This paper is concerned with the semilinear time fractional wave equations on the whole Euclidean space, also known as the super-diffusive equations. Considering the initial data in the fractional Sobolev spaces, we prove the local/global well-posedness results of L2-solutions for linear and semilinear problems. The methods of this paper rely upon the relevant wave operators estimates, Sobolev embedding and fixed point arguments.
Citation: Yong Zhou, Jia Wei He, Ahmed Alsaedi, Bashir Ahmad. The well-posedness for semilinear time fractional wave equations on RN[J]. Electronic Research Archive, 2022, 30(8): 2981-3003. doi: 10.3934/era.2022151
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This paper is concerned with the semilinear time fractional wave equations on the whole Euclidean space, also known as the super-diffusive equations. Considering the initial data in the fractional Sobolev spaces, we prove the local/global well-posedness results of L2-solutions for linear and semilinear problems. The methods of this paper rely upon the relevant wave operators estimates, Sobolev embedding and fixed point arguments.
In this paper, we focus on the following time fractional wave equation
∂βtu−Δu=f(u),t>0, | (1.1) |
supplemented with the initial conditions
u(0,x)=ϕ(x), ∂tu(0,x)=ψ(x),x∈RN, | (1.2) |
where ∂βt stands for the Caputo fractional derivative operator of order β∈(1,2), Δ is the Laplacian operator, and f is the semilinear data to be specified later, initial data ϕ, ψ are given in certain fractional Sobolev spaces, likely (ϕ,ψ)∈Hs(RN)×Hs−1(RN) for s∈R.
From the point of view of physics, it's often better for fractional derivatives to fit practical problems than the integer order setting counterpart in many cases, for example, Hamiltonian chaos [1], biophysics [2,3], control engineering [4,5], viscoelasticity [6,7], anomalous diffusion [8,9,10,11], switched systems [12], and other problems [13,14,15]. In particular, the study of the Eq (1.1) has always been an important topic in the mathematical physics as it represents anomalous diffusion phenomena. The time fractional partial differential equation ∂βtu=Δu of order β∈(0,1) models anomalous diffusion phenomena ensuring the behavior of a subdiffusion process driven by a fractional Brownian motion [16]. The case of order β∈(1,2) will govern intermediate processes between diffusion and wave propagation, for example, see [7], and it also ensures the behavior of superdiffusion process, for instance, see [17]. While the cases β→1+ and β→2− respectively correspond to standard diffusion equation (heat equation) and ballistic diffusion (wave equation). Besides, in anomalous diffusion equations of order β∈(0,1) or β∈(1,2), the mean squared displacement of a diffusive particle behavior likes ⟨x2(t)⟩∼tβ, in contrast to normal diffusion behavior (Brownian motion) ⟨x2(t)⟩∼t.
There are many recent interesting works about time fractional wave equations. One of the most favorable reasons is that the integral kernel in fractional time derivative represents memory of a long-time tail of the power order. The investigation of existence and uniqueness of solutions for a low regularity initial data is a matter of interest in the mathematical analysis. For instance, Kian and Yamamoto [18] investigated a weak solution for semilinear case of (1.1) in bounded domain Ω for dimension n=2,3. By using the technique of eigenvalue expansion together with the properties of Mittag-Leffler functions, they established the existence and uniqueness results, which the solution shall lie in Lp(0,T;Lq(Ω))∩C([0,T];H2r(Ω)) for r=min(1−1/β,γ) with some 1≤p,q≤∞,0<γ<1. Following this technique, Alvarez et al. [19] considered the well-posedness for an abstract Cauchy problem in a Hilbert space, where the solutions will lie in Lrq(0,T;L2r(X)) associated with initial data (ϕ,ψ)∈D(Aγ)×L2(X), D(Aγ) are the fractional power spaces with spectrum form of A (nonnegative self-adjoint operator) for some β∈(1,2), γ=1/β, X is a (relatively) compact metric space and q∈(1/(β−1),∞], r>1. Moreover, Otarola and Salgado [20] studied the time and space regularities of weak solutions for the space-time fractional wave equations, Zhou and He [21] discussed a well-posedness and time regularity of mild solutions for time fractional damped wave equations, etc. For more details, we refer to see the papers [22,23,24,25,26].
However, few results discuss the unbounded domain case, likely the whole Euclidean space RN. This is due the difficulty to establish the relevant estimates on solution operators, also the method of eigenvalue expansion is not appropriate to such problem. As we know, Alemida and Precioso [27] investigated the global existence with large initial data in the framework of Besov-Morrey spaces, Alemida and Viana [28] studied existence, stability, self-similarity and symmetries of solutions with initial data in Sobolev-Morrey space. Zhang and Li [29] studied the local existence on C([0,T],Lq(RN)) for βN(p−1)/2<q for a special semilinear function f∼|u|p, also the critical exponents of the global existence and blow-up solutions are determined when ψ≡0 and ψ≢0. Djida et al. [30] worked on a well-posedness result for semilinear space-time fractional wave equations, by adopting the method of Laplace-Fourier transforms, the properties of Mittag-Leffler functions and Fox H-functions.
