Let $P(X_j=j)=P(X_j=-j)=1/2j^{beta}$ and $P(X_j=0)=1-j^{-beta}$ where $betain(0,1)$, then...












4












$begingroup$



Suppose $P(X_j=j)=P(X_j=-j)=1/2j^{beta}$ and $P(X_j=0)=1-j^{-beta}$, where $beta>0$. Show that:



(i) If $beta>1$ then $S_nto S_infty$ a.s.



(ii) If $betain(0,1)$ then $S_n/n^{(3-beta)/2}Rightarrow cchi$.



(iii) If $beta=1$ then $S_n/nRightarrowaleph$, where
$$Eexp(italeph)=expleft(-int_0^1 x^{-1}(1-cos(xt),mathrm{d}xright).$$




This is problem 3.4.13 in Durrett's Probability text, part (i) was rather trivial, I feel fine about that part. I am having a difficult time on part (ii) though and would like verification for part (iii).



My ideas so far for part (ii) is to define the triangular array as $S_{n,m}=dfrac{X_m}{n^{(3-beta)/2}}$, and then use the Lindeberg-Feller theorem, but I am getting hung up on the details.



For part (iii) consider:



It is a well-known theorem of Levy that if ${X_n}$ is a collection of random variables and $Y$ is another random variable then $X_n Rightarrow Y$ iff $phi_{X_n}(t) rightarrow phi_Y(t)$ as $n rightarrow infty$ and $phi_Y$ is continuous at $t = 0$. Moreover, by properties of Fourier transforms, $phi_{S_n/n}(t) = prodlimits_{1 leq j leq n} phi_{X_j/n}(t)$. Now,
$$phi_{X_j/n}(t) = int_{mathbb{R}} mathrm{d}lambda e^{itlambda} mathbb{P}left(frac{X_j}{n} = lambdaright) = 1-frac{1}{j} + frac{1}{2j}(e^{itfrac{j}{n}} + e^{-itfrac{j}{n}}) = 1-frac{1}{j}(1-cos(tj/n)).
$$

This is clearly real-valued and positive, so that we can write
$$
logphi_{S_n/n}(t) = sum_{j = 1}^n logleft(1-frac{1}{n}cdot frac{n}{j}(1-cos(tj/n)right),
$$

so, up to an $O(1/n)$ error term, we have
$$
log phi_{S_n/n}(t) = frac{1}{n}sum_{j=1}^n frac{n}{j}(1-cos(tj/n)) + Oleft(frac{1}{n}right).
$$

The sum on the right side is a Riemann sum for the exponential, so taking $n rightarrow infty$, we get $phi_{S_n/n}(t) rightarrow Eleft(e^{italeph}right)$, in our notation, the latter of which is continuous at $0$.










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  • $begingroup$
    Use the same technique for part (ii) as you are in part (iii), i.e. characteristic functions.
    $endgroup$
    – maridia
    Dec 9 '18 at 16:30










  • $begingroup$
    @norfair I tried utilizing the same technique but I was not getting the desired result. :-/
    $endgroup$
    – Dragonite
    Dec 9 '18 at 16:47










  • $begingroup$
    Write out the same formula you had for $beta=1$ for general $beta$ and expand out the corresponding $e^{it j/n}$ and $e^{-it j/n}$ terms in Taylor series. You should see some nice cancellation from the constant and linear terms. You're left with a quadratic and higher order terms...from the form of the characteristic function of normal, it should be clear what to do.
    $endgroup$
    – maridia
    Dec 9 '18 at 17:19








  • 2




    $begingroup$
    @Dragonite You might want to explain what $c_{chi}$ actually means... you didn't introduce the notation.
    $endgroup$
    – saz
    Dec 9 '18 at 18:00
















4












$begingroup$



Suppose $P(X_j=j)=P(X_j=-j)=1/2j^{beta}$ and $P(X_j=0)=1-j^{-beta}$, where $beta>0$. Show that:



(i) If $beta>1$ then $S_nto S_infty$ a.s.



(ii) If $betain(0,1)$ then $S_n/n^{(3-beta)/2}Rightarrow cchi$.



(iii) If $beta=1$ then $S_n/nRightarrowaleph$, where
$$Eexp(italeph)=expleft(-int_0^1 x^{-1}(1-cos(xt),mathrm{d}xright).$$




This is problem 3.4.13 in Durrett's Probability text, part (i) was rather trivial, I feel fine about that part. I am having a difficult time on part (ii) though and would like verification for part (iii).



My ideas so far for part (ii) is to define the triangular array as $S_{n,m}=dfrac{X_m}{n^{(3-beta)/2}}$, and then use the Lindeberg-Feller theorem, but I am getting hung up on the details.



