Poincaré metric on the Riemann sphere minus more than two points











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If we omit more than two points from the Riemann sphere, we will obtain a hyperbolic Riemann surface endowed with a canonical metric descending from its universal cover which is the Poincaré disk. Let us denote the hyperbolic metric on this surface by $d_h$, and the usual spherical metric on the Riemann sphere by $d$. Here is my question:
Can we find a constant $C>0$ such that for any two points $x$ and $y$ in this punctured sphere we have:
$$d(x,y)<C d_h(x,y).$$










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up vote
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down vote

favorite
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If we omit more than two points from the Riemann sphere, we will obtain a hyperbolic Riemann surface endowed with a canonical metric descending from its universal cover which is the Poincaré disk. Let us denote the hyperbolic metric on this surface by $d_h$, and the usual spherical metric on the Riemann sphere by $d$. Here is my question:
Can we find a constant $C>0$ such that for any two points $x$ and $y$ in this punctured sphere we have:
$$d(x,y)<C d_h(x,y).$$










share|cite|improve this question




















  • 4




    Welcome to Mathoverflow.
    – Mahdi
    Nov 26 at 18:06










  • @Mahdi Thank you very much.
    – Amin Talebi
    Nov 27 at 9:14















up vote
12
down vote

favorite
1









up vote
12
down vote

favorite
1






1





If we omit more than two points from the Riemann sphere, we will obtain a hyperbolic Riemann surface endowed with a canonical metric descending from its universal cover which is the Poincaré disk. Let us denote the hyperbolic metric on this surface by $d_h$, and the usual spherical metric on the Riemann sphere by $d$. Here is my question:
Can we find a constant $C>0$ such that for any two points $x$ and $y$ in this punctured sphere we have:
$$d(x,y)<C d_h(x,y).$$










share|cite|improve this question















If we omit more than two points from the Riemann sphere, we will obtain a hyperbolic Riemann surface endowed with a canonical metric descending from its universal cover which is the Poincaré disk. Let us denote the hyperbolic metric on this surface by $d_h$, and the usual spherical metric on the Riemann sphere by $d$. Here is my question:
Can we find a constant $C>0$ such that for any two points $x$ and $y$ in this punctured sphere we have:
$$d(x,y)<C d_h(x,y).$$







complex-geometry riemann-surfaces complex-dynamics






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edited Nov 26 at 18:04









Ivan Izmestiev

4,0631238




4,0631238










asked Nov 26 at 15:44









Amin Talebi

634




634








  • 4




    Welcome to Mathoverflow.
    – Mahdi
    Nov 26 at 18:06










  • @Mahdi Thank you very much.
    – Amin Talebi
    Nov 27 at 9:14
















  • 4




    Welcome to Mathoverflow.
    – Mahdi
    Nov 26 at 18:06










  • @Mahdi Thank you very much.
    – Amin Talebi
    Nov 27 at 9:14










4




4




Welcome to Mathoverflow.
– Mahdi
Nov 26 at 18:06




Welcome to Mathoverflow.
– Mahdi
Nov 26 at 18:06












@Mahdi Thank you very much.
– Amin Talebi
Nov 27 at 9:14






@Mahdi Thank you very much.
– Amin Talebi
Nov 27 at 9:14












1 Answer
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Yes. The density of the Poincare metric with respect to the spherical metric is
a positive continuous function which tends to infinity at the punctures. Thus it
is bounded from below by some positive constant. The constant depends only
on the configuration of the punctures. For some special configurations of punctures, the exact constant has been explicitly found:



MR1428102
Bonk, Mario; Cherry, William,
Bounds on spherical derivatives for maps into regions with symmetries.
J. Anal. Math. 69 (1996), 249–274.



The authors of this paper say that for general punctures, the explicit determination of
the optimal constant is hopeless, and I agree with them.






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    1 Answer
    1






    active

    oldest

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    1 Answer
    1






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    16
    down vote



    accepted










    Yes. The density of the Poincare metric with respect to the spherical metric is
    a positive continuous function which tends to infinity at the punctures. Thus it
    is bounded from below by some positive constant. The constant depends only
    on the configuration of the punctures. For some special configurations of punctures, the exact constant has been explicitly found:



    MR1428102
    Bonk, Mario; Cherry, William,
    Bounds on spherical derivatives for maps into regions with symmetries.
    J. Anal. Math. 69 (1996), 249–274.



    The authors of this paper say that for general punctures, the explicit determination of
    the optimal constant is hopeless, and I agree with them.






    share|cite|improve this answer



























      up vote
      16
      down vote



      accepted










      Yes. The density of the Poincare metric with respect to the spherical metric is
      a positive continuous function which tends to infinity at the punctures. Thus it
      is bounded from below by some positive constant. The constant depends only
      on the configuration of the punctures. For some special configurations of punctures, the exact constant has been explicitly found:



      MR1428102
      Bonk, Mario; Cherry, William,
      Bounds on spherical derivatives for maps into regions with symmetries.
      J. Anal. Math. 69 (1996), 249–274.



      The authors of this paper say that for general punctures, the explicit determination of
      the optimal constant is hopeless, and I agree with them.






      share|cite|improve this answer

























        up vote
        16
        down vote



        accepted







        up vote
        16
        down vote



        accepted






        Yes. The density of the Poincare metric with respect to the spherical metric is
        a positive continuous function which tends to infinity at the punctures. Thus it
        is bounded from below by some positive constant. The constant depends only
        on the configuration of the punctures. For some special configurations of punctures, the exact constant has been explicitly found:



        MR1428102
        Bonk, Mario; Cherry, William,
        Bounds on spherical derivatives for maps into regions with symmetries.
        J. Anal. Math. 69 (1996), 249–274.



        The authors of this paper say that for general punctures, the explicit determination of
        the optimal constant is hopeless, and I agree with them.






        share|cite|improve this answer














        Yes. The density of the Poincare metric with respect to the spherical metric is
        a positive continuous function which tends to infinity at the punctures. Thus it
        is bounded from below by some positive constant. The constant depends only
        on the configuration of the punctures. For some special configurations of punctures, the exact constant has been explicitly found:



        MR1428102
        Bonk, Mario; Cherry, William,
        Bounds on spherical derivatives for maps into regions with symmetries.
        J. Anal. Math. 69 (1996), 249–274.



        The authors of this paper say that for general punctures, the explicit determination of
        the optimal constant is hopeless, and I agree with them.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Nov 27 at 1:22

























        answered Nov 26 at 16:11









        Alexandre Eremenko

        48.8k6136253




        48.8k6136253






























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