In dynamical systems on networks, Kirchhoff's first law describes the local conservation of a quantity across edges. Predominantly, Kirchhoff's first law has been conceived as a phenomenological law of continuum physics. We establish its algebraic form as a property that is inherited from fundamental axioms of a network's geometry, instead of a law observed in physical nature. To this end, we extend calculus to networks, modeled as abstract metric spaces, and derive Kirchhoff's first law for hyperbolic conservation laws. In particular, our results show that hyperbolic conservation laws on networks can be stated without explicit Kirchhoff-type boundary conditions.
Citation: Alexandre M. Bayen, Alexander Keimer, Nils Müller. A proof of Kirchhoff's first law for hyperbolic conservation laws on networks[J]. Networks and Heterogeneous Media, 2023, 18(4): 1799-1819. doi: 10.3934/nhm.2023078
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In dynamical systems on networks, Kirchhoff's first law describes the local conservation of a quantity across edges. Predominantly, Kirchhoff's first law has been conceived as a phenomenological law of continuum physics. We establish its algebraic form as a property that is inherited from fundamental axioms of a network's geometry, instead of a law observed in physical nature. To this end, we extend calculus to networks, modeled as abstract metric spaces, and derive Kirchhoff's first law for hyperbolic conservation laws. In particular, our results show that hyperbolic conservation laws on networks can be stated without explicit Kirchhoff-type boundary conditions.
In the present work, we shall consider a chemotaxis-haptotaxis model
{ut=Δu−χ∇⋅(u∇v)−ξ∇⋅(u∇w)+μu(1−u−w),τvt=Δv−v+u,wt=−vw+ηu(1−u−w), | (1.1) |
where χ and ξ are positive parameters. In the model (1.1), u represents the density of cancer cell, v and w denote the density of matrix degrading enzymes (MDEs) and the extracellular matrix (ECM) with the positive sensitivity χ, ξ, respectively. Such an important extension of chemotaxis to a more complex cell migration mechanism has been proposed by Chaplain and Lolas [3] to describe the cancer cell invasion of tissue. In that process, cancer invasion is associated with the degradation of ECM, which is degraded by MDEs secreted by cancer cells. Besides random motion, the migration of invasive cells is oriented both by a chemotaxis mechanism and by a haptotaxis mechanism.
In the past ten more years, the global solvability, boundedness and asymptotic behavior for the corresponding no-flux or homogeneous Neumann boundary-initial value problem in bounded domain and its numerous variants have been widely investigated for certain smooth initial data. For the full parabolic system of (1.1), Pang and Wang [4] studied the global boundedness of classical solution in the case τ=1 in 2D domains, and the global solvability also was established for three dimension. When η=0 and τ=1, Tao and Wang [5] proved the existence and uniqueness of global classical solution for any χ>0 in 1D intervals and for small χμ>0 in 2D domains, and Tao [6] improved the results for any μ>0 in two dimension; Cao [7] proved for small χμ>0, the model (1.1) processes a global and bounded classical solution in 3D domains.
When τ=0, the second equation of (1.1) becomes an elliptic function. In the case of η>0, Tao and Winkler [8] proved the global existence of classical solutions in 2D domains for any μ>0. In the case of η=0, the global existence and boundedness for this simplified model under the condition of μ>(N−2)+Nχ in any N-D domains in [9]. Moreover, the stabilization of solutions with on-flux boundary conditions was discussed in [10]. For the explosion phenomenon, Xiang [11] proved that (1.1) possess a striking feature of finite-time blow-up for N≥3 with μ=η=τ=0; the blow-up results for two dimension was discussed in [2] with wt=−vw+ηw(1−w) and μ=0.
When χ=0, the system (1.1) becomes a haptotaxis-only system. The local existence and uniqueness of classical solutions was proved in [12]. In [13,14,15], the authors respectively established the global existence, the uniform-in-time boundedness of classical solutions and the asymptotic behavior. Very recently, Xiang[11] showed that the pure haptotaxis term cannot induce blow-up and pattern for N≤3 or τ=0 in the case of μ=η=0.
Without considering the effect of the haptotaxis term in (1.1), we may have the extensively-studied Keller-Segel system, which was proposed in [16] to describe the collective behavior of cells under the influence of chemotaxis
{∂tu=Δu−χ∇⋅(u∇v),τ∂tv=Δu−λv+u | (1.2) |
with u and v denoting the cell density and chemosignal concentration, respectively. There have been a lot of results in the past years (see [17,18,19,20,21], for instance). Here we only mention some global existence and blow-up results in two dimensional space. For the parabolic-elliptic case of (1.2) with λ=0, 8πχ was proved to be the mass threshold in two dimension in [22,23,24] (see also [25,26] for related results in the bounded domain); namely, the chemotactic collapse (blowup) should occur if and only if ‖u0‖L1 is greater than 8πχ. If ‖u0‖L1<8πχ, the existence of free-energy solutions were improved in [22]. Furthermore, the asymptotic behavior was given by a unique self-similar profile of the system (see also [27] for radially symmetric results concerning self-similar behavior). For the results in the threshold 8πχ, we refer readers for [28,29,30] for more details. For the parabolic-elliptic model in higher dimensions (N≥3) in (1.2), the solvability results were discussed in [31,32,33,34] with small data in critical spaces like LN2(RN),LN2w(RN),MN2(RN), i.e., those which are scale-invariant under the natural scaling. Blowing up solutions to the parabolic-elliptic model of (1.2) in dimension N≥3 have been studied in [35,36,37,38].