For part (iii) consider:



It is a well-known theorem of Levy that if ${X_n}$ is a collection of random variables and $Y$ is another random variable then $X_n Rightarrow Y$ iff $phi_{X_n}(t) rightarrow phi_Y(t)$ as $n rightarrow infty$ and $phi_Y$ is continuous at $t = 0$. Moreover, by properties of Fourier transforms, $phi_{S_n/n}(t) = prodlimits_{1 leq j leq n} phi_{X_j/n}(t)$. Now,
$$phi_{X_j/n}(t) = int_{mathbb{R}} mathrm{d}lambda e^{itlambda} mathbb{P}left(frac{X_j}{n} = lambdaright) = 1-frac{1}{j} + frac{1}{2j}(e^{itfrac{j}{n}} + e^{-itfrac{j}{n}}) = 1-frac{1}{j}(1-cos(tj/n)).
$$

This is clearly real-valued and positive, so that we can write
$$
logphi_{S_n/n}(t) = sum_{j = 1}^n logleft(1-frac{1}{n}cdot frac{n}{j}(1-cos(tj/n)right),
$$

so, up to an $O(1/n)$ error term, we have
$$
log phi_{S_n/n}(t) = frac{1}{n}sum_{j=1}^n frac{n}{j}(1-cos(tj/n)) + Oleft(frac{1}{n}right).
$$

The sum on the right side is a Riemann sum for the exponential, so taking $n rightarrow infty$, we get $phi_{S_n/n}(t) rightarrow Eleft(e^{italeph}right)$, in our notation, the latter of which is continuous at $0$.










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$endgroup$












  • $begingroup$
    Use the same technique for part (ii) as you are in part (iii), i.e. characteristic functions.
    $endgroup$
    – maridia
    Dec 9 '18 at 16:30










  • $begingroup$
    @norfair I tried utilizing the same technique but I was not getting the desired result. :-/
    $endgroup$
    – Dragonite
    Dec 9 '18 at 16:47










  • $begingroup$
    Write out the same formula you had for $beta=1$ for general $beta$ and expand out the corresponding $e^{it j/n}$ and $e^{-it j/n}$ terms in Taylor series. You should see some nice cancellation from the constant and linear terms. You're left with a quadratic and higher order terms...from the form of the characteristic function of normal, it should be clear what to do.
    $endgroup$
    – maridia
    Dec 9 '18 at 17:19








  • 2




    $begingroup$
    @Dragonite You might want to explain what $c_{chi}$ actually means... you didn't introduce the notation.
    $endgroup$
    – saz
    Dec 9 '18 at 18:00














4












4








4


1



$begingroup$



Suppose $P(X_j=j)=P(X_j=-j)=1/2j^{beta}$ and $P(X_j=0)=1-j^{-beta}$, where $beta>0$. Show that:



(i) If $beta>1$ then $S_nto S_infty$ a.s.



(ii) If $betain(0,1)$ then $S_n/n^{(3-beta)/2}Rightarrow cchi$.



(iii) If $beta=1$ then $S_n/nRightarrowaleph$, where
$$Eexp(italeph)=expleft(-int_0^1 x^{-1}(1-cos(xt),mathrm{d}xright).$$




This is problem 3.4.13 in Durrett's Probability text, part (i) was rather trivial, I feel fine about that part. I am having a difficult time on part (ii) though and would like verification for part (iii).



My ideas so far for part (ii) is to define the triangular array as $S_{n,m}=dfrac{X_m}{n^{(3-beta)/2}}$, and then use the Lindeberg-Feller theorem, but I am getting hung up on the details.



For part (iii) consider:



It is a well-known theorem of Levy that if ${X_n}$ is a collection of random variables and $Y$ is another random variable then $X_n Rightarrow Y$ iff $phi_{X_n}(t) rightarrow phi_Y(t)$ as $n rightarrow infty$ and $phi_Y$ is continuous at $t = 0$. Moreover, by properties of Fourier transforms, $phi_{S_n/n}(t) = prodlimits_{1 leq j leq n} phi_{X_j/n}(t)$. Now,
$$phi_{X_j/n}(t) = int_{mathbb{R}} mathrm{d}lambda e^{itlambda} mathbb{P}left(frac{X_j}{n} = lambdaright) = 1-frac{1}{j} + frac{1}{2j}(e^{itfrac{j}{n}} + e^{-itfrac{j}{n}}) = 1-frac{1}{j}(1-cos(tj/n)).
$$

