action from below
$S'$ is a boosted sequence of $S$ for $(x, c)$, because $\text{red} \circ S'$ gives the same result as $S \circ \text{red}$. Note that $S'$ may be not unique: in the diagram we show a second boosted sequence of $S$ for $(x, c)$, namely $S"$. It is not necessarily the case that $(x', c') : = S'(x, c)$ coincides with $(x", c") : = S"(x, c)$, but merely that their reductions coincide (they must be both $y'$)
A representation of a combinatorial structure $x\in X_n$ with a labelling $\Pi_b.$ The intervals are represented by the bottoms arcs
Outline of the proof of connectivity between $x_1$ and $x_2$, using the labelling method. The sequence $S$ sends $y_1$ to $y_2$, however the intervals containing the gray vertices of $x'_1$ may not be at their correct place, in order to match with those of $x'_2$. The sequence $S_*$ corrects for this. Thus the boosted sequence $S'_* S'$ sends $x'_1$ to $x'_2$, and $x_1$ and $x_2$ are connected
Diagram representations of matchings and permutations, and matrix representation of permutations
Our two main examples of dynamics, in diagram representation. Top: the $\mathcal{M}_n$ case. top: the $\mathcal{S}_n$ case
The Rauzy dynamics concerning permutations, in matrix representation
Left: A reducible permutation in matrix representation; the $\mathcal{S}$ dynamic acts on the first block with $L$ and $R$. Right: A reducible permutation in diagram representation; the $\mathcal{S}$ dynamics acts on the gray part with $L$ and $R$, while leaving the blue part unchanged
Left: an irreducible permutation, $\sigma = [\, 451263\, ]$. Right: the construction of the cycle structure. Different cycles are in different colours. The length of a cycle or path, defined as the number of top (or bottom) arcs, is thus 2 for red and violet, and 1 for blue. The green arrows are the endpoints of the rank path, which is in blue. As a result, in this example $\lambda (\sigma ) = (2, 2)$ (for the cycles of color red and violet), $r(\sigma ) = 1$ (corresponding to the rank path of color blue), and $\ell(\sigma ) = 2$
Left: an example of permutation, $\sigma = [\, 251478396\, ]$. Right: an example of subset $I = \{1, 2, 6, 8, 9\}$ (labels are for the bottom endpoints, edges in $I$ are in blue). There are two crossings, out of the maximal number $\binom{|I|}{2} = 10$, thus $\chi_I = 8$ in this case, and this set contributes $(-1)^{|I|+\chi_I} = (-1)^{5+8} = -1$ to $A(\sigma )$
Two consistent labellings $(\Pi_b, \Pi_t)$ and $(\Pi'_b, \Pi'_t)$ of a permutation $\sigma$ with cycle invariant $(\{2, 2, 2\}, 2)$. Following definition 26 we have $(\Pi'_b, \Pi'_t) = \mathrm{Sh}^1_{2, 1}((\Pi_b, \Pi_t))$
The action on the dynamics (with an $L$ operator) on a consistent labelling
The proof that $\Pi'_b$ verifies property 2 and $(\Pi'_b, \Pi'_t)$ verifies property 3 for the case $\alpha\leq \sigma (1)$
The proof that $\Pi'_b$ verifies property 2 and $(\Pi'_b, \Pi'_t)$ verifies property 3 for the case $\alpha = \sigma (1)+1$
The gray edges of $\sigma$ are transported along the labels of the arcs when applying the boosted dynamics. For instance, $e\in(b, t)$ in $\tau, \Pi$
Illustration that the edges, inserted within the labels of a reduced permutation, follow those labels when applying the boosted dynamic
Left: a schematic representation of a permutation of type $H(r_1, r_2)$. Right: a representation of a permutation of type $X(r, i)$. These configurations have rank $r_1+r_2-1$ and $r$, respectively
The insertion of one edge within the arcs $\alpha$ and $\beta$
The first line represents the case: Top arc : any cycle. Bottom arc: any cycle. The second line represents the case: Top arc : rank path. Bottom arc: principal cycle
The first column represents the permutation $\sigma$ and its the cycle invariant. The second column represents the permutation $\sigma '$ obtained from the insertion of a double-edge within the arcs $\alpha$ and $\beta$ of $\sigma$ and the third column displays and demonstrates the new invariant of $\sigma '$
An example of a shift-irreducible family $S = (\sigma ^i)_{0\leq i \leq n-2}$ with its two unavoidably reducible permutations $d(\sigma ^{n-\sigma (2)})$ and $d(\sigma ^{n-\sigma (n)+1}).$ For a proof of the shift-irreducibility, the characterisation proposition 44 is helpful