In the case τ=1, Calvez and Corrias [1] showed that under hypotheses u0ln(1+|x|2)∈L1(R2) and u0lnu0∈L1(R2), any solution exists globally in time if ‖u0‖L1<8πχ. In [39], the extra assumptions on u0 were removed, while the condition on mass was restricted to ‖u0‖L1<4πχ. The value 4πχ appeared since a Brezis-Merle type inequality played an essential role there. These results were improved in [40,41] to global existence of all solutions with ‖u0‖L1<8πχ by two different method. Furthermore the global existence of solutions was also obtained under some condition on u0 in the critical case ‖u0‖L1=8πχ in[40]. The blow-up results of the parabolic-parabolic case in the whole space were discussed in [42,43] with the second equation was replaced by ∂tv=Δu+u.
However, the global solvability and explosion phenomenon of chemotaxis-haptotaxis model in the whole space have never been touched. Here we consider the global solvability of a simplified model of (1.1)
{ut=Δu−χ∇⋅(u∇v)−ξ∇⋅(u∇w),x∈R2, t>0,vt=Δv−v+u,x∈R2, t>0,wt=−vw,x∈R2, t>0,u(x,0)=u0(x), v(x,0)=v0(x), w(x,0)=w0(x),x∈R2. | (1.3) |
Main results. We assume that the initial data satisfies the following assumptions:
(u0,v0,w0)∈H2(R2)×H3(R2)×H3(R2) and u0,v0,w0 are nonnegative, | (1.4) |
u0∈L1(R2,ln(1+|x|2)dx) and u0lnu0∈L1(R2) | (1.5) |
and
Δw0∈L∞(R2) and ∇√w0∈L∞(R2). | (1.6) |
Theorem 1.1. Let χ>0, ξ>0 and the initial data (u0,v0,w0) satisfy (1.4)–(1.6). If m:=‖u0‖L1<8πχ, then the corresponding chemotaxis-haptotaxis system (1.3) possesses a unique global-in-time, nonnegative and strong solution (u,v,w) fulfilling that for any T<∞
(u,v,w)∈C(0,T;H2(R2)×H3(R2)×H3(R2)). |
Remark 1.1. Our theorem extends the previous results in two aspects. First, our result agrees with that in [1] by setting w=0, which proved that if ‖u0‖L1<8πχ, then the Cauchy problem of the system (1.2) admits a global solution. Secondly, our theorem extends Theorem 1.1 in [2], where the authors proved that 4πχ is the critical mass of the system (1.3) in bounded domains, implying the negligibility of haptotaxis on global existence.
We obtain the critical mass value using the energy method in [1,22]. The energy functional:
F(u,v,w)(t)=∫R2ulnu−χ∫R2uv−ξ∫R2uw+χ2∫R2(v2+|∇v|2),∀t∈(0,Tmax) | (1.7) |
as shown in [2] comes out to be the key ingredient leading to the global existence of solutions under the smallness condition for the mass. Under the assumption
‖u0‖L1<8πχ | (1.8) |
and (1.5), we can derive an integral-type Gronwall inequality for F(t). As a result, we can get a priori estimate for the ∫R2ulnu, which is the key step to establish the global existence of solutions to the system (1.3).
The rest of this paper is organized as follows. In Section 2, we prove local-in-time existence of the solution, and obtain the blow-up criteria for the solution. In Section 3, we give the proof of the Theorem 1.1.
In the following, (u)+ and (u)− will denote the positive and negative part of u as usual, while Lp:=Lp(R2).
We now establish the local existence and uniqueness of strong solutions to system (1.3). Our strategy is first to construct an iteration scheme for (1.3) to obtain the approximate solutions and then to derive uniform bounds for the approximate solutions to pass the limit.
Lemma 2.1. Let χ>0, ξ>0 and u0≥0. Then, there exists a maximal existence time Tmax>0, such that, if the initial data (u0,v0,w0) satisfy (1.4), then there exists a unique solution (u,v,w) of (1.3) satisfying for any T<Tmax, and
(u,v,w)∈C(0,T;H2(R2)×H3(R2)×H3(R2)). | (2.1) |
Furthermore, u, v and w are all nonnegative.
Proof. To obtain the local solution, we follow similar procedures of an iterative scheme developed in [45,46]. We construct the solution sequence (uj,vj,wj)j≥0 by iteratively solving the Cauchy problems of the following system
{∂tuj+1=Δuj+1−χ∇⋅(uj+1∇vj)−ξ∇⋅(uj+1∇wj),x∈R2, t>0,∂tvj+1=Δvj+1−vj+1+uj,x∈R2, t>0,∂twj+1=−vj+1wj+1,x∈R2, t>0,u(x,0)=u0(x), v(x,0)=v0(x), w(x,0)=w0(x),x∈R2. | (2.2) |
We first set (u0(x,t),v0(x,t),w0(x,t))=(u0(x),v0(x),w0(x)). We point out that the system is decouple, then by the linear parabolic equations theory in [44,Theorem Ⅲ.5.2], we can obtain the unique solution u1,v1∈V1,122([0,T]×R2), then we get w1∈C1([0,T],H1(R2)) by directly solving the ordinary equation. Similarly, we define (uj,vj,wj) iteratively.
In the following, we shall prove the convergence of the iterative sequences {uj,vj,wj}j≥1 in C(0,T;X) with X:= H2×H3×H3 for some small T>0. To obtain the uniform estimates, we may use the standard mollifying procedure. However, since the procedure is lengthy, we omit the details, like in the proofs of Theorem 1.1 in [45] and Theorem 2.1 in [46].
Uniform estimates: We will use the induction argument to show that the iterative sequences {uj,vj,wj}j≥1 are in C(0,T;X) with X:= H2×H3×H3 for some small T>0, which means that there exists a constant R>0 such that, for any j, the following inequality holds for a small time interval
sup0≤t≤T(‖uj‖H2+‖vj‖H3+‖wj‖H3)≤R, | (2.3) |
where R=2{‖u0‖H2+‖v0‖H3+‖w0‖H3}+8. Due to the definition of R, the case j=0 is obvious. Then, we need to show that (2.3) is also true for j+1. This will be done by establishing the energy estimate for (uj+1,vj+1,wj+1). First, we begin with the estimate of vj+1.
