Horrible limit envolving floor function












10












$begingroup$


Let $xin [0,1]$, $ellinmathbb{Z}$ and $tau>0$. I want to calculate
$$lim_{Ltoinfty}sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right).$$



I think the result is $exp(-2pi^2ell^2tau)cos(2piell x)$ but I have no idea in how to prove it.



I tried estimating the binomial with Stirling's aproximation but without success.










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$endgroup$








  • 4




    $begingroup$
    Wow, this is really horrible.
    $endgroup$
    – thesagniksaha
    Dec 15 '18 at 21:25






  • 1




    $begingroup$
    Have you tried replacing the cosine by an complex exponential, using the Newton formula and studying the different factors?
    $endgroup$
    – Mindlack
    Dec 15 '18 at 21:36












  • $begingroup$
    @Mindlack Which formula by Newton?
    $endgroup$
    – Gabriel Ribeiro
    Dec 15 '18 at 22:12










  • $begingroup$
    I meant $(1+z)^n=ldots$.
    $endgroup$
    – Mindlack
    Dec 15 '18 at 22:22






  • 1




    $begingroup$
    Factor out everything that does not depend on $k$.
    $endgroup$
    – Mindlack
    Dec 15 '18 at 22:29
















10












$begingroup$


Let $xin [0,1]$, $ellinmathbb{Z}$ and $tau>0$. I want to calculate
$$lim_{Ltoinfty}sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right).$$



I think the result is $exp(-2pi^2ell^2tau)cos(2piell x)$ but I have no idea in how to prove it.



I tried estimating the binomial with Stirling's aproximation but without success.










share|cite|improve this question









$endgroup$








  • 4




    $begingroup$
    Wow, this is really horrible.
    $endgroup$
    – thesagniksaha
    Dec 15 '18 at 21:25






  • 1




    $begingroup$
    Have you tried replacing the cosine by an complex exponential, using the Newton formula and studying the different factors?
    $endgroup$
    – Mindlack
    Dec 15 '18 at 21:36












  • $begingroup$
    @Mindlack Which formula by Newton?
    $endgroup$
    – Gabriel Ribeiro
    Dec 15 '18 at 22:12










  • $begingroup$
    I meant $(1+z)^n=ldots$.
    $endgroup$
    – Mindlack
    Dec 15 '18 at 22:22






  • 1




    $begingroup$
    Factor out everything that does not depend on $k$.
    $endgroup$
    – Mindlack
    Dec 15 '18 at 22:29














10












10








10


1



$begingroup$


Let $xin [0,1]$, $ellinmathbb{Z}$ and $tau>0$. I want to calculate
$$lim_{Ltoinfty}sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right).$$



I think the result is $exp(-2pi^2ell^2tau)cos(2piell x)$ but I have no idea in how to prove it.



I tried estimating the binomial with Stirling's aproximation but without success.










share|cite|improve this question









$endgroup$




Let $xin [0,1]$, $ellinmathbb{Z}$ and $tau>0$. I want to calculate
$$lim_{Ltoinfty}sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right).$$



I think the result is $exp(-2pi^2ell^2tau)cos(2piell x)$ but I have no idea in how to prove it.



I tried estimating the binomial with Stirling's aproximation but without success.







real-analysis limits






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share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 15 '18 at 21:24









Gabriel RibeiroGabriel Ribeiro

1,454523




1,454523








  • 4




    $begingroup$
    Wow, this is really horrible.
    $endgroup$
    – thesagniksaha
    Dec 15 '18 at 21:25






  • 1




    $begingroup$
    Have you tried replacing the cosine by an complex exponential, using the Newton formula and studying the different factors?
    $endgroup$
    – Mindlack
    Dec 15 '18 at 21:36












  • $begingroup$
    @Mindlack Which formula by Newton?
    $endgroup$
    – Gabriel Ribeiro
    Dec 15 '18 at 22:12










  • $begingroup$
    I meant $(1+z)^n=ldots$.
    $endgroup$
    – Mindlack
    Dec 15 '18 at 22:22






  • 1




    $begingroup$
    Factor out everything that does not depend on $k$.
    $endgroup$
    – Mindlack
    Dec 15 '18 at 22:29














  • 4




    $begingroup$
    Wow, this is really horrible.
    $endgroup$
    – thesagniksaha
    Dec 15 '18 at 21:25






  • 1




    $begingroup$
    Have you tried replacing the cosine by an complex exponential, using the Newton formula and studying the different factors?
    $endgroup$
    – Mindlack
    Dec 15 '18 at 21:36












  • $begingroup$
    @Mindlack Which formula by Newton?
    $endgroup$
    – Gabriel Ribeiro
    Dec 15 '18 at 22:12










  • $begingroup$
    I meant $(1+z)^n=ldots$.
    $endgroup$
    – Mindlack
    Dec 15 '18 at 22:22






