Suppose that $\{\xi_i, -\infty<i<\infty\}$ is a doubly infinite sequence of random variables, and $\{a_i, -\infty<i<\infty\}$ is an absolutely summable sequence of real numbers. Let
be the moving average process based on $\{\xi_i, -\infty<i<\infty\}$. So far, there were detailed studies about the asymptotic behavior of the moving average process $\{X_k, k\geq1\}$.
Throughout the paper, let $N$ be the standard normal random variable. We denote by $C$ a positive constant which may vary from place to place, $\xrightarrow{d}$ means convergence in distribution, and $\lfloor x\rfloor=\sup\{m:m\leq x, m\in\mathbb{Z}^+\}$. Also we let $\log x=\ln(x\vee e)$ and $\log\log x=\ln(\ln(x\vee e^e))$.
We begin with a brief review of the definition of $\varphi$-mixing. Let $\mathbb{F}_k^l$ denote the $\sigma$-field generated by $X_k, X_{k+1}, \cdots, X_l$ and define
A sequence $\{X_n\}_{n\geq1}$ of random variables is said to be $\varphi$-mixing if $\varphi(n)\rightarrow0$ and $\varrho$-mixing if $\varrho(n)\rightarrow0$. It is well known that a $\varphi$-mixing sequence is $\varrho$-mixing, since $\varrho(n)\leq 2\varphi^{1/2}(n)$.
In the sequel, we suppose $\{\xi_i, -\infty<i<\infty\}$ is a sequence of identically distributed and $\varphi$-mixing random variables with zero mean and finite variance with $0<\sigma^2=\mathbb{E}\xi_1^2+2\sum\limits_{k=2}^\infty\mathbb{E}\xi_1\xi_k<\infty$ and $\sum\limits_{m=1}^\infty\varphi^{1/2}(m)<\infty$. For the moving average processes $\{X_k, k\geq1\}$ defined in (1.1), where $\{a_i, -\infty<i<\infty\}$ is a sequence of real numbers with $\sum\limits_{i=-\infty}^\infty|a_i|<\infty$, we set $S_n=\sum\limits_{k=1}^nX_k$ and $\tau=\sigma\cdot\sum\limits_{i=-\infty}^\infty a_i$.
Li and Zhang [1] showed the precise rates in the law of the iterated logarithm of the moving average process defined in (1.1) for $\varphi$-mixing or negatively associated sequences under conditions above. For any $\delta\geq0$, if $\mathbb{E}\xi_1^2(\log\log|\xi_1|)^{\delta-1}<\infty$, they proved that
where $\Gamma(\cdot)$ is a Gamma function.
In this paper, we consider the general law of complete convergence rates of the moving average process $\{X_k, k\geq1\}$ defined in (1.1) for $\varphi$-mixing sequences, and we have the following results.
Theorem 1.1 Suppose that $g(x)$ is a positive and differentiable function defined on $[n_0, \infty)$, which is strictly increasing to $\infty$. For $b>0$, assume that $\phi(x)=g'(x)g^{b-1}(x)$ is monotone nondecreasing or monotone nonincreasing on $[n_0, \infty)$, and if $\phi(x)$ is monotone nondecreasing, we assume that $\lim\limits_{x\rightarrow\infty}\phi(x+1)/\phi(x)=1$. If $\mathbb{E}|\xi_1|^{2+\delta}<\infty$ for some $\delta\geq0$, then we have
where $a_n=o(g^{-s}(n))$ as $n\rightarrow\infty$.
Theorem 1.2 Suppose that $g(x)$ is a positive and differentiable function defined on $[n_0, \infty)$, which is strictly increasing to $\infty$. Assume that $\phi(x)=g'(x)g^{-1}(x)$ is monotone nondecreasing or monotone nonincreasing on $[n_0, \infty)$, and if $\phi(x)$ is monotone nondecreasing, we assume that $\lim\limits_{x\rightarrow\infty}\phi(x+1)/\phi(x)=1$. Then for $s>0$, we have
Remark 1.1 Applying Lemma 2.3 of [1], Theorems 1.1 and 1.2 are still true when $\{\xi_i, -\infty<i<\infty\}$ is a sequence of identically distributed negatively associated random variables with $\mathbb{E}\xi_1=0$, $\mathbb{E}\xi^2_1<\infty$ and $0<\sigma^2=\mathbb{E}\xi_1^2+2\sum\limits_{k=2}^\infty\mathbb{E}\xi_1\xi_k<\infty$.