Our goal in this work is to establish the well-posedness results for time fractional wave equations with initial conditions in certain function spaces. More precisely, we consider linear and semilinear problems on RN, and present a general assumption in semilinear function and a special case λ|u|αu to deal with the current problem. In addition, this paper is devoted to studying the well-posedness of mild solutions to the current problems. In order to obtain the solution operators for time fractional wave equation and their properties, we shall establish some useful estimates which depend on that of wave operators. We remark that our proofs and results are completely different from the previous mentioned works. We will concern about the local/global well-posedness of solutions. Let us now enlist the main results presented in this paper:
Ⅰ. The solution operators of Eq (1.1). Let ϖ=(−Δ)1/2, we know that the wave operators can be given by cos(ϖt) and ϖ−1sin(ϖt). Concerned with the principle of subordination, two inherent relationships between probability density function and wave operators are presented. More precisely, we get
Cσ(t)=∫∞0Mσ(θ)cos(ϖtσθ)dθ,Sσ(t)=σtσ−1∫∞0θMσ(θ)ϖ−1sin(ϖtσθ)dθ. |
To the best of our knowledge, these forms of solution operators are completely different from the existing literatures, relying upon the estimates of wave operators, some useful estimates of solution operators can be derived, which provide very helpful tools for the proofs. Observe that, the probability density function builds a bridge between integer wave equations and the fractional ones.
Ⅱ. The well-posedness results on RN. We first establish some estimates on several spaces for the linear problem with initial data (ϕ,ψ)∈Hs(RN)×Hs−1(RN), and then an existence of L2-solution is established on a space of continuous functions. Next, under the case that the semilinear function f:L2(RN)∩Lr(RN)→Lr′(RN), and satisfies
‖f(u)−f(v)‖Lr′(RN)≤Cα(R)(‖u‖αLr(RN)+‖v‖αLr(RN))‖u−v‖Lr(RN), |
where constants r and α satisfy some restriction requirements, the local well-posedness results of mild solutions are established in the framework of Lγ(Lγ′) spaces, here γ and γ′ are the conjugate indices. By concerning with a special semilinear data f∼λ|u|αu, we show that the local well-posedness on Besov space Bsr,2(RN). Furthermore, when ψ≡0, we also show the global existence to the Cauchy problem (1.1)–(1.2).
The rest of this paper is divided into three sections. In Section 2, some basic notations and useful preliminaries are introduced. In Section 3, for the linear problem, we derive the solution operators and establish their some properties. In addition, two existence of L2-solutions are given. In Section 4, we prove some local/global well-posedness results on Lebesgue and Besov spaces for the semilinear problems.
In this section, some notations and preliminaries related to our work will be introduced.
Denote by Lq(RN) (q≥1) the Lebesgue space of q-integrable functions with the norm ‖⋅‖Lq(RN). Let S(RN) be the Schwartz space and let S′(RN) denote its topological dual, for any v∈S(RN), F represents the Fourier transform
ˆv(ξ)=F(v)(ξ)=(2π)−N/2∫RNe−ix⋅ξv(x)dx, |
with its inverse
ˇv(x)=F−1(v)(x)=(2π)−N/2∫RNeix⋅ξv(x)dx. |
Define the Sobolev space by
Hs,q(RN)={u∈S′(RN): F−1[(1+|ξ|2)s2F(u)]∈Lq(RN)}, |
equipped with the norm
‖u‖Hs,q(RN):=‖F−1[(1+|ξ|2)s2F(u)]‖Lq(RN), |
for s∈R, 1≤q≤∞. We also use the Besov space Bsp,q:=Bsp,q(RN) and homogeneous Besov space ˙Bsp,q:=˙Bsp,q(RN), for s∈R, 1≤p,q≤∞. For the definitions and properties of Besov spaces, we refer to see [31,32]. In particular, it yields H0,p(RN)=Lp(RN) and Hs(RN)=Hs,2(RN)=Bs2,2(RN) for p≥1 and s∈R. Throughout this paper, we denote the notation a≲b that stands for a≤Cb, with a positive generic constant C that does not depend on a,b, the notations ∨, ∧ stand for a∨b=max{a,b} and a∧b=min{a,b}, respectively. Let p and p′ be the conjugate indices such that 1/p+1/p′=1.
Let T>0 (or T=+∞) and let X be a usual Banach space. For any u∈L1(0,T;X) and v∈L1(0,T;X), denote ∗ the convolution by
(u∗v)(t)=∫t0u(t−s)v(s)ds,t≥0, |
and for β≥0 let the weak singular kernel gβ(⋅) be defined by
gβ(t)=tβ−1/Γ(β),t>0, |
where Γ(⋅) is the usual gamma function. We denote g0(t)=δ(t), the Dirac measure is concentrated at the origin. Next, let us recall the concepts of fractional calculus and Mittag-Leffler functions. The Riemann-Liouville fractional integral of order β≥0 for a function v∈L1(0,T;X) is defined as
Jβtv(t)=1Γ(β)∫t0(t−s)β−1v(s)ds=(gβ∗v)(t), t>0. |
Definition 2.1. Let β∈(1,2). Consider a function v∈L1(0,T;X) such that convolution g2−β∗v∈W2,1(0,T;X). The representation
∂βtv(t)=∂2tt(g2−β∗[v(t)−v(0)−t∂tv(0)]),t>0, |
is called the Caputo fractional derivative of order β.