  • 1




    $begingroup$
    Factor out everything that does not depend on $k$.
    $endgroup$
    – Mindlack
    Dec 15 '18 at 22:29








4




4




$begingroup$
Wow, this is really horrible.
$endgroup$
– thesagniksaha
Dec 15 '18 at 21:25




$begingroup$
Wow, this is really horrible.
$endgroup$
– thesagniksaha
Dec 15 '18 at 21:25




1




1




$begingroup$
Have you tried replacing the cosine by an complex exponential, using the Newton formula and studying the different factors?
$endgroup$
– Mindlack
Dec 15 '18 at 21:36






$begingroup$
Have you tried replacing the cosine by an complex exponential, using the Newton formula and studying the different factors?
$endgroup$
– Mindlack
Dec 15 '18 at 21:36














$begingroup$
@Mindlack Which formula by Newton?
$endgroup$
– Gabriel Ribeiro
Dec 15 '18 at 22:12




$begingroup$
@Mindlack Which formula by Newton?
$endgroup$
– Gabriel Ribeiro
Dec 15 '18 at 22:12












$begingroup$
I meant $(1+z)^n=ldots$.
$endgroup$
– Mindlack
Dec 15 '18 at 22:22




$begingroup$
I meant $(1+z)^n=ldots$.
$endgroup$
– Mindlack
Dec 15 '18 at 22:22




1




1




$begingroup$
Factor out everything that does not depend on $k$.
$endgroup$
– Mindlack
Dec 15 '18 at 22:29




$begingroup$
Factor out everything that does not depend on $k$.
$endgroup$
– Mindlack
Dec 15 '18 at 22:29










1 Answer
1






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10












$begingroup$

You are correct. We have that
$$begin{aligned}
& sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right) \
= &sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}Releft(exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
=& frac{1}{2^{lfloor tau L^2rfloor}}Releft(sum_{k=0}^{lfloor tau L^2rfloor}binom{lfloor tau L^2rfloor}{k}exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
=& frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piellfrac{lfloor xLrfloor -lfloortau L^2rfloor}{L}right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).
end{aligned}$$

Since we are taking the limit as $Ltoinfty$ we have that
$$frac{lfloor xLrfloor -lfloortau L^2rfloor}{L}to x-tau L$$
so we can consider the expression
$$frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piell(x-tau L)right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).$$
Again since we only care about $Ltoinfty$ we can let $Lmapsto L/sqrt{tau}$ so that we are considering the expression
$$begin{aligned}
&frac{1}{2^{L^2}}Releft(expleft(i2piell(x-sqrt{tau} L)right)left(1+expleft(ifrac{4piellsqrt{tau}}{L}right)right)^{L^2}right) \
=&Releft(exp(i2piell x)left(frac{expleft(-ifrac{2piellsqrt{tau}}{L}right)+expleft(ifrac{2piellsqrt{tau}}{L}right)}{2}right)^{L^2}right) \
=&cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}Releft(exp(i2piell x)right) \
=& cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x).
end{aligned}$$

Taking the limit we indeed get
$$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$






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    10












    $begingroup$

    You are correct. We have that
    $$begin{aligned}
    & sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right) \
    = &sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}Releft(exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
    =& frac{1}{2^{lfloor tau L^2rfloor}}Releft(sum_{k=0}^{lfloor tau L^2rfloor}binom{lfloor tau L^2rfloor}{k}exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
    =& frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piellfrac{lfloor xLrfloor -lfloortau L^2rfloor}{L}right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).
    end{aligned}$$

    Since we are taking the limit as $Ltoinfty$ we have that
    $$frac{lfloor xLrfloor -lfloortau L^2rfloor}{L}to x-tau L$$
    so we can consider the expression
    $$frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piell(x-tau L)right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).$$
    Again since we only care about $Ltoinfty$ we can let $Lmapsto L/sqrt{tau}$ so that we are considering the expression
    $$begin{aligned}
    &frac{1}{2^{L^2}}Releft(expleft(i2piell(x-sqrt{tau} L)right)left(1+expleft(ifrac{4piellsqrt{tau}}{L}right)right)^{L^2}right) \
    =&Releft(exp(i2piell x)left(frac{expleft(-ifrac{2piellsqrt{tau}}{L}right)+expleft(ifrac{2piellsqrt{tau}}{L}right)}{2}right)^{L^2}right) \
    =&cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}Releft(exp(i2piell x)right) \
    =& cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x).
    end{aligned}$$

    Taking the limit we indeed get
    $$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$






    share|cite|improve this answer









    $endgroup$


















      10












      $begingroup$

      You are correct. We have that
      $$begin{aligned}
      & sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right) \
      = &sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}Releft(exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
      =& frac{1}{2^{lfloor tau L^2rfloor}}Releft(sum_{k=0}^{lfloor tau L^2rfloor}binom{lfloor tau L^2rfloor}{k}exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
      =& frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piellfrac{lfloor xLrfloor -lfloortau L^2rfloor}{L}right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).
      end{aligned}$$