(ⅰ) Estimates of vj+1. Taking the L2 inner product of the second equation of (2.2) with vj+1, integrating by parts and using Young's inequality, we have
12ddt‖vj+1(t)‖2L2+‖∇vj+1‖2L2=−∫R2(vj+1)2+∫R2vj+1uj≤−12‖vj+1‖2L2+12‖uj‖2L2. | (2.4) |
To show the H1 estimate of vj+1, we will multiply the second equation of (2.2) by ∂tvj+1, integrating by parts and then obtain
12ddt‖∇vj+1(t)‖2L2+‖∂tvj+1‖2L2=−∫R2vj+1∂tvj+1+∫R2uj∂tvj+1≤12‖∂tvj+1‖2L2+‖vj+1‖2L2+‖uj‖2L2. | (2.5) |
For the H2 estimate of vj+1, by Young's inequality, we have
12ddt‖∇2vj+1(t)‖2L2+‖∇Δvj+1‖2L2=−∫R2(Δvj+1)2+∫R2Δvj+1Δuj≤−12‖Δvj+1‖2L2+12‖Δuj‖2L2. | (2.6) |
Similarly, integrating by parts, it is clear that for all t∈(0,T)
ddt‖∇3vj+1(t)‖2L2=2∫R2∇3vj+1∇3(Δvj+1−vj+1+uj)=−2∫R2|∇4vj+1|2−2∫R2|∇3vj+1|2−2∫R2∇4vj+1∇2uj≤−‖∇4vj+1‖2L2−2‖∇3vj+1‖2L2+‖∇2uj‖2L2, |
togethering with (2.3)–(2.6) and adjusting the coefficients carefully, we can find a positive constant α such that
ddt‖vj+1(t)‖2H3+α(‖vj+1‖2H4+‖∂tvj+1‖2L2)≤c1(‖vj+1‖2H3+‖uj‖2H2) | (2.7) |
with c1>0. Here after ci(i=2,3...) denotes the constant independent of R. Integrating on (0,t), we can obtain for all t∈(0,T)
‖vj+1(t)‖2H3+α∫t0(‖vj+1(s)‖2H4+‖∂tvj+1(s)‖2L2)≤ec1T‖v0‖2H3+ec1TTsupt∈(0,T)‖uj(t)‖2H2≤ec1T‖v0‖2H3+ec1TTR2≤2‖v0‖2H3+2, | (2.8) |
by choosing T>0 small enough to satisfy ec1T<2 and TR2<1.
(ⅱ) The estimate of wj+1. In fact, the third component of the above solution of (2.2) can be expressed explicitly in terms of vj+1. This leads to the representation formulae
wj+1(x,t)=w0(x)e−∫t0vj+1(x,s)ds, | (2.9) |
∇wj+1(x,t)=∇w0(x)e−∫t0vj+1(x,s)ds−w0(x)e−∫t0vj+1(x,s)ds∫t0∇vj+1(x,s)ds | (2.10) |
as well as
Δwj+1(x,t)=Δw0(x)e−∫t0vj+1(x,s)ds−2e−∫t0vj+1(x,s)ds∇w0(x)⋅∫t0∇vj+1(x,s)ds+w0(x)e−∫t0vj+1(x,s)ds|∫t0∇vj+1(x,s)ds|2−w0(x)e−∫t0vj+1(x,s)ds∫t0Δvj+1(x,s)ds. | (2.11) |
From (2.9), we can easily get for t∈(0,T)
‖wj+1‖Lp≤‖w0‖Lp,∀p∈(1,∞]. | (2.12) |
From (2.10), by (2.8), the definition of R and the following inequality
‖∫t0f(x,s)ds‖Lp={∫R2|∫t0f(x,s)ds|pdx}1p≤{tp−1∫R2∫t0|f|pdsdx}1p≤tsups∈(0,t)‖f(s)‖Lp,for all p∈(1,∞), | (2.13) |
we can obtain
‖∇wj+1‖L2≤‖∇w0‖L2+‖w0e−∫t0vj+1ds∫t0∇vj+1ds‖L2≤‖∇w0‖L2+‖w0‖L∞‖∫t0∇vj+1ds‖L2≤‖∇w0‖L2+‖w0‖L∞Tsupt∈(0,T)‖∇vj+1‖L2≤‖∇w0‖L2+c2R2T≤‖∇w0‖L2+1 | (2.14) |
by setting T small enough to satisfy c2R2T<1.
Similarly, by the embedding H2↪W1,4 and (2.13), we can obtain from (2.11) for all t∈(0,T)
‖Δwj+1‖L2≤‖Δw0‖L2+2‖∇w0∫t0∇vj+1ds‖L2+‖w0|∫t0∇vj+1ds|2‖L2 +‖w0∫t0Δvj+1ds‖L2≤‖Δw0‖L2+2‖∇w0‖L4‖∫t0∇vj+1ds‖L4+‖w0‖L∞‖|∫t0∇vj+1ds|2‖L2 +‖w0‖L∞‖∫t0Δvj+1ds‖L2≤‖Δw0‖L2+2‖∇w0‖L4Tsupt∈(0,T)‖∇vj+1‖L4+‖w0‖L∞T2supt∈(0,T)‖∇vj+1‖2L2 +‖w0‖L∞Tsupt∈(0,T)‖Δvj+1‖L2≤‖Δw0‖L2+c3R2T+c3R3T2≤‖Δw0‖L2+2 | (2.15) |
by setting T small enough to satisfy c3R2T<1 and c3R3T2<1.