This is clearly real-valued and positive, so that we can write
$$
logphi_{S_n/n}(t) = sum_{j = 1}^n logleft(1-frac{1}{n}cdot frac{n}{j}(1-cos(tj/n)right),
$$

so, up to an $O(1/n)$ error term, we have
$$
log phi_{S_n/n}(t) = frac{1}{n}sum_{j=1}^n frac{n}{j}(1-cos(tj/n)) + Oleft(frac{1}{n}right).
$$

The sum on the right side is a Riemann sum for the exponential, so taking $n rightarrow infty$, we get $phi_{S_n/n}(t) rightarrow Eleft(e^{italeph}right)$, in our notation, the latter of which is continuous at $0$.










share|cite|improve this question











$endgroup$





Suppose $P(X_j=j)=P(X_j=-j)=1/2j^{beta}$ and $P(X_j=0)=1-j^{-beta}$, where $beta>0$. Show that:



(i) If $beta>1$ then $S_nto S_infty$ a.s.



(ii) If $betain(0,1)$ then $S_n/n^{(3-beta)/2}Rightarrow cchi$.



(iii) If $beta=1$ then $S_n/nRightarrowaleph$, where
$$Eexp(italeph)=expleft(-int_0^1 x^{-1}(1-cos(xt),mathrm{d}xright).$$




This is problem 3.4.13 in Durrett's Probability text, part (i) was rather trivial, I feel fine about that part. I am having a difficult time on part (ii) though and would like verification for part (iii).



My ideas so far for part (ii) is to define the triangular array as $S_{n,m}=dfrac{X_m}{n^{(3-beta)/2}}$, and then use the Lindeberg-Feller theorem, but I am getting hung up on the details.



For part (iii) consider:



It is a well-known theorem of Levy that if ${X_n}$ is a collection of random variables and $Y$ is another random variable then $X_n Rightarrow Y$ iff $phi_{X_n}(t) rightarrow phi_Y(t)$ as $n rightarrow infty$ and $phi_Y$ is continuous at $t = 0$. Moreover, by properties of Fourier transforms, $phi_{S_n/n}(t) = prodlimits_{1 leq j leq n} phi_{X_j/n}(t)$. Now,
$$phi_{X_j/n}(t) = int_{mathbb{R}} mathrm{d}lambda e^{itlambda} mathbb{P}left(frac{X_j}{n} = lambdaright) = 1-frac{1}{j} + frac{1}{2j}(e^{itfrac{j}{n}} + e^{-itfrac{j}{n}}) = 1-frac{1}{j}(1-cos(tj/n)).
$$

This is clearly real-valued and positive, so that we can write
$$
logphi_{S_n/n}(t) = sum_{j = 1}^n logleft(1-frac{1}{n}cdot frac{n}{j}(1-cos(tj/n)right),
$$

so, up to an $O(1/n)$ error term, we have
$$
log phi_{S_n/n}(t) = frac{1}{n}sum_{j=1}^n frac{n}{j}(1-cos(tj/n)) + Oleft(frac{1}{n}right).
$$

The sum on the right side is a Riemann sum for the exponential, so taking $n rightarrow infty$, we get $phi_{S_n/n}(t) rightarrow Eleft(e^{italeph}right)$, in our notation, the latter of which is continuous at $0$.







probability-theory central-limit-theorem probability-limit-theorems






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edited Dec 10 '18 at 13:42









Saad

19.7k92352




19.7k92352










asked Dec 7 '18 at 16:12









DragoniteDragonite

1,045420




1,045420












  • $begingroup$
    Use the same technique for part (ii) as you are in part (iii), i.e. characteristic functions.
    $endgroup$
    – maridia
    Dec 9 '18 at 16:30










  • $begingroup$
    @norfair I tried utilizing the same technique but I was not getting the desired result. :-/
    $endgroup$
    – Dragonite
    Dec 9 '18 at 16:47










  • $begingroup$
    Write out the same formula you had for $beta=1$ for general $beta$ and expand out the corresponding $e^{it j/n}$ and $e^{-it j/n}$ terms in Taylor series. You should see some nice cancellation from the constant and linear terms. You're left with a quadratic and higher order terms...from the form of the characteristic function of normal, it should be clear what to do.
    $endgroup$
    – maridia
    Dec 9 '18 at 17:19








  • 2




    $begingroup$
    @Dragonite You might want to explain what $c_{chi}$ actually means... you didn't introduce the notation.
    $endgroup$
    – saz
    Dec 9 '18 at 18:00


















  • $begingroup$
    Use the same technique for part (ii) as you are in part (iii), i.e. characteristic functions.
    $endgroup$
    – maridia
    Dec 9 '18 at 16:30