Remark 1.2 Specially, for $k\geq1$, if we let $a_{2k}=1$ and $a_i=0, -\infty<i<\infty$ for $i\neq 2k$, that is to say, $X_k=\xi_k$ with $\mathbb{E}\xi_1=0$, $\mathbb{E}\xi^2_1<\infty$ and $0<\sigma^2=\mathbb{E}\xi_1^2+2\sum\limits_{k=2}^\infty\mathbb{E}\xi_1\xi_k<\infty$, then for $S_n=\sum\limits_{k=1}^n\xi_k$, Theorems 1.1 and 1.2 are still valid for $\tau=\sigma$.
Remark 1.3 The conditions about $\phi(x)$ and $g(x)$ in the above two theorems are mild for many common functions like $g(x)=x^\gamma$, $(\log x)^\gamma$ and $(\log\log x)^\gamma$ with $\gamma>0$, and the corresponding results were obtained by many researchers.
For proving of our main results, we introduce the following lemmas.
Lemma 2.1(see [2]) Let $\sum\limits_{i=-\infty}^\infty a_i$ be an absolutely convergent series of real numbers with $a=\sum\limits_{i=-\infty}^\infty a_i$ and $k\geq1$, then
Lemma 2.2 Suppose $\{\xi_i, -\infty<i<\infty\}$ is a sequence of identically distributed and $\varphi$-mixing random variables with $\mathbb{E}\xi_1=0$, $\mathbb{E}\xi^2_1<\infty$ and $\sum\limits_{m=1}^\infty\varphi^{1/2}(m)<\infty$, and suppose $0<\sigma^2=\mathbb{E}\xi_1^2+2\sum\limits_{k=2}^\infty\mathbb{E}\xi_1\xi_k<\infty$. For the moving average processes $\{X_k, k\geq1\}$ defined in (1.1) with $\sum\limits_{i=-\infty}^\infty|a_i|<\infty$, we set $S_n=\sum\limits_{k=1}^nX_k$. Then we have $\frac{S_n}{\tau\sqrt{n}}\xrightarrow{d}N, $ where $\tau=\sigma\cdot\sum\limits_{i=-\infty}^\infty a_i.$
Proof The proof is similar to that of Theorem 1 in [3], so we omit it.
Lemma 2.3(see [4]) Let $\{\xi_i; i\geq 1\}$ be a $\varphi$-mixing sequence. $Y_n=\sum\limits_{i=1}^n\xi_i$, $n\geq1$. Suppose that there exists a sequence $\{C_n\}$ of positive numbers such that $\rm{ma}{{\rm{x}}_{1\le i\le \mathit{n}}}\mathbb{E}\mathit{Y}_{\mathit{i}}^{2}\le {{\mathit{C}}_{\mathit{n}}}$. Then for any $q\geq 2$, there exists some constant $C=C(q, \varphi(\cdot))$ such that
In this section, for $M>1$ and $0<\varepsilon<1$, we define
Without loss of generality, we assume $\tau=1$. Next we calculate the left hand side of (1.3) and (1.4) by approximation of partial sums about the tail probability of standard normal random variable $N$.