An important special function for the fractional differential equations involving the Caputo fractional derivative is the Mittag-Leffler function, which is defined by
Eν,μ(z)=∞∑k=0zkΓ(νk+μ),z∈C, ν>0, μ∈R. |
Note that, if w(t):=Eν,1(atν), ν∈(0,2), a∈R, then one can check that w is the solution of the equation ∂νtw(t)=aw(t).
Let the probability density function Mυ(⋅) (also called Mainardi's Wright function) be defined by
Mυ(z)=∞∑k=0(−z)kk!Γ(1−υ(k+1)),υ∈(0,1), z∈C. |
For θ>0, this probability density function has the properties
Mυ(θ)≥0,∫∞0θδMυ(θ)dθ=Γ(1+δ)Γ(1+υδ),for −1<δ<∞. | (2.1) |
Next, we see that the probability density function can be viewed as a bridge between the classical and fractional wave equations.
Lemma 2.1. Let β∈(1,2) [21]. Then for z∈C, the following formulas expressing the Mittag-Leffler function in term of probability density function hold:
Eβ,1(−z2)=∫∞0Mβ/2(θ)cos(zθ)dθ,Eβ,β(−z2)=β2z∫∞0θMβ/2(θ)sin(zθ)dθ. |
Lemma 2.2. Let f∈L1(0,T;R) [33]. The unique solution of the fractional order problem
{∂βtu(t)+au(t)=f(t),a≥0, t≥0,u(0)=u0, u′(0)=u1, |
is give by
u(t)=Eβ,1(−atβ)u0+tEβ,2(−atβ)u1+∫t0(t−s)β−1Eβ,β(−a(t−s)β)f(s)ds. |
In particular,
u(t)=cos(√at)u0+1√asin(√at)u1+1√a∫t0sin(√a(t−s))f(s)ds, |
which is the unique solution to the corresponding classical wave equation, i.e., β=2.
In this section, we are concerned with the following linear Cauchy problem
{∂βtu(t,x)−Δu(t,x)=f(t,x),t>0, x∈RN,u(0,x)=ϕ(x), ∂tu(0,x)=ψ(x). | (3.1) |
Without loss of generality, the solutions in this paper are defined as mild solutions associated with the corresponding initial data.
We first establish the solution representation of linear problem (3.1). Let u satisfies (3.1), taking the Fourier transform of both sides in (3.1) with respect to x∈RN, we obtain
{∂βtˆu(t,ξ)+|ξ|2ˆu(t,ξ)=ˆf(t,ξ),t>0,ˆu(0,ξ)=ˆϕ(ξ), ∂tˆu(0,ξ)=ˆψ(ξ). |
It follows from [4,(1.100)] that
tEβ,2(−tβ|ξ|2)=1Γ(2−β)∫t0(t−s)1−βEβ,β(−sβ|ξ|2)sβ−1ds. |
Therefore, by virtue of Lemma 2.2, we get
ˆu(t,ξ)=ˆφ(t,ξ)ˆϕ(ξ)+(ˆϑ(⋅,ξ)∗g2−β)(t)ˆψ(ξ)+(ˆϑ(⋅,ξ)∗ˆf)(t), |
where
ˆφ(t,ξ)=Eβ,1(−tβ|ξ|2),ˆϑ(t,ξ)=tβ−1Eβ,β(−tβ|ξ|2). |
By using the inverse Fourier transform, we get
u(t,x)=∫RNφ(t,x−y)ϕ(y)dy+∫t0∫RNg2−β(t−s)ϑ(s,x−y)ψ(y)dyds+∫t0∫RNϑ(t−s,x−y)f(s,y)dyds, |
where
φ(t,x)=(2π)−N/2∫RNeix⋅ξEβ,1(−tβ|ξ|2)dξ,ϑ(t,x)=(2π)−N/2∫RNeix⋅ξtβ−1Eβ,β(−tβ|ξ|2)dξ. |
On the other hand, set σ=β/2∈(1/2,1), it follows from Lemma 2.1 that
φ(t,x)=(2π)−N/2∫∞0∫RNeix⋅ξMσ(θ)cos(tσθ|ξ|)dξdθ, |
and
ϑ(t,x)=tσ−1(2π)−N/2∫∞0∫RNeix⋅ξσθMσ(θ)sin(tσθ|ξ|)|ξ|dξdθ. |
Let
˙K(t)v=F−1[cos(t|ξ|)ˆv(ξ)],K(t)v=F−1[sin(t|ξ|)|ξ|ˆv(ξ)], |
and
Sσ(t)v=σtσ−1∫∞0θMσ(θ)K(tσθ)vdθ. |
Rewriting u(t) for the function u(t,⋅), we get an equivalent integral representation for problem (3.1) by
u(t)=Cσ(t)ϕ+Pσ(t)ψ+∫t0Sσ(t−s)f(s)ds, | (3.2) |
where solution operators Cσ(⋅) and Pσ(⋅) are defined by
Cσ(t)ϕ=∫∞0Mσ(θ)˙K(tσθ)ϕdθ,Pσ(t)ψ=(g2−2σ∗Sσ)(t)ψ. |
Let α(r)=12−1r for r∈[2,∞], and
β(r)=N+12α(r),γ(r)=(N−1)α(r),δ(r)=Nα(r). |
Let us recall the following two results in [34,35], which play a key role in proving the general results on the solution operators.