  • $begingroup$
    @norfair I tried utilizing the same technique but I was not getting the desired result. :-/
    $endgroup$
    – Dragonite
    Dec 9 '18 at 16:47










  • $begingroup$
    Write out the same formula you had for $beta=1$ for general $beta$ and expand out the corresponding $e^{it j/n}$ and $e^{-it j/n}$ terms in Taylor series. You should see some nice cancellation from the constant and linear terms. You're left with a quadratic and higher order terms...from the form of the characteristic function of normal, it should be clear what to do.
    $endgroup$
    – maridia
    Dec 9 '18 at 17:19








  • 2




    $begingroup$
    @Dragonite You might want to explain what $c_{chi}$ actually means... you didn't introduce the notation.
    $endgroup$
    – saz
    Dec 9 '18 at 18:00
















$begingroup$
Use the same technique for part (ii) as you are in part (iii), i.e. characteristic functions.
$endgroup$
– maridia
Dec 9 '18 at 16:30




$begingroup$
Use the same technique for part (ii) as you are in part (iii), i.e. characteristic functions.
$endgroup$
– maridia
Dec 9 '18 at 16:30












$begingroup$
@norfair I tried utilizing the same technique but I was not getting the desired result. :-/
$endgroup$
– Dragonite
Dec 9 '18 at 16:47




$begingroup$
@norfair I tried utilizing the same technique but I was not getting the desired result. :-/
$endgroup$
– Dragonite
Dec 9 '18 at 16:47












$begingroup$
Write out the same formula you had for $beta=1$ for general $beta$ and expand out the corresponding $e^{it j/n}$ and $e^{-it j/n}$ terms in Taylor series. You should see some nice cancellation from the constant and linear terms. You're left with a quadratic and higher order terms...from the form of the characteristic function of normal, it should be clear what to do.
$endgroup$
– maridia
Dec 9 '18 at 17:19






$begingroup$
Write out the same formula you had for $beta=1$ for general $beta$ and expand out the corresponding $e^{it j/n}$ and $e^{-it j/n}$ terms in Taylor series. You should see some nice cancellation from the constant and linear terms. You're left with a quadratic and higher order terms...from the form of the characteristic function of normal, it should be clear what to do.
$endgroup$
– maridia
Dec 9 '18 at 17:19






2




2




$begingroup$
@Dragonite You might want to explain what $c_{chi}$ actually means... you didn't introduce the notation.
$endgroup$
– saz
Dec 9 '18 at 18:00




$begingroup$
@Dragonite You might want to explain what $c_{chi}$ actually means... you didn't introduce the notation.
$endgroup$
– saz
Dec 9 '18 at 18:00










1 Answer
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$begingroup$

$defe{mathrm{e}}defi{mathrm{i}}defd{mathrm{d}}$As is written at the start of exercise section, $X_1, X_2, cdots$ are independent.



Define $X_{n, k} = dfrac{X_k}{n^{frac{3 - β}{2}}}$ for $1 leqslant k leqslant n$. Since Lindeberg's condition does not apply for ${X_{n, k} mid 1 leqslant k leqslant n}$, so the proposition has to be proved directly. Since$$
φ_{n, k}(t) := E(exp(i t X_{n, k})) = frac{1}{k^β} cosfrac{kt}{n^{frac{3 - β}{2}}} + left( 1 - frac{1}{k^β} right), quad forall t in mathbb{R}
$$

it suffices to prove that there exists a constant $c$ that$$lim_{t → ∞} prod_{k = 1}^n φ_{n, k}(t) = expleft( -frac{1}{2} c^2 t^2 right). quad forall t in mathbb{R}
$$



For a fixed $t$, in order to apply Exercise 3.1.1., denote $c_{n, k} = φ_{n, k}(t) - 1 = dfrac{1}{k^β} left( cosdfrac{kt}{n^{frac{3 - β}{2}}} - 1 right)$, it suffices to prove that$$
lim_{n → ∞} max_{1 leqslant k leqslant n} |c_{n, k}| = 0, quad lim_{n → ∞} sum_{k = 1}^n c_{n, k} = -frac{1}{2} c^2 t^2, quad sup_{n geqslant 1} sum_{k = 1}^n |c_{n, k}| < +∞.
$$

Since $|c_{n, k}| leqslant dfrac{1}{k^β} · dfrac{1}{2} left( dfrac{kt}{n^{frac{3 - β}{2}}} right)^2 = dfrac{k^{2 - β} t^2}{2n^{3 - β}} leqslant dfrac{t^2}{2n}$, then $limlimits_{n → ∞} maxlimits_{1 leqslant k leqslant n} |c_{n, k}| = 0$ and$$
sum_{k = 1}^n |c_{n, k}| leqslant sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} leqslant frac{t^2}{2n^{3 - β}} int_1^{n + 1} x^{2 - β} ,d x leqslant frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
$$

which implies $suplimits_{n geqslant 1} sumlimits_{k = 1}^n |c_{n, k}| < +∞$.