Now we deduce the L2 norm of ∇3wj+1. According to the equation of w and Hölder inequality, we can easily get for all t∈(0,T)
ddt‖∇3wj+1(t)‖2L2=2∫R2∇3wj+1∇3wj+1t=−2∫R2∇3wj+1∇3(vj+1wj+1)≤2‖∇3wj+1‖L2‖∇3(vj+1wj+1)‖L2≤c4‖∇3wj+1‖L2‖∇3wj+1vj+1+3∇2wj+1∇vj+1+3∇2vj+1∇wj+1+∇3vj+1wj+1‖L2≤c4{‖∇3wj+1‖2L2‖vj+1‖L∞+‖∇3wj+1‖L2‖∇2wj+1‖L4‖∇vj+1‖L4+‖∇3wj+1‖L2‖∇2vj+1‖L2‖∇wj+1‖L∞+‖∇3wj+1‖L2‖∇3vj+1‖L2‖wj+1‖L∞}. | (2.16) |
By Galiardo-Nirenberg inequality, we have
‖∇2wj+1‖L4≤c5‖∇3wj+1‖34L2‖wj+1‖14L∞,‖∇wj+1‖L∞≤c5‖∇3wj+1‖12L2‖wj+1‖12L∞. |
Together with Young's inequality, (2.8), (2.12) and (2.16), we can get
ddt‖∇3wj+1(t)‖2L2≤c6(‖vj+1‖2H3+1)‖∇3wj+1‖2L2+c6‖∇wj+1‖2L∞≤c7R2‖∇3wj+1‖2L2+c6‖∇w0‖2L∞≤c7R2‖∇3wj+1‖2L2+c7R2. | (2.17) |
Then, we can deduce from Gronwall's inequality that for all t∈(0,T)
‖∇3wj+1‖2L2≤ec7R2T3‖∇3w0‖2L2+ec7R2T3c7R2T≤2‖∇3w0‖2L2+2 | (2.18) |
by setting T small enough to satisfy ec7R2T3<2 and c7R2T<1.
Combining with (2.12)–(2.15) and (2.18), we can see that
‖wj+1‖H3≤2‖w0‖H3+5. | (2.19) |
(ⅲ) Estimates of uj+1. Taking the L2 inner product of the equation of uj+1 in (2.2), integrating by part we obtain
12ddt‖uj+1(t)‖2L2+‖∇uj+1‖2L2=χ∫R2uj+1∇vj⋅∇uj+1+ξ∫R2uj+1∇wj⋅∇uj+1≤‖uj+1‖L2‖∇vj‖L∞‖∇uj+1‖L2+‖uj+1‖L2‖∇wj‖L∞‖∇uj+1‖L2. | (2.20) |
By (2.8), (2.19) and (2.20) and the embedding H3↪W1,∞, we can see for all t∈(0,T)
12ddt‖uj+1(t)‖2L2+‖∇uj+1‖2L2≤c8R2‖uj+1‖2L2+12‖∇uj+1‖2L2. | (2.21) |
Now we turn to show the L2-estimate of ∇uj+1. Multiplying −Δuj+1 to both sides of the first equation of (2.3) and integrating by parts, we obtain for all t∈(0,T)
12ddt‖∇uj+1(t)‖2L2+‖Δuj+1‖2L2=χ∫R2Δuj+1∇⋅(uj+1∇vj)+ξ∫R2Δui+1∇⋅(ui+1∇wj)=I1+I2. | (2.22) |
By Hölder inequality and Young's inequality, it yields that
I1≤c9‖Δuj+1‖L2‖∇(uj+1∇vj)‖L2≤c10‖Δuj+1‖L2{‖uj+1‖H1‖vj‖H2}≤14‖Δuj+1‖2L2+c210‖vj‖2H2‖uj+1‖2H1. |
Applying the similar procedure to I2, we can obtain
I2≤14‖Δuj+1‖2L2+c210‖wj‖2H2‖uj+1‖2H1, |
which entails that for all t∈(0,T)
12ddt‖∇uj+1(t)‖2L2+12‖Δuj+1‖2L2≤c11R2‖uj+1‖2H1. | (2.23) |
Similar as (2.16), we can get
ddt‖∇2uj+1(t)‖2L2=2∫R2∇2uj+1∇2uj+1t=2∫R2∇2uj+1{∇2(Δuj+1−χ∇⋅(uj+1∇vj)−ξ∇⋅(uj+1∇wj))}=2∫R2∇2uj+1∇2Δuj+1−2χ∫R2∇2uj+1∇3(uj+1∇vj)−2ξ∫R2∇2ui+1∇3(uj+1∇wj)=−2‖∇3uj+1‖2L2+2χ∫R2∇3uj+1∇2(uj+1∇vj)+2ξ∫R2∇3ui+1∇2(ui+1∇wj)≤−‖∇3uj+1‖2L2+c12{‖∇2(uj+1∇vj)‖2L2+‖∇2(ui+1∇wj)‖2L2}≤−‖∇3uj+1‖2L2+c13R2‖uj+1‖2H2. | (2.24) |
Together with (2.21), (2.23) and (2.24), and adjusting the coefficients carefully, we can find a positive constant β such that
ddt‖uj+1(t)‖2H2+β‖uj+1‖2H3≤c14R2‖uj+1‖2H2, | (2.25) |
which implies from Gronwall's inequality that
‖uj+1‖2H2+β∫t0‖uj+1‖2H3⩽ec14R2T‖u0‖2H2≤2‖u0‖2H2,for all t∈(0,T), | (2.26) |
by choosing T small enough to satisfy ec14R2T<2.