Lemma 3.1. Let 2≤p<∞ and 2β(p)≤ν≤2δ(p). Then for t≠0, it follows that
‖F−1[|ξ|−νeit|ξ|ˆv(ξ)]‖Lp(RN)≲|t|ν−2δ(p)‖v‖Lp′(RN). |
Lemma 3.2. Let 2≤p<∞ and 2β(p)≤ν≤2δ(p), 0≤μ≤s+ν. Then for 1≤q≤∞, t≠0, it follows that
‖F−1[|ξ|−μeit|ξ|ˆv(ξ)]‖Bsp,q(RN)≲|t|ν−2δ(p)‖v‖Bs+ν−μp′,q(RN). |
In the sequel, we set ϖ=(−Δ)12 and U(t)=exp(iϖt)=F−1[exp(i|ξ|t)F], so that K(t)=ϖ−1sin(ϖt), ˙K(t)=cos(ϖt). Hence, it follows that
K(t)=ϖ−1U(t)−U(−t)2i,˙K(t)=U(t)+U(−t)2. |
Lemma 3.3. Let N≥2, 2N/(N−1)≤p≤2(N+1)/(N−1), then Sσ(t):Lp′(RN)→Lp(RN) and moreover
‖Sσ(t)v‖Lp(RN)≲t2σ−2σδ(p)−1‖v‖Lp′(RN),v∈Lp′(RN), t>0. |
Proof. Obviously, the condition 2NN−1≤p≤2(N+1)N−1 for N≥2 implies that 1/2≤δ(p)≤N/(N+1). Hence, it follows from Lemma 3.1 that
‖K(t)v‖Lp(RN)≲t1−2δ(p)‖v‖Lp′(RN). |
By the definition of Sσ(t), and (2.1) we obtain
‖Sσ(t)v‖Lp(RN)≤σtσ−1∫∞0θMσ(θ)‖K(tσθ)v‖Lp(RN)dθ≲σtσ−1+σ(1−2δ(p))∫∞0θ2−2δ(p)Mσ(θ)dθ‖v‖Lp′(RN)≲t2σ−2σδ(p)−1‖v‖Lp′(RN). |
Consequently, we get the desired inequality.
Lemma 3.4. Let 2≤p<∞, 1≤q≤∞, s∈R, t>0.
If (−s)∨(2δ(p)−1)∨(2β(p))≤ν≤2δ(p), then
‖Cσ(t)v‖Bsp,q(RN)≲tσ(ν−2δ(p))‖v‖Bs+νp′,q(RN). |
If (1−s)∨(2δ(p)−2)∨(2β(p))≤ν≤2δ(p), then
‖Sσ(t)v‖Bsp,q(RN)≲tσ(ν−2δ(p)+1)−1‖v‖Bs+ν−1p′,q(RN). |
Proof. From Lemma 3.2, for t>0 we have the following estimates
‖˙K(t)v‖Bsp,q(RN)≲tν−2δ(p)‖v‖Bs+νp′,q(RN),0≤s+ν, |
‖K(t)v‖Bsp,q(RN)≲tν−2δ(p)‖v‖Bs+ν−1p′,q(RN),1≤s+ν. |
Therefore, using the same argument as employed in Lemma 3.3, we obtain the desired results.
Let σ=β/2 for β∈(1,2). We now introduce an operator Qσ defined by
(Qσf)(t)=∫t0Sσ(t−s)f(s)ds. |
In order to obtain the existence of L2-solutions, we need the following lemma.
Lemma 3.5. For each h∈Lp(0,T;X) with 1≤p<+∞ [36], we have
limτ→0∫T0‖h(t+τ)−h(t)‖pXdt=0, |
where we suppose that h(s)=0 for s not belonging to [0,T].
Lemma 3.6. For any q and μ with 1<q≤2 and μ=N(1/q−1/2−1/N), N≥1, let f∈Lr(0,T;Hμ,q(RN)), then
‖Qσf‖L∞(0,T;L2(RN))≲‖f‖Lr(0,T;Hμ,q(RN)), |
for r>1/σ. Furthermore, let q=2N/(N+2) for N≥3, and f∈Lr(0,T;Lq(RN)), then
‖Qσf‖L∞(0,T;L2(RN))≲‖f‖Lr(0,T;Lq(RN)). |
Moreover the operator Qσ maps Lr(0,T;H−1(RN)) into C([0,T];L2(RN)).