Now, since $cos x = 1 - dfrac{x^2}{2} + dfrac{x^4}{24} + o(x^5) (x → 0)$, there exists $δ > 0$ such that$$
1 - frac{x^2}{2} < cos x < 1 - frac{x^2}{2} + frac{x^4}{23}. quad forall |x| < δ
$$

For $n > left( dfrac{t}{δ} right)^{frac{2}{1 - β}}$,begin{align*}
sum_{k = 1}^n c_{n, k} &leqslant sum_{k = 1}^n frac{1}{k^β} left( -frac{k^2 t^2}{2n^{3 - β}} + frac{k^4 t^4}{23n^{2(3 - β)}} right) = -sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} + sum_{k = 1}^n frac{k^{4 - β} t^4}{23n^{2(3 - β)}}\
&leqslant -frac{t^2}{2n^{3 - β}} int_0^n x^{2 - β} ,d x + n · frac{n^{4 - β} t^4}{23n^{2(3 - β)}} = -frac{t^2}{2(3 - β)} + frac{t^4}{23n^{1 - β}},
end{align*}
$$
sum_{k = 1}^n c_{n, k} geqslant -sum_{k = 1}^n frac{1}{k^β} · frac{k^2 t^2}{2n^{3 - β}} geqslant -frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
$$

thus $limlimits_{n → ∞} sumlimits_{k = 1}^n c_{n, k} = -dfrac{t^2}{2(3 - β)}$. Applying Exercise 3.1.1., $dfrac{S_n}{n^{frac{3 - β}{2}}} Rightarrow cχ$, where $c = dfrac{1}{sqrt{3 - β}}$.






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    $begingroup$

    $defe{mathrm{e}}defi{mathrm{i}}defd{mathrm{d}}$As is written at the start of exercise section, $X_1, X_2, cdots$ are independent.



    Define $X_{n, k} = dfrac{X_k}{n^{frac{3 - β}{2}}}$ for $1 leqslant k leqslant n$. Since Lindeberg's condition does not apply for ${X_{n, k} mid 1 leqslant k leqslant n}$, so the proposition has to be proved directly. Since$$
    φ_{n, k}(t) := E(exp(i t X_{n, k})) = frac{1}{k^β} cosfrac{kt}{n^{frac{3 - β}{2}}} + left( 1 - frac{1}{k^β} right), quad forall t in mathbb{R}
    $$

    it suffices to prove that there exists a constant $c$ that$$lim_{t → ∞} prod_{k = 1}^n φ_{n, k}(t) = expleft( -frac{1}{2} c^2 t^2 right). quad forall t in mathbb{R}
    $$



    For a fixed $t$, in order to apply Exercise 3.1.1., denote $c_{n, k} = φ_{n, k}(t) - 1 = dfrac{1}{k^β} left( cosdfrac{kt}{n^{frac{3 - β}{2}}} - 1 right)$, it suffices to prove that$$
    lim_{n → ∞} max_{1 leqslant k leqslant n} |c_{n, k}| = 0, quad lim_{n → ∞} sum_{k = 1}^n c_{n, k} = -frac{1}{2} c^2 t^2, quad sup_{n geqslant 1} sum_{k = 1}^n |c_{n, k}| < +∞.
    $$

    Since $|c_{n, k}| leqslant dfrac{1}{k^β} · dfrac{1}{2} left( dfrac{kt}{n^{frac{3 - β}{2}}} right)^2 = dfrac{k^{2 - β} t^2}{2n^{3 - β}} leqslant dfrac{t^2}{2n}$, then $limlimits_{n → ∞} maxlimits_{1 leqslant k leqslant n} |c_{n, k}| = 0$ and$$
    sum_{k = 1}^n |c_{n, k}| leqslant sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} leqslant frac{t^2}{2n^{3 - β}} int_1^{n + 1} x^{2 - β} ,d x leqslant frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
    $$

    which implies $suplimits_{n geqslant 1} sumlimits_{k = 1}^n |c_{n, k}| < +∞$.