Combining (2.8), (2.19) and (2.26), we can get for all t∈(0,T)
‖uj+1‖H2+‖vj+1‖H2+‖wj+1‖H3≤2{‖u0‖H1+‖v0‖H2+‖w0‖H3}+7≤R, | (2.27) |
by the definition of R.
Convergence: The derivation of the relevant estimates of uj+1−uj, vj+1−vj and wj+1−wj are similar to the ones of uj+1, vj+1 and wj+1, so we omit the details. For simplicity, we denote δfj+1:=fj+1−fj. Subtracting the j-th equations from the (j+1)-th equations, we have the following equations for δuj+1,δvj+1 and δwj+1 :
{∂tδuj+1=Δδuj+1−χ∇⋅(δuj+1∇vj)−χ∇⋅(uj∇δvj)−ξ∇⋅(δuj+1∇wj)−ξ∇⋅(uj∇δwj),∂tδvj+1=Δδvj+1−δvj+1+δuj,[1mm]∂tδwj+1=−vj+1δwj+1−δvj+1wj+1. | (2.28) |
(ⅰ) Estimates of δvj+1. Using the same procedure as proving (2.8), we can obtain that for all t∈(0,T)
‖δvj+1(t)‖2H3+α∫t0(‖δvj+1(s)‖2H4+‖∂tδvj+1(s)‖2L2)ds≤ec15Tc15Tsup0≤t≤T‖δuj(t)‖2H2. | (2.29) |
(ⅱ) Estimates of δwj+1. According to the third equation of (2.28), we have for all t∈(0,T)
δwj+1(t)=−∫t0e−∫tsvj+1(τ)dτwj+1(s)δvj+1(s)ds. | (2.30) |
Using the same procedure as proving (2.19) entails that for all t∈(0,T)
‖δwj+1‖L2⩽supt∈(0,T)‖wj+1‖L∞supt∈(0,T)‖δvj+1‖L2T⩽c16RTsupt∈(0,T)‖δvj+1‖L2, |
‖∇δwj+1‖L2⩽‖∫t0∇wj+1(s)δvj+1(s)ds‖L2+‖∫t0wj+1(s)∇δvj+1(s)ds‖L2+‖∫t0wj+1δvj+1∫ts∇vj+1dτds‖L2⩽c17RTsupt∈(0,T)‖δvj+1‖L2+c17RTsupt∈(0,T)‖∇δvj+1‖L2+c17R2T2supt∈(0,T)‖δvj+1‖L2⩽c18(R2+R)Tsupt∈(0,T)‖δvj+1‖H1,‖Δδwj+1‖L2=‖−∫t0Δ{e−∫tsvj+1dτwj+1δvj+1}ds‖L2⩽Tsupt∈(0,T)‖Δ{e−∫tsvj+1dτwj+1δvj+1}‖L2⩽Tsupt∈(0,T){(‖vj+1‖2H2T2+‖vj+1‖H2T)‖wj+1‖H2‖δvj+1‖H2}⩽c19(R3+R2)Tsupt∈(0,T)‖δvj+1‖H2 |
and
‖∇3δwj+1‖2L2⩽ec20R2TTsupt∈(0,T)‖δvj+1‖H3, |
which imply that for all t∈(0,T)
‖δwj+1‖H3≤c21(R3+R2+R)Tsup0≤t≤T‖δuj(t)‖H2. | (2.31) |
by setting T<1.
(ⅲ) Estimates of δuj+1. Using the same procedure as proving (2.26) entails that for all t∈(0,T)
sup0≤t≤T‖δuj+1‖2H2+β∫t0‖δuj+1‖2H3⩽ec22RTc22RTsup0≤t≤T(‖δvj‖2H3+‖δwj‖2H2). | (2.32) |
Combining with (2.29), (2.31) and (2.32), we can obtain for all t∈(0,T)
sup0≤t≤T(‖δuj+1‖H2+‖δvj+1‖H3+‖δwj+1‖H3)≤ec23(R3+R2+R)Tc23(R3+R2+R)Tsup0≤t≤T(‖δuj‖H2+‖δvj‖H3+‖δwj‖H3). | (2.33) |
Taking T>0 small enough, we can find a constant r∈(0,1) such that
sup0≤t≤T(‖δuj+1‖H2+‖δvj+1‖H3+‖δwj+1‖H3)≤rsup0≤t≤T(‖δuj‖H2+‖δvj‖H3+‖δwj‖H3) | (2.34) |
for any j≥1 and t∈(0,T). From the above inequality, we find that (uj,vj,wj) is a Cauchy sequence in the Banach space C(0,T;X) for some small T>0, and thus its corresponding limit denoted by (u,v,w) definitely exists in the same space.
Uniqueness: If (ˉu,ˉv,ˉw) is another local-in-time solution of system (1.3), (˜u,˜v,˜w):=(u−ˉu,v−ˉv,w−ˉw) solves
{∂t˜u=Δ˜u−χ∇⋅(˜u∇ˉv)−χ∇⋅(u∇˜v)−ξ∇⋅(˜u∇ˉw)−ξ∇⋅(u∇˜w),x∈R2, 0<t≤T,∂t˜v=Δ˜v−˜v+˜u,x∈R2, 0<t≤T,∂t˜w=−v˜w−˜vˉw,x∈R2, 0<t≤T,˜u(x,0)=˜v(x,0)=˜w(x,0)=0,x∈R2, |
where T is any time before the maximal time of existence. Following a same procedure as (2.34), we can deduce that ˜u=˜v=˜w=0, which implies the uniqueness of the local solution.
