We consider a functional stochastic delay semilinear Rayleigh–Stokes equation involving Riemann–Liouville derivative. Our aim is using the resolvent theory, fixed point argument to prove the global solvability and gives some sufficient conditions to ensure the asymptotic stability of mild solutions in the mean square moment.
Fractional differential equations are naturally used to model anomalous diffusion processes [8]. This is due to both the rapid development of fractional calculus theory and its applications in various fields, such as physics, fluid mechanics, viscoelasticity, heat conduction in materials with memory, chemistry, and engineering [4,11,12,16,23]. As a result, fractional differential equations have gained significant attention in recent years.
Stochastic differential equations are commonly used in many different fields, including physics, chemistry, economics, social sciences, finance, and engineering. The existence of solutions for fractional stochastic differential equations has been studied in the literature [7,22,24]. Fractional Brownian motion (fBm) is a type of Gaussian process with continuous sample paths. It is indexed by the Hurst parameter . fBm is a self-similar process with stationary increments, and it has long-range memory when . These properties make fBm a naturally occurring noise component in many physical phenomena, as well as in mathematical finance, communication networks, hydrology, and medicine. For more information on fBm, please see the following articles [1,5–7,13,14,21,27].
The Rayleigh–Stokes equation is a mathematical equation that describes the flow of non-Newtonian fluids in cylinders. It is a nonlinear equation, which makes it difficult to solve analytically. However, there are a number of numerical methods that have been developed to solve the equation, and there are also some analytic solutions available for special cases. Let be a bounded domain with smooth boundary and be a probability space. We consider following problem
where , , , stands for the Riemann–Liouville derivative of order α defined by
where for , . In this model, is defined by with h being a continuous function on such that and . For convenience, throughout this paper we denote then is a nonlinear map, which will be specified in Section 2, is a fractional Brownian motion with on a real line and a separable Hilbert space , is given.
Equation (1.1) comes from a generalized Rayleigh–Stokes problem, which was described in [15,26]. The fractional derivative term in the equation is important for describing the viscoelasticity of the fluids. In the literature, various numerical methods have been developed for solving the Rayleigh–Stokes problem in the linear case, such as those described in [2,3,9,10,25,30]. Analytic solutions to this problem have also been obtained in some special cases [15,17,26,29,31]. Recently, some inverse problems involving equation (1.1) have been addressed in [19,20,28], where the state function is identified from its terminal value.
In this work, we study a nonlinear stochastic model with a delayed term in the nonlinearity f. This model describes a situation in which the external force depends on the history state of the system. It is important to note that the presence of a delayed term can reduce the performance and affect the stability of the system. A typical example of a delayed term is , , where . This is called a proportional delay. For this model, the long-time behavior of solutions has not been addressed in the literature, and we aim to fill this gap. We first prove that the problem is globally solvable. Then, we analyze some sufficient conditions for the asymptotic stability of our system.
Our paper is structured as follows. In the next section, we review the key properties of fractional Brownian motion, relaxation functions, and resolvent operators. Section 3 is devoted to proving the solvability of our problem. In the final section, we present the mean square moment asymptotic stability of the solutions.
Preliminaries
Fractional Brownian motion
We first recall the definition of Wiener integrals with respect to an infinite dimensional fractional Brownian motion with Hurst index (see [21]).
Let be a complete probability space with a normal filtration and be an arbitrary fixed horizon. An one-dimensional fractional Brownian motion (fBm) with Hurst parameter is a centered Gaussian process with the covariance function
It is known that with admits the following Volterra representation
where β is a standard Brownian motion and the Volterra kernel is given by
For the deterministic function , the fractional Wiener integral of φ with respect to is defined by
where .
Let and be two real, separable Hilbert spaces and be the space of bounded linear operators from to . For the sake of convenience, we shall use the same notation to denote the norms in , and . Let be a complete orthonormal basis in and be an operator defined by with finite trace , where are non-negative real numbers. We define the infinite dimensional fBm on Y with covariance Q as where are real, independent fBm’s. This process is a -valued Gaussian, it starts from 0, has zero mean and covariance:
In order to define Wiener integrals with respect to the Q-fBm , we introduce the space of all Q-Hilbert–Schmidt operators . We recall that is called a Q-Hilbert–Schmidt operator if
and that the space equipped with the inner product is a separable Hilbert space.
The fractional Wiener integral of the function with respect to Q-fBm is defined by
where is the standard Brownian motion used to present . We have the following fundamental inequality which was proved in [6].
Ifsatisfiesthen the sum in (2.1) is well defined as an-valued random variable and we have
Resolvent operators and representation of solutions
In this section give a representation of solutions to (1.1)–(1.3) by using a resolvent operator and recall some basic properties of this operator.
Consider the relaxation problem
where the unknown ω is a scalar function, β and θ are positive parameters. We collect some properties of ω in Proposition 2.2
The function ω is completely monotone for, i.e.forand. Consequently, ω is a nonincreasing function.