      Since we are taking the limit as $Ltoinfty$ we have that
      $$frac{lfloor xLrfloor -lfloortau L^2rfloor}{L}to x-tau L$$
      so we can consider the expression
      $$frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piell(x-tau L)right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).$$
      Again since we only care about $Ltoinfty$ we can let $Lmapsto L/sqrt{tau}$ so that we are considering the expression
      $$begin{aligned}
      &frac{1}{2^{L^2}}Releft(expleft(i2piell(x-sqrt{tau} L)right)left(1+expleft(ifrac{4piellsqrt{tau}}{L}right)right)^{L^2}right) \
      =&Releft(exp(i2piell x)left(frac{expleft(-ifrac{2piellsqrt{tau}}{L}right)+expleft(ifrac{2piellsqrt{tau}}{L}right)}{2}right)^{L^2}right) \
      =&cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}Releft(exp(i2piell x)right) \
      =& cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x).
      end{aligned}$$

      Taking the limit we indeed get
      $$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$






      share|cite|improve this answer









      $endgroup$
















        10












        10








        10





        $begingroup$

        You are correct. We have that
        $$begin{aligned}
        & sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right) \
        = &sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}Releft(exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
        =& frac{1}{2^{lfloor tau L^2rfloor}}Releft(sum_{k=0}^{lfloor tau L^2rfloor}binom{lfloor tau L^2rfloor}{k}exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
        =& frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piellfrac{lfloor xLrfloor -lfloortau L^2rfloor}{L}right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).
        end{aligned}$$

        Since we are taking the limit as $Ltoinfty$ we have that
        $$frac{lfloor xLrfloor -lfloortau L^2rfloor}{L}to x-tau L$$
        so we can consider the expression
        $$frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piell(x-tau L)right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).$$
        Again since we only care about $Ltoinfty$ we can let $Lmapsto L/sqrt{tau}$ so that we are considering the expression
        $$begin{aligned}
        &frac{1}{2^{L^2}}Releft(expleft(i2piell(x-sqrt{tau} L)right)left(1+expleft(ifrac{4piellsqrt{tau}}{L}right)right)^{L^2}right) \
        =&Releft(exp(i2piell x)left(frac{expleft(-ifrac{2piellsqrt{tau}}{L}right)+expleft(ifrac{2piellsqrt{tau}}{L}right)}{2}right)^{L^2}right) \
        =&cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}Releft(exp(i2piell x)right) \
        =& cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x).
        end{aligned}$$

        Taking the limit we indeed get
        $$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$






        share|cite|improve this answer









        $endgroup$



        You are correct. We have that
        $$begin{aligned}
        & sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}cosleft(2pi ellfrac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L}right) \
        = &sum_{k=0}^{lfloor tau L^2rfloor}frac{1}{2^{lfloor tau L^2rfloor}}binom{lfloor tau L^2rfloor}{k}Releft(exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
        =& frac{1}{2^{lfloor tau L^2rfloor}}Releft(sum_{k=0}^{lfloor tau L^2rfloor}binom{lfloor tau L^2rfloor}{k}exp(i2piell frac{lfloor xLrfloor-lfloor tau L^2rfloor+2k}{L})right) \
        =& frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piellfrac{lfloor xLrfloor -lfloortau L^2rfloor}{L}right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).
        end{aligned}$$

        Since we are taking the limit as $Ltoinfty$ we have that
        $$frac{lfloor xLrfloor -lfloortau L^2rfloor}{L}to x-tau L$$
        so we can consider the expression
        $$frac{1}{2^{lfloor tau L^2rfloor}}Releft(expleft(i2piell(x-tau L)right)left(1+expleft(ifrac{4piell}{L}right)right)^{lfloor tau L^2rfloor}right).$$
        Again since we only care about $Ltoinfty$ we can let $Lmapsto L/sqrt{tau}$ so that we are considering the expression
        $$begin{aligned}
        &frac{1}{2^{L^2}}Releft(expleft(i2piell(x-sqrt{tau} L)right)left(1+expleft(ifrac{4piellsqrt{tau}}{L}right)right)^{L^2}right) \
        =&Releft(exp(i2piell x)left(frac{expleft(-ifrac{2piellsqrt{tau}}{L}right)+expleft(ifrac{2piellsqrt{tau}}{L}right)}{2}right)^{L^2}right) \
        =&cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}Releft(exp(i2piell x)right) \
        =& cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x).
        end{aligned}$$

        Taking the limit we indeed get
        $$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Dec 16 '18 at 0:27









        Will FisherWill Fisher

        4,04811032




        4,04811032






























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