Nonnegativity: The nonnegativity of wj can be easily obtained by (2.9) and the nonnegativity of w0. We will use the induction argument to show that uj and vj are nonnegative for all j>0. We assume that uj and vj are nonnegative. If we apply the maximum principle to the second equation of (2.2), we find vj+1 is nonnegative (uj is nonnegative). Then we turn to deal with uj+1. Let us decompose uj+1=uj+1+−uj+1−, where uj+1+={uj+1uj+1≥00uj+1<0anduj+1−={−uj+1uj+1≤00uj+1>0. Now multiplying the negative part uj+1− on both sides of the first equation of (2.2) and integrating over [0,t]×R2, we can get
∫t0∫R2∂tuj+1uj+1−dxds=−∫t0‖∇(uj+1−)‖2L2ds+χ∫t0∫R2uj+1∇vj⋅∇uj+1−+ξ∫R2uj+1∇wj⋅∇uj+1−≤C∫t0‖(uj+1)−‖2L2(‖∇vj‖2L∞+‖∇wj‖2L∞)+12‖∇(uj+1−)‖2L2ds |
by Young's inequality and the fact the weak derivative of uj+1− is −∇uj+1 if uj+1−<0, otherwise zero. Since uj+1−, ∂tuj+1∈L2(0,T;L2(R2)), we can have
∫t0∫R2∂tuj+1(uj+1)−dxds=12(‖(uj+1−)(0)‖L2−‖(uj+1−)(0)‖L2), |
together with the above inequality, it holds that
‖(uj+1−)(t)‖2L2≤‖(uj+1−)(0)‖2L2exp(C∫t0(‖∇vj‖2L∞+‖∇wj‖2L∞)ds). |
Due to the fact uj+1−(0) is nonnegative, we can deduce that uj+1 is nonnegative. This completes the proof of Lemma 2.1.
Remark 2.1. Since the above choice of T depends only on ‖u0‖H2(R2), ‖v0‖H3(R2) and ‖w0‖H3(R2), it is clear by a standard argument that (u,v,w) can be extended up to some Tmax≤∞. If Tmax<∞ in Lemma 2.1, then
lim supt→Tmax(‖u(t)‖H2(R2)+‖v(t)‖H3(R2)+‖w(t)‖H3(R2))=∞. | (2.35) |
In order to show the H2×H3×H3-boundedness of (u,v,w), it suffices to estimate a suitable Lp-norm of u, with some large, but finite p.
Lemma 2.2. Suppose that χ,ξ>0 and the initial data (u0,v0,w0) satisfy all the assumptions presented in Lemma 2.1. Then for every K>0 there is C>0 such that whenever (u,v,w)∈C(0,T;H2(R2)×H3(R2)×H3(R2)) solves (1.3) for some T>0 and q0>2 satisfies
‖u(⋅,t)‖Lq0≤K,for allt∈(0,T), | (2.36) |
then
‖u(t)‖H2(R2)+‖v(t)‖H3(R2)+‖w(t)‖H3(R2)≤Cfor allt∈(0,T). | (2.37) |
Proof. Firstly, we suppose that for some q0>2 and K>0
‖u(t)‖Lq0(R2)≤K,for all t∈(0,T). | (2.38) |
By the Duhamel principle, we represent u and v of the following integral equations
u(t)=etΔu0−χ∫t0e(t−τ)Δ∇⋅(u∇v)(τ)dτ−ξ∫t0e(t−τ)Δ∇⋅(u∇w)(τ)dτ | (2.39) |
and
v(t)=e−t(−Δ+1)v0+∫t0e−(t−τ)(−Δ+1)u(τ)dτ. | (2.40) |
where etΔf(x)=∫R2G(x−y,t)f(y)dy and
G(x,t)=Gt(x):=1(4πt)exp(−|x|24t) |
is the Gaussian heat kernel. The following well-known Lp−Lq estimates of the heat semigroup play an important role in the proofs [47,48]. For 1⩽p⩽q⩽∞ and f∈Lq(R2), we have
‖etΔf‖Lp⩽(4πt)−(1q−1p)‖f‖Lq,‖∇etΔf‖Lp⩽C3t−12−(1q−1p)‖f‖Lq, |
where C3 is a constant depending on p and q. Then, according to (2.39), we can see that for q0>2 and all t∈(0,T)
‖u(t)‖L∞≤‖u0‖L∞+χ∫t0(t−τ)−12−1q0‖u⋅∇v‖Lq0dτ+ξ∫t0(t−τ)−12−1q0‖u⋅∇w‖Lq0dτ≤‖u0‖L∞+Kχ∫t0(t−τ)−12−1q0‖∇v‖L∞dτ+Kξ∫t0(t−τ)−12−1q0‖∇w‖L∞dτ. | (2.41) |
From (2.40) and the above Lp−Lq estimates of the heat semigroup, we have
‖v(t)‖Lq≤‖v0‖Lq+∫t0e−(t−τ)(t−τ)−(1q0−1q)‖u(t)‖Lq0dτ≤C4,∀q∈(1,∞] | (2.42) |
and by the embedding H3(R2)↪W1,∞(R2)
‖∇v(t)‖L∞≤‖∇v0‖L∞+∫t0e−(t−τ)(t−τ)−12−1q0‖u(t)‖Lq0dτ≤C5, | (2.43) |
where C4 and C5 depend on ‖v0‖H3 and K in (2.38).
According to the equation of w, we can see that for some C6=C6(‖w0‖H3,‖v0‖H2,K,T)>0 and all t∈(0,T)
‖∇w(t)‖L∞≤‖∇w0e−∫t0v(s)ds‖L∞+‖w0e−∫t0v(s)ds∫t0∇v(s)ds‖L∞≤‖w0‖H3+‖w0‖L∞supt∈(0,T)‖∇v(t)‖L∞T≤C6 | (2.44) |
by the embedding H3(R2)↪W1,∞(R2). Inserting (2.43) and (2.44) into (2.41), this yields for all t∈(0,T)
‖u(t)‖L∞≤‖u0‖L∞+KC5χ∫t0(t−τ)−12−1q0dτ+KC6ξ∫t0(t−τ)−12−1q0dτ≤C7, | (2.45) |
where C7 depends on ‖u0‖H2, ‖v0‖H2, ‖w0‖H3, K and T.