    Now, since $cos x = 1 - dfrac{x^2}{2} + dfrac{x^4}{24} + o(x^5) (x → 0)$, there exists $δ > 0$ such that$$
    1 - frac{x^2}{2} < cos x < 1 - frac{x^2}{2} + frac{x^4}{23}. quad forall |x| < δ
    $$

    For $n > left( dfrac{t}{δ} right)^{frac{2}{1 - β}}$,begin{align*}
    sum_{k = 1}^n c_{n, k} &leqslant sum_{k = 1}^n frac{1}{k^β} left( -frac{k^2 t^2}{2n^{3 - β}} + frac{k^4 t^4}{23n^{2(3 - β)}} right) = -sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} + sum_{k = 1}^n frac{k^{4 - β} t^4}{23n^{2(3 - β)}}\
    &leqslant -frac{t^2}{2n^{3 - β}} int_0^n x^{2 - β} ,d x + n · frac{n^{4 - β} t^4}{23n^{2(3 - β)}} = -frac{t^2}{2(3 - β)} + frac{t^4}{23n^{1 - β}},
    end{align*}
    $$
    sum_{k = 1}^n c_{n, k} geqslant -sum_{k = 1}^n frac{1}{k^β} · frac{k^2 t^2}{2n^{3 - β}} geqslant -frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
    $$

    thus $limlimits_{n → ∞} sumlimits_{k = 1}^n c_{n, k} = -dfrac{t^2}{2(3 - β)}$. Applying Exercise 3.1.1., $dfrac{S_n}{n^{frac{3 - β}{2}}} Rightarrow cχ$, where $c = dfrac{1}{sqrt{3 - β}}$.






    share|cite|improve this answer









    $endgroup$


















      1





      +200







      $begingroup$

      $defe{mathrm{e}}defi{mathrm{i}}defd{mathrm{d}}$As is written at the start of exercise section, $X_1, X_2, cdots$ are independent.



      Define $X_{n, k} = dfrac{X_k}{n^{frac{3 - β}{2}}}$ for $1 leqslant k leqslant n$. Since Lindeberg's condition does not apply for ${X_{n, k} mid 1 leqslant k leqslant n}$, so the proposition has to be proved directly. Since$$
      φ_{n, k}(t) := E(exp(i t X_{n, k})) = frac{1}{k^β} cosfrac{kt}{n^{frac{3 - β}{2}}} + left( 1 - frac{1}{k^β} right), quad forall t in mathbb{R}
      $$

      it suffices to prove that there exists a constant $c$ that$$lim_{t → ∞} prod_{k = 1}^n φ_{n, k}(t) = expleft( -frac{1}{2} c^2 t^2 right). quad forall t in mathbb{R}
      $$



      For a fixed $t$, in order to apply Exercise 3.1.1., denote $c_{n, k} = φ_{n, k}(t) - 1 = dfrac{1}{k^β} left( cosdfrac{kt}{n^{frac{3 - β}{2}}} - 1 right)$, it suffices to prove that$$
      lim_{n → ∞} max_{1 leqslant k leqslant n} |c_{n, k}| = 0, quad lim_{n → ∞} sum_{k = 1}^n c_{n, k} = -frac{1}{2} c^2 t^2, quad sup_{n geqslant 1} sum_{k = 1}^n |c_{n, k}| < +∞.
      $$

      Since $|c_{n, k}| leqslant dfrac{1}{k^β} · dfrac{1}{2} left( dfrac{kt}{n^{frac{3 - β}{2}}} right)^2 = dfrac{k^{2 - β} t^2}{2n^{3 - β}} leqslant dfrac{t^2}{2n}$, then $limlimits_{n → ∞} maxlimits_{1 leqslant k leqslant n} |c_{n, k}| = 0$ and$$
      sum_{k = 1}^n |c_{n, k}| leqslant sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} leqslant frac{t^2}{2n^{3 - β}} int_1^{n + 1} x^{2 - β} ,d x leqslant frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
      $$

      which implies $suplimits_{n geqslant 1} sumlimits_{k = 1}^n |c_{n, k}| < +∞$.



      Now, since $cos x = 1 - dfrac{x^2}{2} + dfrac{x^4}{24} + o(x^5) (x → 0)$, there exists $δ > 0$ such that$$
      1 - frac{x^2}{2} < cos x < 1 - frac{x^2}{2} + frac{x^4}{23}. quad forall |x| < δ
      $$