Proof. The inequality |sinα|≤1∧|α| for any α∈R implies
|(sin(t|ξ|)⟨ξ⟩)/|ξ||≤(1∧(t|ξ|))⋅⟨ξ⟩/|ξ|≤(1+t)⋅(1∧|ξ|)⋅⟨ξ⟩/|ξ|≤√2(1+t), | (3.3) |
where ⟨ξ⟩=√1+|ξ|2, for t≥0, ξ∈RN. Let g(t,s,ξ)=sin((t−s)σθ|ξ|)ˆf(s,ξ)/|ξ| and y(t,x)=⟨x⟩−1ˆf(s,x), for s∈(0,t), ξ,x∈RN. It is clear by (3.3) that
|g(t,s,ξ)|≤√2(1+(t−s)σθ)|y(s,ξ)|,for s∈(0,t), ξ∈RN. |
Then it follows from the Plancherel theorem (see e.g. [37]) that
‖ˇg(t,s,⋅)‖L2(RN)=‖g(t,s,⋅)‖L2(RN)≤√2(1+(t−s)σθ)‖y(s,⋅)‖L2(RN)=√2(1+(t−s)σθ)‖ˇy(s,⋅)‖L2(RN). |
Therefore, we have
‖(Qσf)(t)‖L2(RN)≤∫t0‖Sσ(t−s)f(s,⋅)‖L2(RN)ds≤∫t0∫∞0σ(t−s)σ−1θMσ(θ)‖ˇg(t,s,⋅)‖L2(RN)dθds≤√2∫t0∫∞0σ(t−s)σ−1θMσ(θ)(1+(t−s)σθ)‖ˇy(s,⋅)‖L2(RN)dθds. |
Due to ‖ˇy(s,⋅)‖L2(RN)=‖F−1[⟨ξ⟩−1ˆf(s,ξ)]‖L2(RN)=‖f(s,⋅)‖H−1(RN), in view of (2.1) and Hölder inequality, for r>1/σ we get
‖(Qσf)(t)‖L2(RN)≤√2∫t0∫∞0σ(t−s)σ−1θMσ(θ)‖f(s,⋅)‖H−1(RN)dθds+√2∫t0∫∞0σ(t−s)2σ−1θ2Mσ(θ)‖f(s,⋅)‖H−1(RN)dθds=√2∫t0gσ(t−s)‖f(s,⋅)‖H−1(RN)ds+√2∫t0g2σ(t−s)‖f(s,⋅)‖H−1(RN)ds≤C‖f‖Lr(0,T;H−1(RN)), | (3.4) |
where C depends on σ,r,N and T.
Now, let us show the first estimate. Due to the embedding Hs,p(RN)↪Hs1,p1(RN) for 1<p≤p1<∞, s,s1∈R, and s−N/p=s1−N/p1, (see e.g., [31,32]) we know that Hμ,q(RN)↪H−1(RN) for μ=N(1/q−1/2−1/N). This means that
‖(Qσf)(t)‖L2(RN)≲‖f‖Lr(0,T;Hμ,q(RN)). |
Hence, the first estimate holds. On the other hand, since Bs2,2(RN)=Hs,2(RN) for s∈R, recall the embedding Hs,q(RN)↪Bsq,2(RN) for 1<q≤2 and the embedding (see e.g., [31,32])
Bs0p0,q0(RN)↪Bs1p1,q1(RN), for s0−N/p0=s1−N/p1, | (3.5) |
for any s0,s1∈R, 1≤p0≤p1≤∞, 1≤q0≤q1≤∞, we have
B0q,2(RN)↪B−12,2(RN),for q=2NN+2, |
by virtue of H0,q(RN)↪B0q,2(RN), and H0,q(RN)=Lq(RN) for q≥1, we obtain the embedding Lq(RN)↪H−1(RN) when q=2N/(N+2). These means that
‖(Qσf)(t)‖L2(RN)≲‖f‖Lr(0,T;Lq(RN)). |
Hence, the second estimate holds.