Proposition 3.1 For $b, s>0$, we have
Proof Since $\lim\limits_{n\rightarrow\infty}a_ng^{s}(n)=0$, then for any $\tilde{\delta}>0$ there exists a positive integer $N_0>n_0$ such that for any $n\geq N_0$, we have $ -\tilde{\delta} \leq a_ng^{s}(n)\leq \tilde{\delta}$. If $\phi(x)=g'(x)g^{b-1}(x)$ is monotone nonincreasing, we have
Let $\tilde{\delta}\downarrow0$, we obtain
If $\phi(x)$ is monotone nondecreasing, by $\lim\limits_{x\rightarrow\infty}\phi(x+1)/\phi(x)=1$, for the $\tilde{\delta}$ mentioned above, there exists a positive integer $N_1$ such that for any $x\geq N_1$, we have $(1-\tilde{\delta})\phi(x+1)\leq\phi(x)\leq(1+\tilde{\delta})\phi(x-1)$. Let $N_2=\max\{N_0, N_1\}$, thus it holds that
similarly, it holds that
Let $\tilde{\delta}\downarrow0$, we obtain (3.3).
Let $y=\varepsilon g^{s}(x)$, we have
Thus proof of this proposition is completed.
Proposition 3.2 For $b, s>0$, we have
Proof Let
then $\Delta_n\rightarrow0$ as $n\rightarrow\infty$ from Lemma 2.2. Using the Toeplitz lemma [5], we have
Proposition 3.3 For $b, s>0$, we have
Proof Since $a_n\rightarrow0$ as $n>b_M(\varepsilon)\rightarrow\infty$, it is enough to show
Let $y=\varepsilon g^{s}(x)/2$, we have
Therefore this proposition is proved.
Proposition 3.4 If $\mathbb{E}|\xi_1|^{2+\delta}<\infty$ for some $\delta\geq0$, then for $0<b<(2+\delta)s$, we have
Proof It suffices to show that
Note that $S_n=\sum\limits_{i=-\infty}^\infty\sum\limits_{k=1}^n a_{k+i}\xi_i=\sum\limits_{i=-\infty}^\infty a_{ni}\xi_i$, where $a_{ni}=\sum\limits_{k=1}^n a_{k+i}$. From Lemma 2.1, we can suppose that
Next, for $x\geq0$, we set
Since $\mathbb{E}\xi_i=0$, then by (3.9) and Markov's inequality, we have
Since
and for $M$ large enough, it holds that
then we obtain
Set
then $\cup_{j\geq1}I_{nj}=\mathbb{Z}$, where $\mathbb{Z}$ is the set of all integers. Note that (referred by [6])
On the one hand, we have
On the other hand, since $\sum\limits_{m=1}^\infty\varphi^{1/2}(m)<\infty$, then we have
Thus using Markov's inequality and Lemma 2.3, we have
where we take $q=2+\delta$ (actually, the above inequality holds for any $q\geq2$).
However, from (3.14), it holds that
then we have
Therefore, using (3.17), we have
Thus we have
then using (3.13), (3.15), (3.16) and (3.18) with $x=0$, it holds that
Therefore we complete the proof of (3.8).
From Propositions 3.1-3.4, applying the triangle inequality, we complete the proof of (1.3). Next we show (1.4). For simplicity, we let $a_n=0$ and omit the discussion of $\phi(x)$, but the process is similar to that of Proposition 3.1.
Proposition 3.5 For $b, s>0$, one has that
Proof We calculate that
Proposition 3.6 For $b, s>0$, one has that
Proof It holds that
where $l(n)=g^{-s}(n)\Delta_n^{-1/2}$ and $\Delta_n$ is defined in (3.5). It is easy to see that
Next for $J_3$, from (3.10)-(3.12) and the fact that for $x\geq l(n)$ and $n$ large enough, we have
Thus using (3.15), (3.16) and (3.18) with $\delta=0$, we have
Therefore using (3.21) and the Toeplitz lemma, we have
Proposition 3.7 For $b, s>0$, one has that
Proof It is easy from the proof of Proposition 3.5.
Proposition 3.8 For $0<b<(1+\delta)s$, one has that
Proof For $M$ large enough and any $x\geq0$, it holds that
Then using (3.22), (3.15), (3.16) and (3.18), we have
Hence it holds that
where $b<(1+\delta)s$.
Finally, the proof of (1.4) is completed by combining Propositions 3.5-3.8 together and using the triangle inequality. We omit the proof of Theorem 1.2 since the idea is similar, and we only need to replace $\varepsilon^{b/s}$ by $1/(-\log\varepsilon)$ with $b=0$.