      For $n > left( dfrac{t}{δ} right)^{frac{2}{1 - β}}$,begin{align*}
      sum_{k = 1}^n c_{n, k} &leqslant sum_{k = 1}^n frac{1}{k^β} left( -frac{k^2 t^2}{2n^{3 - β}} + frac{k^4 t^4}{23n^{2(3 - β)}} right) = -sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} + sum_{k = 1}^n frac{k^{4 - β} t^4}{23n^{2(3 - β)}}\
      &leqslant -frac{t^2}{2n^{3 - β}} int_0^n x^{2 - β} ,d x + n · frac{n^{4 - β} t^4}{23n^{2(3 - β)}} = -frac{t^2}{2(3 - β)} + frac{t^4}{23n^{1 - β}},
      end{align*}
      $$
      sum_{k = 1}^n c_{n, k} geqslant -sum_{k = 1}^n frac{1}{k^β} · frac{k^2 t^2}{2n^{3 - β}} geqslant -frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
      $$

      thus $limlimits_{n → ∞} sumlimits_{k = 1}^n c_{n, k} = -dfrac{t^2}{2(3 - β)}$. Applying Exercise 3.1.1., $dfrac{S_n}{n^{frac{3 - β}{2}}} Rightarrow cχ$, where $c = dfrac{1}{sqrt{3 - β}}$.






      share|cite|improve this answer









      $endgroup$
















        1





        +200







        1





        +200



        1




        +200



        $begingroup$

        $defe{mathrm{e}}defi{mathrm{i}}defd{mathrm{d}}$As is written at the start of exercise section, $X_1, X_2, cdots$ are independent.



        Define $X_{n, k} = dfrac{X_k}{n^{frac{3 - β}{2}}}$ for $1 leqslant k leqslant n$. Since Lindeberg's condition does not apply for ${X_{n, k} mid 1 leqslant k leqslant n}$, so the proposition has to be proved directly. Since$$
        φ_{n, k}(t) := E(exp(i t X_{n, k})) = frac{1}{k^β} cosfrac{kt}{n^{frac{3 - β}{2}}} + left( 1 - frac{1}{k^β} right), quad forall t in mathbb{R}
        $$

        it suffices to prove that there exists a constant $c$ that$$lim_{t → ∞} prod_{k = 1}^n φ_{n, k}(t) = expleft( -frac{1}{2} c^2 t^2 right). quad forall t in mathbb{R}
        $$



        For a fixed $t$, in order to apply Exercise 3.1.1., denote $c_{n, k} = φ_{n, k}(t) - 1 = dfrac{1}{k^β} left( cosdfrac{kt}{n^{frac{3 - β}{2}}} - 1 right)$, it suffices to prove that$$
        lim_{n → ∞} max_{1 leqslant k leqslant n} |c_{n, k}| = 0, quad lim_{n → ∞} sum_{k = 1}^n c_{n, k} = -frac{1}{2} c^2 t^2, quad sup_{n geqslant 1} sum_{k = 1}^n |c_{n, k}| < +∞.
        $$

        Since $|c_{n, k}| leqslant dfrac{1}{k^β} · dfrac{1}{2} left( dfrac{kt}{n^{frac{3 - β}{2}}} right)^2 = dfrac{k^{2 - β} t^2}{2n^{3 - β}} leqslant dfrac{t^2}{2n}$, then $limlimits_{n → ∞} maxlimits_{1 leqslant k leqslant n} |c_{n, k}| = 0$ and$$
        sum_{k = 1}^n |c_{n, k}| leqslant sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} leqslant frac{t^2}{2n^{3 - β}} int_1^{n + 1} x^{2 - β} ,d x leqslant frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
        $$

        which implies $suplimits_{n geqslant 1} sumlimits_{k = 1}^n |c_{n, k}| < +∞$.



        Now, since $cos x = 1 - dfrac{x^2}{2} + dfrac{x^4}{24} + o(x^5) (x → 0)$, there exists $δ > 0$ such that$$
        1 - frac{x^2}{2} < cos x < 1 - frac{x^2}{2} + frac{x^4}{23}. quad forall |x| < δ
        $$

        For $n > left( dfrac{t}{δ} right)^{frac{2}{1 - β}}$,begin{align*}
        sum_{k = 1}^n c_{n, k} &leqslant sum_{k = 1}^n frac{1}{k^β} left( -frac{k^2 t^2}{2n^{3 - β}} + frac{k^4 t^4}{23n^{2(3 - β)}} right) = -sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} + sum_{k = 1}^n frac{k^{4 - β} t^4}{23n^{2(3 - β)}}\
        &leqslant -frac{t^2}{2n^{3 - β}} int_0^n x^{2 - β} ,d x + n · frac{n^{4 - β} t^4}{23n^{2(3 - β)}} = -frac{t^2}{2(3 - β)} + frac{t^4}{23n^{1 - β}},
        end{align*}
        $$
        sum_{k = 1}^n c_{n, k} geqslant -sum_{k = 1}^n frac{1}{k^β} · frac{k^2 t^2}{2n^{3 - β}} geqslant -frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
        $$

        thus $limlimits_{n → ∞} sumlimits_{k = 1}^n c_{n, k} = -dfrac{t^2}{2(3 - β)}$. Applying Exercise 3.1.1., $dfrac{S_n}{n^{frac{3 - β}{2}}} Rightarrow cχ$, where $c = dfrac{1}{sqrt{3 - β}}$.






        share|cite|improve this answer









        $endgroup$



        $defe{mathrm{e}}defi{mathrm{i}}defd{mathrm{d}}$As is written at the start of exercise section, $X_1, X_2, cdots$ are independent.



