Poincaré metric on the Riemann sphere minus more than two points











up vote
12
down vote

favorite
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




















  • 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












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






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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
1






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























    Your Answer





    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("mathjaxEditing", function () {
    StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
    StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
    });
    });
    }, "mathjax-editing");

    StackExchange.ready(function() {
    var channelOptions = {
    tags: "".split(" "),
    id: "504"
    };
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function() {
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled) {
    StackExchange.using("snippets", function() {
    createEditor();
    });
    }
    else {
    createEditor();
    }
    });

    function createEditor() {
    StackExchange.prepareEditor({
    heartbeatType: 'answer',
    convertImagesToLinks: true,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: 10,
    bindNavPrevention: true,
    postfix: "",
    imageUploader: {
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    },
    noCode: true, onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    });


    }
    });














    draft saved

    draft discarded


















    StackExchange.ready(
    function () {
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathoverflow.net%2fquestions%2f316243%2fpoincar%25c3%25a9-metric-on-the-riemann-sphere-minus-more-than-two-points%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    1 Answer
    1






    active

    oldest

    votes








    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






























            draft saved

            draft discarded




















































            Thanks for contributing an answer to MathOverflow!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            Use MathJax to format equations. MathJax reference.


            To learn more, see our tips on writing great answers.





            Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


            Please pay close attention to the following guidance:


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathoverflow.net%2fquestions%2f316243%2fpoincar%25c3%25a9-metric-on-the-riemann-sphere-minus-more-than-two-points%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown







            Popular posts from this blog

            How to change which sound is reproduced for terminal bell?

            Title Spacing in Bjornstrup Chapter, Removing Chapter Number From Contents

            Can I use Tabulator js library in my java Spring + Thymeleaf project?