The two permutations $\sigma ^{n-\sigma (2)}$ and $\sigma ^{n-\sigma (n)+1}$ are of type $H(1, r)$ and $H(r, 1)$ respectively
The permutation $\sigma _{1, 4, \{(0.1, 0.1), (0.2, 3.1), (0.3, 1.1), (1.1, 4.1), (1.2, 2.1)\}}$. We cannot show the full matrix $Q$ for such a big example, but we can give one row, for the edge which has the label $e$ in the drawing. The row $Q_e$ reads $(Q_e)_{11, 12, \ldots, 15, 21, \ldots, 25} = (1, 1, 0, 0, 0, \, 0, 0, 1, 1, 1)$
From left to right: the permutations $\sigma$, $\sigma |_{1, \alpha, \beta}$ and $\sigma |_{1, \alpha, \beta'}$. The proposition 53 says that $\sigma |_{1, \alpha, \beta}$ and $\sigma |_{1, \alpha, \beta'}$ have same cycle invariant and opposite sign invariant
Left: a permutation $\sigma$. Right : the permutation $\sigma _i(C_p)$
The base cases of the induction of the proof of proposition 58
Left: a permutation with a $C_p$ structure containing two even cycles of even length. Middle and Right: By proposition 53 the two permutations have same cycle invariant and opposite sign
Two families of base permutations with their respective cycle invariant
The permutation resulting from adding a $C_p$ on the last edge of $\sigma ^0$ or one of its successors. It is clearly also $I_2X$
The two constructed $I_2X$-permutations $\sigma _1 = \sigma _{|\sigma |}(C_p)$ and $\sigma '_1 = \sigma _{|\sigma |}(C_{p-(2), 2})$ with invariant $(\lambda , r, \pm s)$. In this case $p = 4k+1$
The scheme to add two different even cycles to a permutation
Two families of $I_2X$-permutations with their respective cycle invariant
The case $\sigma$ has type $X(r, i)$ of lemma 68. We have $\sigma _1 = \tau_{1, \alpha_1 = 1, \beta} = \tau_{1, t^{rk}_0, b^{rk}_{i\!-\!1}}$
The case $\sigma$ has type $H(i, r_2)$ of lemma 69. We have $\sigma _1 = \tau_{1, \alpha_1 = 1, \beta} = \tau_{1, t^{rk}_0, b_{i-1, i\!-\!1, 1}}$
The construction of the proof of the lemma. We have $\sigma _1 = \tau_{1, 1, \beta'}$, $\sigma _1' = \tau_{1, 1, \beta}$ and $\sigma _1 = B(S)(\sigma _1)$ moreover ${\bar A}(\tau) = \pm 2^{\frac{n}{2}}$
The construction of the proof of the lemma 73. Both $\sigma$ and $\sigma _{next}$ have type $X(r, i)$ and ${\bar A}(d(\sigma )) = -{\bar A}(d(\sigma _{next}))$
$(\sigma '_4, \sigma '_2)$ are both permutations with invariant $(\lambda , r, s\neq 0)$ and $\tau_2$ has sign invariant 0. However proposition 19 applied to $\sigma '_4, \sigma '_2$ and $\tau_2$ implies that $\sigma '_4$ should have invariant $-s$, This is contradictory and thus the case $\Pi'''_b(\beta) = b_{0, i-1, 1}$ cannot happen
$\sigma$ and $\sigma '$ are standard of type $X(r, i)$. The labels of the principal cycle of $\sigma$ are send to the arcs of the principal cycle of $\sigma '$ by the sequence $R^{-1}B(S)R$. Indeed they are attached to the $i$th first labels of the rank of $\tau$ and the labels of the rank are fixed. Thus they are also attached to $i$th first the labels of the rank of $\tau'$ which are the arcs of the principal cycle of $\sigma '$.
In the figure, we choose $\Pi_t(\alpha) = t_{0, i, j}$ instead of $t_{c, i, j}$ for some $c$ for space-saving purpose
The case of the cycle 1-shift. We apply proposition 28 on $\sigma '$ and $\sigma$
The case of the cycle jump. We apply proposition 76 on $\sigma '$ and $\sigma$
The case of the cycle 2-shift. We apply proposition 28 on $\sigma '$ and $\sigma$
Description of the structure of configurations in the identity class ${\rm{Id}}_n$, besides the configuration ${\rm{id}}_n$
Cycle, rank and sign invariants of the exceptional classes. The sign $s \in \{-1, 0, +1\}$ is shortened into $\{-, 0, +\}$
List of invariants $(\lambda , r, s)$ for $n \leq 8$, for which the corresponding class in the $\mathcal{S}_n$ dynamics exist. We shorten $s$ to $\{-, +\}$ if valued $\{-1, +1\}$, and omit it if valued 0
Modification to the cycle invariant between $\sigma$ and $\sigma |_{1, 0, i}$, for every possible $i$. In green, the newly-added edge. In red, parts that get added to the rank path. In blue, parts which are singled out to form a new cycle
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