Horrible limit envolving floor function
$begingroup$
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.
real-analysis limits
$endgroup$
|
show 3 more comments
$begingroup$
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.
real-analysis limits
$endgroup$
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
|
show 3 more comments
$begingroup$
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.
real-analysis limits
$endgroup$
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
real-analysis limits
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
|
show 3 more comments
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
|
show 3 more comments
1 Answer
1
active
oldest
votes
$begingroup$
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
$$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$
$endgroup$
add a comment |
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1 Answer
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$begingroup$
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
$$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$
$endgroup$
add a comment |
$begingroup$
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
$$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$
$endgroup$
add a comment |
$begingroup$
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
$$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$
$endgroup$
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
$$lim_{Ltoinfty} cosleft(frac{2piellsqrt{tau}}{L}right)^{L^2}cos(2piell x)=exp(-2pi^2ell^2tau)cos(2piell x).$$
answered Dec 16 '18 at 0:27
Will FisherWill Fisher
4,04811032
4,04811032
add a comment |
add a comment |
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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