Research Article | Open Access

# Moderate Deviations for Stochastic Fractional Heat Equation Driven by Fractional Noise

**Academic Editor:**Maricel Agop

#### Abstract

We consider a class of stochastic fractional heat equations driven by fractional noises. A central limit theorem is given, and a moderate deviation principle is established.

#### 1. Introduction

Since the work of Freidlin and Wentzell [1], the large deviation principle (LDP) has been extensively developed for small noise systems and other types of models (such as interacting particle systems) (see [2–7]). Cardon-Weber [2] proved a LDP for a Burgers-type SPDE driven by white noise. Marquez-Carreras and Sarra [3] proved a LDP for a stochastic heat equation with spatially correlated noise, and Mellali and Mellouk [4] extended Marquez-Carreras and Sarra’s [3] to a fractional operator. Jiang et al. [5] proved a LDP for a fourth-order stochastic heat equation driven by fractional noise. Budhiraja et al. [6] studied large deviation properties of systems of weakly interacting particles. Budhiraja et al. [7] proved a large deviation for Brownian particle systems with killing.

Similar to the large deviation, the moderate deviation problems also come from the theory of statistical inference. Using the moderate deviation principle (MDP), we can get the rate of convergence and an important method to construct asymptotic confidence intervals, for example, Liming [8], Guillin and Liptser [9], Cattani and Ciancio [10], and other references therein. There are also many works about MDP about stochastic (partial) differential equations; some surveys and literatures could be found in Budhiraja et al. [11], Wang and Zhang [12], Li et al. [13], Yang and Jiang [14], and the references therein. On the other hand, fractional equations have attracted many physicists and mathematicians due to various applications in risk management, image analysis, and statistical mechanics (see Droniou and Imbert [15], Bakhoum and Toma [16], Levy and Pinchas [17], Mardani et al. [18], Niculescu et al. [19], Paun [20], and Pinchas [21] for a survey of applications). Stochastic partial differential equations involving a fractional Laplacian operator have been studied by many authors; see Mueller [22], Wu [23], Liu et al. [24], Wu [25], and the references therein.

Motived above, we investigated the moderate deviations about the stochastic fractional heat equation with fractional noise as follows: where , , is the fractional Laplacian operator which is defined in Appendix, and denotes a fractional noise which is fractional in time and white in space with Hurst parameter ; that is, is a mean zero Gaussian random field on with covariance.

Assume that the coefficients satisfy the following.

*Assumption 1. *Function is Lipschitz; that is, there exist an satisfying

Under the conditions of Assumption 1, (1) possesses a unique solution in the sense of Walsh [26] as follows:

As the parameter , the solution of (1) will tend to which is the solution to the following equation:

This paper mainly devotes to investigate the deviations of from the deterministic solution , as , that is, the asymptotic behavior of the trajectories. where is the same deviation scale that strongly influences the asymptotic behavior of .

If , we are in the domain of large deviation estimate, which can be proved similarly to Jiang et al. [5].

The case provides the central limit theorem. As , we will prove that converges to a random field in this paper.

To fill the gap between scale and scale , we mainly devote to the moderate deviation when the scale satisfies the following:

This paper is organized as follows. In Section 2, the definition of the fractional noise is given. In Section 3, the main result is given and proved. In Appendix, some results about the Green kernel are given.

#### 2. Fractional Noise

Let , and is a centered Gaussian family of random variables with the covariance satisfying with and covariance kernel , where denotes the Lebesgue measure of the set and denotes the class of Borel sets in .

We denote as the set of step functions on . Let be the Hilbert space defined as the closure of with respect to the scalar product.

According to Nualart and Ouknine [27], the mapping can be extended to an isometry between and the Gaussian space associated with and denoted by

Define the linear operator by where is defined by with , and one can get

Moreover, satisfies the following:

Then, since one can get where and in are any step functions. So the operator gives an isometry between the Hilbert space and . Hence, defined by is a space-time white noise, and has the following form:

Therefore, the mild formulation of (4) has the following form:

That is, the last term of (4) is equal to

The following embedding proposition is given by Nualart and Ouknine [27].

Lemma 1. *Set **, then, we have*

#### 3. Main Results and Their Proof

##### 3.1. Main Results

For any function defined on , let where , , and . Let

Let be the functions which satisfy , endowed with the .