Finally, in order to obtain the conclusion that the operator Qσ maps Lr(0,T;H−1(RN)) into C([0,T];L2(RN)), by (3.4) it suffices to prove the continuity of operator Qσ for any f∈Lr(0,T;H−1(RN)), i.e., we next to check that for 0≤t<t+h≤T
‖(Qσf)(t+h)−(Qσf)(t)‖L2(RN)→0,as h→0. |
In fact, we first have
‖(Qσf)(t+h)−(Qσf)(t)‖L2(RN)≤‖∫t+htSσ(t+h−s)f(s,⋅)ds‖L2(RN)+‖∫t0∫∞0σ((t+h−s)σ−1−(t−s)σ−1)θMσ(θ)ˇg(t+h,s,⋅)dθds‖L2(RN)+‖∫t0∫∞0σ(t−s)σ−1θMσ(θ)(ˇg(t+h,s,⋅)−ˇg(t,s,⋅))dθds‖L2(RN):=I1+I2+I3. |
Obviously, applying (2.1) and Hölder inequality, it follows that
I1≤√2∫t+ht∫∞0σ(t+h−s)σ−1θMσ(θ)‖f(s,⋅)‖H−1(RN)dθds+√2∫t+ht∫∞0σ(t+h−s)2σ−1θ2Mσ(θ)‖f(s,⋅)‖H−1(RN)dθds=√2∫t+htgσ(t+h−s)‖f(s,⋅)‖H−1(RN)ds+√2∫t+htg2σ(t+h−s)‖f(s,⋅)‖H−1(RN)ds≲hσ−1/r‖f‖Lr(0,T;H−1(RN))+h2σ−1/r‖f‖Lr(0,T;H−1(RN))→0,as h→0. |
For the second term I2, similarly to (3.4) we have
I2≤√2∫t0∫∞0σ|(t+h−s)σ−1−(t−s)σ−1|θMσ(θ)‖f(s,⋅)‖H−1(RN)dθds+√2∫t0∫∞0σ|(t+h−s)σ−1−(t−s)σ−1|(t+h−s)σθ2Mσ(θ)‖f(s,⋅)‖H−1(RN)dθds=Ch∫t0|(t+h−s)σ−1−(t−s)σ−1|‖f(s,⋅)‖H−1(RN)ds≤Ch(∫t0|(t+h−s)σ−1−(t−s)σ−1|r/(r−1)ds)1−1/r‖f‖Lr(0,T;H−1(RN)), |
where Ch=(√2/Γ(σ)+√2(T+h)σ/Γ(2σ)). Using the Lebesgue dominated convergence theorem and Lemma 3.5 we find that I2→0 as h→0.
For estimating the third term I3, by virtue of (3.3), we first have
‖ˇg(t+h,s,⋅)−ˇg(t,s,⋅)‖L2(RN)≤2√2(1+(t+h−s)σθ)‖f(s,⋅)‖H−1(RN), |
which means that
I3≤∫t0∫∞0σ(t−s)σ−1θMσ(θ)‖ˇg(t+h,s,⋅)−ˇg(t,s,⋅)‖L2(RN)dθds≤2√2∫t0∫∞0σ(t−s)σ−1θMσ(θ)‖f(s,⋅)‖H−1(RN)dθds+2√2∫t0∫∞0σ(t−s)σ−1(t+h−s)σθ2Mσ(θ)‖f(s,⋅)‖H−1(RN)dθds≲‖f‖Lr(0,T;H−1(RN)). |
Hence, by the Lebesgue dominated convergence theorem, we conclude that I3→0 as h→0. Similarly, for any 0≤t−h<t≤T, it is not difficult to verify that ‖(Qσf)(t)−(Qσf)(t−h)‖L2(RN)→0, as h→0. Thus, the proof is completed.
Lemma 3.7. For any s∈R, operators Cσ(⋅) and Pσ(⋅) satisfy
‖Cσ(⋅)ϕ‖C([0,T];Hs(RN))≤‖ϕ‖Hs(RN), ‖Pσ(⋅)ψ‖C([0,T];Hs(RN))≲‖ψ‖Hs−1(RN), |
for any (ϕ,ψ)∈Hs(RN)×Hs−1(RN).
Proof. By virtue of |cos(t|x|)|≤1 for all t≥0, x∈RN, it is easy to get ‖˙K(t)ϕ‖Hs(RN)≤‖ϕ‖Hs(RN). In fact, for any ϕ∈Hs(RN), we have
‖˙K(t)ϕ‖Hs(RN)=‖F−1[cos(t|⋅|)ˆϕ]‖Hs(RN)=‖F−1[(1+|⋅|2)s/2cos(t|⋅|)ˆϕ]‖L2(RN). |
The Plancherel theorem implies
‖F−1[(1+|⋅|2)s/2cos(t|⋅|)ˆϕ]‖L2(RN)=‖(1+|⋅|2)s/2cos(t|⋅|)ˆϕ‖L2(RN)≤‖(1+|⋅|2)s/2ˆϕ‖L2(RN)=‖F−1[(1+|⋅|2)s/2ˆϕ]‖L2(RN), |
which means that ‖˙K(t)ϕ‖Hs(RN)≤‖ϕ‖Hs(RN). In addition, by virtue of the inequality |sin(t|x|)|≤t|x|, for all t≥0, x∈RN, as repeating the above processes, by (3.3) it is easy to check that ‖K(t)ψ‖Hs(RN)≤√2(1+t)‖ψ‖Hs−1(RN) for any ψ∈Hs−1(RN).