, for all.
, for any.
For fixedand, the functionis nonincreasing on.
Denote by the solution of (2.2)–(2.3), respecting to parameter β. In what follows, we use the notation to express the Laplace convolution of u and v, i.e.
We now concern with the inhomogeneous problem
where , and g is a continuous function. The representation of v is given in Proposition 2.2.
Recall that , let be the orthonormal basis of consisting of the eigenfunctions of the Laplacian subject to homogeneous Dirichlet boundary condition, that is
where we can assume that is an increasing sequence, and as . Then one can give a representation of solution to the linear problem
where and . Indeed, let
Then
Employing Proposition 2.3, we get
This implies
where is the resolvent operator defined by
We recall some properties of the resolvent operator in the following lemma.
We are in a position to prove a Halanay type inequality for the stability analysis in the next section.
Let w be a continuous and nonnegative function satisfying
where,andwhich is nondecreasing. Then
In addition, ifis bounded onthen
In particular, ifthenas.
We use the following result [21]: if is a nonnegative function satisfying
where is a nondecreasing function and , then
We can conclude from (2.12) that
here we employed Proposition 2.2(4). Since is nondecreasing, it is evident that the function is nondecreasing as well. Applying inequality (2.16) for and , we get (2.14) as desired. Now assume that is bounded on . Then by (2.14), is bounded by
and therefore the limit exists. Since as , for any , one can find such that
According to the last estimate, we see that
provided t chosen such that
which is possible since as and . Equation (2.17) yields that
which implies that
Hence
thanks to the fact that is an arbitrarily positive number.
To prove the main results, we use the following hypothesis throughout this work
G
The map satisfies
Solvability results
Based on representation (2.9), we give Definition 3.1.
Let be given. An -valued stochastic process is said to be a mild solution to the problem (1.1)–(1.3) on the interval iff
,
for ,
, .
For given , denote .
For , we define as follows
Hence, we have
In what follows, we use the notation for the sup norm in the spaces and . For example if then .
Let be the operator defined by
which will be referred to as the solution operator. This operator is continuous if f is a continuous map. Obviously, u is a fixed point of Σ iff is a mild solution of (1.1)–(1.3).
We show global existence for (1.1)–(1.3) in the next theorems.
Letbe a continuous mapping such that
(F1)for alland, whereis a nonnegative function andis a continuous and nonnegative function obeying that
Then there existssuch that the problem (1.1)–(1.3) has at least one mild solution on, provided.
Let
Then by assumption, one can take such that . In addition, there is such that
and
Let
then provided that . Indeed, we see that
then
Denote by the closed ball in centered at origin with radius τ. Considering , we have
where
Put . If such that , then
So
And
thanks to Lemma 2.4(4) and assumption (F1).
For , we have
Combining (3.2), (3.3), (3.4) we arrive at
We have shown that , provided . Consider . In order to apply the Schauder fixed point theorem, it remains to check that Σ is a compact operator. It should be noted that, Σ admits the representation
where . According to the compactness of stated in Lemma 2.4, we conclude that Σ is compact. The proof is complete.
In the following theorem, we prove the existence and uniqueness of the solution.
Letbe a continuous mapping such that
(F2)andfor allandsuch that, whereis a nonnegative function and η is a continuous function obeying that
Then there existssuch that the problem (1.1)–(1.3) has a unique mild solution on, provided.
The existence result can be obtained by applying Theorem 3.1 with . It remains to prove the uniqueness. Assume that , are solutions of (1.1)–(1.3). Let , then
thanks to Lemma 2.4(4) and the fact that for . It is evident that the last integral does not decrease in t then
by means of the Halanay inequality in Lemma 2.6 with , implies that . a.e. The proof is complete.
Mean square moment asymptotic stability
Our goal of this section is to prove the mean square moment asymptotic stability of zero solution to (1.1).
Letbe a continuous mapping such that
(F3)andfor allandsuch that, whereis a nonnegative function and η is a continuous function satisfying that
Then the zero solution of (1.1) is mean square asymptotically stable.
Let . Choosing such that , we can find such that for all . Reasoning as in the proof of Theorems 3.1 and 3.2, there exists such that the problem (1.1)–(1.3) has a unique mild solution as long as , which is defined on for all .
The final step involves proving that the obtained solution is asymptotically stable. Let be the solution of (1.1) with respect to the initial datum such that . Moreover, we have
Employing Lemma 2.6 again with , , we obtain
and
which implies the mean square asymptotic stability of the zero solution of (1.1). The proof is complete.
To show that a function g exists such that it satisfies the Assumptions (G) in (2.18), we consider function define by
with . For , we have
thanks to the fact that , , and Proposition 2.2 (1), (3). Hence the Assumptions (G) is fulfilled.
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