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**Growth of the number of simple closed geodesics on hyperbolic surfaces.**
*(English)*
Zbl 1177.37036

Let \(X\) be a complete hyperbolic Riemann surface of finite area, genus \(g\), with \(n\) cusps, and let \(\gamma_0\) be a simple closed geodesic on \(X\). Let then \(s(L)\) (respectively, \(s_X(L,\gamma_0)\)) denote the number of simple closed geodesics \(\gamma\) of length \(\leq L\) (respectively, having the same type as \(\gamma_0\), i.e. such that \(X-\gamma\) be homeomorphic to \(X\setminus\gamma_0\)).

We call a multi-curve on \(X\) any formal linear combination \(\gamma:= \sum^k_{i=1} a_i\gamma_i\), with positive rational coefficients \(a_i\), of disjoint, essential, non-peripheral, simple closed curves \(\gamma_i\), which belong to distinct homotopy classes. The length of such \(\gamma\) is defined by \[ \ell_X(\gamma):= \sum^k_{i=1} a_i\ell_X(\gamma_i). \] Then the main result is that, for any multicurve \(\gamma\), \(\lim_{L\to\infty} s_X(L,\gamma)/L^{6g- 6+2n}= N_\gamma(X)\), where \(N_\gamma\) is a positive continuous proper function on the moduli space of complete hyperbolic Riemann surfaces of genus \(g\) with \(n\) cusps. Moreover, the contributions of \(\gamma\) and \(X\) is \(N_\gamma(X)\) separate into: \(N_\gamma(X)= c(\gamma)\times\beta(X)\), where \(c(\gamma)\) is a positive rational depending only on the type \(\gamma\), and \(\beta(X)\) is a normalized measure of the set of multi-curves on \(X\) having integral coefficients and length \(\leq 1\).

Central ingredients of the proof are:

– The Weil-Petersson volume form on the moduli space, and the integral of \(s_X(L,\gamma)\) with respect to it;

– the ergodic action of the mapping class group on the space of measured geodesic laminations (endowed with the Thurston measure), over a closed surface of genus \(g\) with \(n\) boundary components.

We call a multi-curve on \(X\) any formal linear combination \(\gamma:= \sum^k_{i=1} a_i\gamma_i\), with positive rational coefficients \(a_i\), of disjoint, essential, non-peripheral, simple closed curves \(\gamma_i\), which belong to distinct homotopy classes. The length of such \(\gamma\) is defined by \[ \ell_X(\gamma):= \sum^k_{i=1} a_i\ell_X(\gamma_i). \] Then the main result is that, for any multicurve \(\gamma\), \(\lim_{L\to\infty} s_X(L,\gamma)/L^{6g- 6+2n}= N_\gamma(X)\), where \(N_\gamma\) is a positive continuous proper function on the moduli space of complete hyperbolic Riemann surfaces of genus \(g\) with \(n\) cusps. Moreover, the contributions of \(\gamma\) and \(X\) is \(N_\gamma(X)\) separate into: \(N_\gamma(X)= c(\gamma)\times\beta(X)\), where \(c(\gamma)\) is a positive rational depending only on the type \(\gamma\), and \(\beta(X)\) is a normalized measure of the set of multi-curves on \(X\) having integral coefficients and length \(\leq 1\).

Central ingredients of the proof are:

– The Weil-Petersson volume form on the moduli space, and the integral of \(s_X(L,\gamma)\) with respect to it;

– the ergodic action of the mapping class group on the space of measured geodesic laminations (endowed with the Thurston measure), over a closed surface of genus \(g\) with \(n\) boundary components.

Reviewer: Jacques Franchi (Strasbourg)

### MSC:

37D40 | Dynamical systems of geometric origin and hyperbolicity (geodesic and horocycle flows, etc.) |

53C22 | Geodesics in global differential geometry |

32G15 | Moduli of Riemann surfaces, Teichmüller theory (complex-analytic aspects in several variables) |

37J50 | Action-minimizing orbits and measures (MSC2010) |