Integrating by parts and by Young's inequality, we can obtain from the second equation of (1.3) that for all t∈(0,T)
12ddt‖∂tv(t)‖2L2+‖∇∂tv‖2L2=−∫R2(∂tv)2+∫R2∂tv∂tu≤−12‖∂tv‖2L2+12‖∂tu‖2L2 | (2.46) |
and
12ddt‖∇v(t)‖2L2+‖Δv‖2L2=∫R2vΔv+∫R2uΔv≤−‖∇v‖2L2+12‖u‖2L2+12‖Δv‖2L2. | (2.47) |
Similarly, according to the first equation of (1.3), (2.43) and (2.44), we have for all t∈(0,T)
12ddt‖u(t)‖2L2+‖∇u‖2L2=χ∫R2u∇v⋅∇u+ξ∫R2u∇w⋅∇u≤χ22‖u∇v‖2L2+ξ22‖u∇w‖2L2+12‖∇u‖2L2≤χ2C252‖u‖2L2+ξ2C262‖u‖2L2+12‖∇u‖2L2 | (2.48) |
and by (2.27) for some θ>0
12ddt‖∂tu(t)‖2L2+‖∇∂tu‖2L2=χ∫R2∂tu∇v⋅∇∂tu+χ∫R2u∂t∇v⋅∇∂tu+ξ∫R2∂tu∇w⋅∇∂tu+ξ∫R2u∇∂tw⋅∇∂tu≤C8(‖∇v‖L∞+‖∇w‖L∞)(θ‖∂tu‖2L2+1θ‖∇∂tu‖2L2)+C8‖u‖L∞(θ‖∇∂tv‖2L2+θ‖∇∂tw‖2L2+2θ‖∇∂tu‖2L2)≤C8(C5+C6)θ‖∂tu‖2L2+C8(C5+C6+2C7)θ‖∇∂tu‖2L2+θC8C7(‖∇∂tv‖2L2+‖∇∂tw‖2L2). | (2.49) |
Now we turn to estimate the last term of the right side of (2.49). According to the third equation of (1.3), (2.42) and (2.43), we obtain for some C9>0
‖∇∂tw‖2L2≤‖∇v‖2L2‖w‖2L∞+‖∇w‖2L2‖v‖2L∞≤‖w0‖2L∞‖∇v‖2L2+C24(‖∇w0‖2L2+T2supt∈(0,T)‖∇v‖2L∞‖w0‖2L2)≤C9‖∇v‖2L2+C9. | (2.50) |
Combining with (2.46)–(2.50) and setting θ>0 to satisfy C8(C5+C6+2C7)θ<12, we can obtain such Gronwall-type inequality
ddt{‖∂tv(t)‖2L2+‖∇v(t)‖2L2+‖u(t)‖2L2+‖∂tu(t)‖2L2}+C10(‖∇∂tv‖2L2+‖∇u‖2L2+‖∇v‖2L2+‖Δv‖2L2+‖∇∂tu‖2L2)≤C10(‖∂tv‖2L2+‖∇v‖2L2+‖u‖2L2+‖∂tu‖2L2+1),for all t∈(0,T), | (2.51) |
then by direct integration, we can have for some C11=C11(‖w0‖H3,‖v0‖H2,‖u0‖H2,K,T)>0
∫t0(‖∇∂tv‖2L2+‖∇u‖2L2+‖∇v‖2L2+‖Δv‖2L2+‖∇∂tu‖2L2) +{‖∂tv‖2L2+‖∇v‖2L2+‖u‖2L2+‖∂tu‖2L2}≤C11,for all t∈(0,T). | (2.52) |
By (2.42) and (2.52) and the second equation of v in (1.3), we have
‖Δv‖L2≤‖∂tv‖L2+‖v‖L2+‖u‖L2≤C12,for all t∈(0,T), | (2.53) |
where C12=C12(‖w0‖H3,‖v0‖H2,‖u0‖H2,K,T). Hence by the equation of w and Young's inequality, we obtain for some C13=C13(‖w0‖H3,‖v0‖H2,‖u0‖H2,K,T)>0
‖Δw‖L2≤‖Δw0‖L2+2‖∇w0‖L4‖∫t0∇v‖L4+‖w0‖L∞‖|∫t0∇v|2‖L2+‖w0‖L∞‖∫t0Δv‖L2≤‖Δw0‖L2+2‖∇w0‖L4supt∈(0,T)‖∇v‖L4T+‖w0‖L∞supt∈(0,T)‖∇v‖2L4T+‖w0‖L∞supt∈(0,T)‖Δv‖L2T≤C13,for all t∈(0,T). | (2.54) |
By (2.42) and (2.43) and the embedding W2,2(R2)↪W1,4(R2), we can see
ddt‖∇3w(t)‖2L2⩽C{‖∇3w‖2L2‖v‖L∞+‖∇3w‖L2‖∇2w‖L4‖∇v‖L4+‖∇3w‖L2‖∇2v‖L2‖∇w‖L∞+‖∇3w‖L2‖w‖L∞‖∇3v‖L2}≤C14‖∇3w‖2L2+C14‖∇3v‖2L2+C14,for all t∈(0,T). | (2.55) |
Integrating on (0,t), we have
‖∇3w‖2L2≤C15‖∇3w0‖2L2+C15∫t0‖∇3v‖2L2+C15,for all t∈(0,T). | (2.56) |
Now we turn to estimate the second integral of the right side of (2.56). Applying ∇ to the second equation of (1.3) and rewriting the equation as ∇Δv=∇vt+∇v−∇u, then by (2.52) we have that
∫t0‖∇3v‖2L2≤∫t0‖∇vt‖2L2+∫t0‖∇v‖2L2+∫t0‖∇u‖2L2≤C11,for all t∈(0,T). | (2.57) |
Inserting (2.57) into (2.56), we can obtain that for some C16>0
‖∇3w‖L2≤C16,for all t∈(0,T). | (2.58) |