        Define $X_{n, k} = dfrac{X_k}{n^{frac{3 - β}{2}}}$ for $1 leqslant k leqslant n$. Since Lindeberg's condition does not apply for ${X_{n, k} mid 1 leqslant k leqslant n}$, so the proposition has to be proved directly. Since$$
        φ_{n, k}(t) := E(exp(i t X_{n, k})) = frac{1}{k^β} cosfrac{kt}{n^{frac{3 - β}{2}}} + left( 1 - frac{1}{k^β} right), quad forall t in mathbb{R}
        $$

        it suffices to prove that there exists a constant $c$ that$$lim_{t → ∞} prod_{k = 1}^n φ_{n, k}(t) = expleft( -frac{1}{2} c^2 t^2 right). quad forall t in mathbb{R}
        $$



        For a fixed $t$, in order to apply Exercise 3.1.1., denote $c_{n, k} = φ_{n, k}(t) - 1 = dfrac{1}{k^β} left( cosdfrac{kt}{n^{frac{3 - β}{2}}} - 1 right)$, it suffices to prove that$$
        lim_{n → ∞} max_{1 leqslant k leqslant n} |c_{n, k}| = 0, quad lim_{n → ∞} sum_{k = 1}^n c_{n, k} = -frac{1}{2} c^2 t^2, quad sup_{n geqslant 1} sum_{k = 1}^n |c_{n, k}| < +∞.
        $$

        Since $|c_{n, k}| leqslant dfrac{1}{k^β} · dfrac{1}{2} left( dfrac{kt}{n^{frac{3 - β}{2}}} right)^2 = dfrac{k^{2 - β} t^2}{2n^{3 - β}} leqslant dfrac{t^2}{2n}$, then $limlimits_{n → ∞} maxlimits_{1 leqslant k leqslant n} |c_{n, k}| = 0$ and$$
        sum_{k = 1}^n |c_{n, k}| leqslant sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} leqslant frac{t^2}{2n^{3 - β}} int_1^{n + 1} x^{2 - β} ,d x leqslant frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
        $$

        which implies $suplimits_{n geqslant 1} sumlimits_{k = 1}^n |c_{n, k}| < +∞$.



        Now, since $cos x = 1 - dfrac{x^2}{2} + dfrac{x^4}{24} + o(x^5) (x → 0)$, there exists $δ > 0$ such that$$
        1 - frac{x^2}{2} < cos x < 1 - frac{x^2}{2} + frac{x^4}{23}. quad forall |x| < δ
        $$

        For $n > left( dfrac{t}{δ} right)^{frac{2}{1 - β}}$,begin{align*}
        sum_{k = 1}^n c_{n, k} &leqslant sum_{k = 1}^n frac{1}{k^β} left( -frac{k^2 t^2}{2n^{3 - β}} + frac{k^4 t^4}{23n^{2(3 - β)}} right) = -sum_{k = 1}^n frac{k^{2 - β} t^2}{2n^{3 - β}} + sum_{k = 1}^n frac{k^{4 - β} t^4}{23n^{2(3 - β)}}\
        &leqslant -frac{t^2}{2n^{3 - β}} int_0^n x^{2 - β} ,d x + n · frac{n^{4 - β} t^4}{23n^{2(3 - β)}} = -frac{t^2}{2(3 - β)} + frac{t^4}{23n^{1 - β}},
        end{align*}
        $$
        sum_{k = 1}^n c_{n, k} geqslant -sum_{k = 1}^n frac{1}{k^β} · frac{k^2 t^2}{2n^{3 - β}} geqslant -frac{t^2}{2(3 - β)} left( frac{n + 1}{n} right)^β,
        $$

        thus $limlimits_{n → ∞} sumlimits_{k = 1}^n c_{n, k} = -dfrac{t^2}{2(3 - β)}$. Applying Exercise 3.1.1., $dfrac{S_n}{n^{frac{3 - β}{2}}} Rightarrow cχ$, where $c = dfrac{1}{sqrt{3 - β}}$.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Dec 10 '18 at 13:38









        SaadSaad

        19.7k92352




        19.7k92352






























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