Let us show that ‖Cσ(t)ϕ‖Hs(RN)≤‖ϕ‖Hs(RN). In fact, from the definition of operator Cσ(⋅), we have
‖Cσ(t)ϕ‖Hs(RN)≤∫∞0Mσ(θ)‖˙K(tσθ)ϕ‖Hs(RN)dθ≤‖ϕ‖Hs(RN), |
where we have used the identity (2.1). Moreover, from the definition of operator Sσ(⋅) and the semigroup (ga∗gb)(t)=ga+b(t) for a,b>0, t>0, we have
‖Pσ(t)ψ‖Hs(RN)≤∫t0g2−2σ(t−s)‖Sσ(s)ψ‖Hs(RN)ds≤∫t0∫∞0g2−2σ(t−s)σsσ−1θMσ(θ)‖K(sσθ)ψ‖Hs(RN)dθds≤√2∫t0∫∞0g2−2σ(t−s)σsσ−1(1+sσθ)θMσ(θ)‖ψ‖Hs−1(RN)dθds=√2((g2−2σ∗gσ)(t)+(g2−2σ∗g2σ)(t))‖ψ‖Hs−1(RN)≲‖ψ‖Hs−1(RN), |
where (g2−2σ∗gσ)(t)+(g2−2σ∗g2σ)(t)≤g2−σ(T)+g2(T). Hence, it yields ‖Pσ(t)ψ‖Hs(RN)≲‖ψ‖Hs−1(RN). To end this proof, it suffices to check the continuity of Cσ(t)ϕ and Pσ(t)ψ.
By the continuity of cos(t|ξ|), for h>0 and 0≤t<t+h≤T, by the Plancherel theorem we know that
‖˙K((t+h)σθ)ϕ−˙K(tσθ)ϕ‖Hs(RN)=‖F−1[⟨⋅⟩s(cos((t+h)σθ|⋅|)−cos(tσθ|⋅|))ˆϕ]‖L2(RN)=‖⟨⋅⟩s(cos((t+h)σθ|⋅|)−cos(tσθ|⋅|))ˆϕ‖L2(RN)≤2‖⟨⋅⟩sˆϕ‖L2(RN)=2‖ϕ‖Hs(RN). |
Hence, passing to the Fourier representation and the Lebesgue dominated convergence theorem, we have the pointwise convergence
‖˙K((t+h)σθ)ϕ−˙K(tσθ)ϕ‖Hs(RN)→0,as h→0, a.e. θ∈(0,∞). |
On the other hand, by (2.1) we have
is integrable for a.e. , hence Lebesgue dominated convergence theorem implies
This means that . Furthermore, as repeating the above processes, we also get the continuity of . Hence, . The proof is completed.
Remark 3.1. Obviously, in view of the inequality , for all , , from the same way as in Lemma 3.7, we get,
By using Lemma 3.6 and Lemma 3.7, it is not difficult to obtain the existence theorem to linear problem (3.1).
Theorem 3.1. Let . Given for , let and , for any and satisfying and , , then there exists a unique solution to the linear problem (3.1), and moreover
Let for , if , then
In the sequel, we consider the global existence to the linear Cauchy problem (3.1).
Theorem 3.2. Given , for . Let , for and , where the Banach space
equipped with its natural norm . Then there exists a unique solution to the linear problem (3.1), and moreover
(3.6) |
Proof. Observe that, for , from Lemma 3.7, . Moreover, for any , Lemma 3.4 implies
which means that . Hence, we have
where we have used the semigroup for . Similarly to Lemma 3.6 and Lemma 3.7, the continuity is easy to check, where we can use the Plancherel theorem and Lebesgue dominated convergence theorem. Consequently, there exists a solution satisfying (3.2) and its values lie in , and then (3.6) holds. Moreover, the uniqueness follows (3.6). The proof is completed.
In this section, we focus on the well-posedness results of the semilinear problem, we first establish a local well-posed result of -solutions that also belong to the setting of , furthermore, by a similar way in Theorem 3.2, another conclusion will be given in the framework of . In the sequel, for a given semilinear function, we obtain the well-posedness results in the setting of Besov space .
Theorem 4.1. Let and for any . Assume that
where index satisfying
For every , there exist and constant such that
for all with . Let be an element to the admissible set
for Then there exists a unique mild solutionof the problem (1.1)–(1.2) for some . Moreover, depends continuously on in the following sense. If in and in and if denotes the solution of problem (1.1)-(1.2) with the initial value , then for all sufficiently large , converges to in .
Proof. Let for . We want to construct a local (in ) solution to the integral equation
By applying Lemma 3.6, Lemma 3.7, and Remark 3.1, it is clear that is well defined. We next shall use a fixed point argument to verify this proof. Fixed and set a space by
where is the distance to the space given by
Obviously, is a complete metric space.
For and every , from the assumption of nonlinearity , by the trigonometric inequality we first have
Therefore, for , Hölder inequality yields
Similarly, for , we have
which, by Hölder inequality and Minkowski inequality, leads to
(4.1) |
Next, we obtain the existence and uniqueness results for small. By Lemma 3.3, for and , we have
Observe that the embedding for , , we have
Therefore, we have
On the other hand, by Lemma 3.6, for any , we have
Therefore, there exists a constant such that
Fixed satisfying , let be small enough such that
(4.2) |
and
Hence, it follows that for any . Moreover, from Lemma 3.6 we deduce
where , for and for . Hence, by selecting small enough so that (4.2) holds, by virtue of (4.1), we have
for all . Since
for small enough so that (4.2) holds, by virtue of (4.1), it follows that
Consequently, is a strict contraction on . From the similar proof of continuity in Theorem 3.1 taking on , it follows that has a fixed point , which is the unique mild solution of problem (1.1)–(1.2).