Define which is a Cameron-Martin space endowed with the norm

Suppose . Now, let be the solution of the following deterministic equation: where satisfies the following:

The above (26) shows a unique mild solution.

Similar to Jiang et al. [5], one can get the following:

Theorem 1. *Let **. Under Assumption*1*, the law of the solution to* (1) *satisfies a deviation principle on ** with the good rate function:**with the convention **, where *.*More precisely, for any Borel measurable subset B of *,
*where ** and ** denote the interior and the closure of **, respectively.*

We furthermore suppose that the coefficients satisfy the following.

*Assumption 2. * is differentiable, and the derivative of is Lipschitz. That is to say, there exist positive constant and which satisfy the following:

Together with the Lipschitz of , we conclude that

Now, we give the following central limit theorem.

Theorem 2. *Let ** and its derivative ** satisfy* Assumptions 1* and *2*. Then, for *,* converges in ** to a random field ** on ** with **, determined by**for all, *.

Let the function be the solution to the following partial differential equation:

Under Assumptions 1 and 2, by Theorem 1, one can get which satisfies large deviation principles on with the speed and the good rate function satisfies the following:

Now, the second result is given as follows:

Theorem 3. *In moderate deviation principle, let **. Under the Assumptions*1*and*2*, then, the random field ** satisfies a large deviation principle on the space ** with speed ** and the good rate function ** defined by* (36)*, where *.

##### 3.2. Convergence of the Solution

Lemma 2. *Let **. Under Assumption*1*, then, there exists a unique solution to* (1)*. Moreover, for any *,

As , we get the convergence of as follows:

Proposition 1. *Let ** and **. By Assumption *1*, there exists a constant * which *satisfies* the following:

*Proof. *Note that
One can get
Set
Together with Hölder’s inequality, the Lipschitz condition (C) and (5) of Lemma A.1, for , we have
where .

For any , , , by (21) and (A.9), there exist which satisfies the following:
Similarly,
By (21), (A.10), and (A.11), for , one can get
and
Together with (43), (44), (45), and (46), one can get for any and , there exists a constant such that
where is independent of . For , by Garsis-Rodemich-Rumser’s Lemma, there exist a constant *C* and a random variable satisfying
and
If we choose in (48), we can get
By Gronwall’s inequality and (40), (42), and (48), there exists a constant satisfying
The proof is completed.

##### 3.3. The Proof of the Main Results

We first prove Theorem 1. Our proof is based on the following proposition (see Doss and Priouret [28]).

Proposition 2. * Suppose and are two families of random variables and and are two Polish spaces. Suppose that*(1)

*(2)*

*There exists a map**such that**is continuous for any*.*(3)*

*satisfies with the rate functions*.

*For any**, there exist**and**satisfying**and**for all**, and*

*Then,**satisfies a LDP with the rate functions*To prove Theorem 1, one only needs to prove (i)Under some topology, is continuous for any .(ii)In Freidin-Wentzell inequality, for any , , and , there exist a satisfying

Theorem 4. *When the level set ** is endowed with the topology of uniform convergence on ** ,**is the continuous map for any *.

*Proof. *One only needs to prove that for fixed , ,
Note that
Using (A.5) in Appendix with , then , one can get
Now, we deal with , together with
Recalling is defined by (11), one can get
Using Gronwall’s inequality, we can get
The proof of the theorem is completed.

We now prove the Freidin-Wentizell inequality as follows:

Suppose is a Brownian sheet. For and , we define and

Using Girsanov’s theorem, the process is a Brownian sheet under . Suppose is a solution of (1) under . Then,

Now, one can prove (36). Note that, under , then,

So under , by Gronwall’s Lemma, one can get

Now, one can change (34) the proof to the following theorem.

Theorem 5. *Suppose ** and **. For each **, **, and **, there exists a constant ** satisfying*

In the following, we give a key Lemma to prove Theorem 1, which is similar to Candon-Weber [2], and the proof is omitted.

Lemma 3. *Suppose *, * and ** satisfying*for any and . Suppose which *is an almost surely continuous, **-adapted process satisfying *, *and for **, suppose**Then, for any ** , there exist a positive constant C, Ç*(), *and **such that for all**Ç*(),

*Proof of Theorem 5. *Suppose
Then, there exists satisfying