For the choice of (independent to and ), as before, is determined only by the size of the norm of initial data. Hence and are independent of for sufficiently large. Suppose in and in when . Now, let be large enough, then
By the same argument, one can conclude that
Then, (4.1) implies that
Let be chosen so small such that (4.2) holds, then
Consequently, we deduce that as . The proof is completed.
Remark 4.1. From the above theorem, we have the following remarks:
The admissible set of is not empty. Indeed, for , , and taking initial values , for a suitable semilinear function satisfying the assumptions in Theorem 4.1, likely for , it follows that for .
Let and satisfy the assumptions of Theorem 4.1, noting that if , then the restrictions in the admissible set of reduce to , or if and , then the restrictions in the admissible set of reduce to .
By the embedding for , the requirement implies that the conclusion fails for , and thus the assumption is needed in the initial value conditions.
Noting that, by virtue of the critical embedding , for , , we obtain a weaken requirement of index in Theorem 4.1.
Corollary 4.1. Let and . Assume that
where index satisfying
For every , there exist and constant such that
for all with . Let be an element to the admissible set
Then the problem (1.1)–(1.2) is local well-posed on .
By using the embedding
one can prove the following result by employing the arguments used in the proof of Theorem 4.1.
Corollary 4.2. Let . Assume that for every , there exists such that
for all and that of , . Then for , problem (1.1)–(1.2) is local well-posed on for some , where is the metric space given by
equipped with the distance
In the sequel, we consider a semilinear function of the form for and for , a well-posed result on Besov setting is also established. In order to complete the proof, we need the following result [38].
Lemma 4.1. Let for and for . If , , then
and
for any .
Proof. Noting that
we obtain by Hölder inequality and the embedding that
and
In view of the inequality
and the interpolation property , the desired inequalities follow.
Theorem 4.2. Let , and for , . Let
given .Moreover, let being an element tothe set
(4.3) |
Then the problem (1.1)–(1.2) is local well-posed on .
Proof. Following the method of proof for Theorem 4.1, we just need to construct a local solution to the operator equation for in a suitable ball in , where the ball is defined by
with the radius . Note that this space is not trivial. Indeed, for , by virtue of Lemma 3.4, and the embedding (3.5) and for , we have
which shows that . Similarly, by we also get . Thus, is in if and is small enough. Endowed with the metric
then is a complete metric space. Indeed, since is reflexive, the closed ball of radius is weakly compact, for details, see [38].
From Lemma 4.1, we get for all . Thus, it remains to consider the case for some . Noting that from the requirements of index , it yields , consider any , then it follows from (4.3) that
Noting that
we obtain
On the other hand, from Lemma 4.1, for any , we get
Therefore, we have
This means that there exists a constant such that
and
Hence, fixed , let be small enough such that
Then maps into itself and we obtain
Thus, there exists a unique solution of problem (1.1)–(1.2). The remaining proof is similar to that of Theorem 4.1. So we omit its details. The proof is completed.
Remark 4.2. Observe that, the embedding , Lemma 3.4 yields
which belongs to . Hence, it shall possess a decay rate , which implies that the solution does not belong to .
Concerning with Remark 4.2, in the sequel, we establish the global well-posed result for a special initial data.
Theorem 4.3. Let , , , and for .Given with for some and .Then the problem (1.1)–(1.2) is global well-posed on , where is the metric space given by
equipped with the distance
Proof. Let for . We next verity the operator maps into itself. Indeed, for , by virtue of Lemma 3.4, we have
where we have used the embedding for , . Therefore, there exists a constant such that
Taking for , due to the Fourier representation of operators and the Lebesgue dominated convergence theorem, the proof of the continuity of is similar to Lemma 3.6. Hence, we deduce that for any .
Next, we show that is a contraction on . Indeed, for any , combined the requirement of and Lemma 3.3 imply
Lemma 4.1 shows that
Therefore, there exists a constant for such that
Consequently, is a strict contraction on . This means that has a unique fixed point . The proof is completed.
In this paper, we proved some well-posedness results for linear and semilinear time fractional wave equations, which are also called super-diffusive equations. Under the probability density function and wave operators, we construct the solution operators that are differential to the references therein, which are also useful to establish the local well-posedness of -solutions as well as the local well-posedness on Besov space. Moreover, based on the standard fixed point arguments, the initial data and are taking in the more regularity fractional Sobolev spaces, respectively. Finally, we also establish the global existence results for a linear and a spacial semilinear fractional Cauchy problems.
The work was supported by National Natural Science Foundation of China (Nos. 12071396, 12101142).
The authors declare there is no conflicts of interest.
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