Now we deduce the L2-norm of ∇u and ∇2u. We multiply the first equation of (1.3) by −Δu, integrate by parts and then obtain
12ddt‖∇u(t)‖2L2+‖Δu‖2L2=χ∫R2Δu∇⋅(u∇v)+ξ∫R2Δu⋅∇(u∇w)=χ∫R2Δu∇u⋅∇v+χ∫R2uΔuΔv+ξ∫R2Δu∇u⋅∇w+ξ∫R2uΔuΔw=I1+I2+I3+I4. | (2.59) |
Then by (2.42), (2.45) and (2.52), Hölder's inequality and Young's inequality, we have
I1+I2⩽χ‖∇v‖L∞‖Δu‖L2‖∇u‖L2+χ‖Δv‖L2‖Δu‖L2‖u‖L∞⩽14‖Δu‖2L2+C17‖∇u‖2L2+C17. |
Similarly, according to (2.43), (2.45) and (2.56), we can obtain
I3+I4⩽χ‖∇v‖L∞‖Δu‖L2‖∇u‖L2+χ‖Δv‖L2‖Δu‖L2‖u‖L∞⩽14‖Δu‖2L2+C17‖∇u‖2L2+C18. |
Then we have
12ddt‖∇u(t)‖2L2+12‖Δu‖2L2≤C19‖∇u‖2L2+C19. |
Integrating on (0,t), we have for some C20>0
‖∇u‖L2≤C20,for all t∈(0,T). | (2.60) |
Rewriting the first equation of (1.3) as Δu=ut+χ∇⋅(u∇v)+ξ∇⋅(u∇w), and by (2.42)–(2.44), (2.53) and (2.54), we have for some C21>0
‖Δu‖L2≤‖∂tu‖L2+χ‖∇u⋅∇v‖L2+χ‖uΔv‖L2+ξ‖∇u⋅∇w‖L2+ξ‖uΔw‖L2≤‖∂tu‖L2+χ‖∇u‖L2‖∇v‖L∞+χ‖Δv‖L2‖u‖L∞+ξ‖∇u‖L2‖∇w‖L∞+ξ‖Δw‖L2‖u‖L∞≤C21. | (2.61) |
For the L2-norm of ∇3v, integrating by parts, we deduce that
ddt‖∇3v(t)‖2L2=2∫R2∇3v∇3(Δv−v+u)=−2∫R2|∇4v|2−2∫R2|∇3v|2+2∫R2∇3v∇3u≤−‖∇4v‖2L2−2‖∇3v‖2L2+‖∇2u‖2L2, |
then by (2.61) and Gronwall's inequality, we can see that for all t∈(0,T)
‖∇3v(t)‖L2≤C22. | (2.62) |
Putting (2.52)–(2.54), (2.58) and (2.60)–(2.62) together, we conclude that for some C>0
‖u(t)‖H2(R2)+‖v(t)‖H3(R2)+‖w(t)‖H3(R2)≤C,∀t∈(0,T), | (2.63) |
which completes the proof.
As a preparation, we first state some results concerning the system which will be used in the proof of Theorem 1.1.
Lemma 3.1. The local-in-time classical solution (u,v,w) of system (1.3) satisfies
‖u(t)‖L1=‖u0‖L1:=M,∀t∈(0,Tmax) | (3.1) |
and
‖v(t)‖L1=‖u0‖L1+(‖v0‖L1−‖u0‖L1)e−t,∀t∈(0,Tmax). | (3.2) |
Proof. Integrating the first and second equation of (1.3) on R2, we can obtain
ddt∫R2u=∫R2Δu−χ∫R2∇⋅(u∇v)−ξ∫R2∇⋅(u∇w)=0 |
and
ddt∫R2v=∫R2Δv−∫R2v+∫R2u=−∫R2v+∫R2u, |
which can easily yield (3.1) and (3.2).
The following energy
F(t)=∫R2ulnu−χ∫R2uv−ξ∫R2uw+χ2∫R2(v2+|∇v|2) |
plays a key role in the proof. The main idea of the proof is similar to the strategy introduced in [2].
Lemma 3.2. Assume that (1.4) and (1.5) holds. Let (u,v,w) be the local-in-time classical solution of system (1.3). Then F(t) satisfies
F(t)+χ∫t0∫R2v2t+∫t0∫R2u|∇(lnu−χv−ξw)|2=F(0)+ξ∫t0∫R2uvw,∀t∈(0,Tmax). | (3.3) |
Proof. We use the same ideas as in the proofs of [45,Theorem 1.3], [46,Lemma 3.1] and [1,Theorem 3.2]. The equation of u can be written as ut=∇⋅(u∇(lnu−χv−ξw)). Multiplying by lnu−χv−ξw and integrating by parts, we obtain
−∫R2u|∇(lnu−χv−ξw)|2=∫R2ut(lnu−χv−ξw)=ddt∫R2(ulnu−χuv−ξuw)+χ∫R2uvt+ξ∫R2uwt. | (3.4) |
Substituting the second and third equation of (1.3) into (3.4) and integrating by parts, we have
−∫R2u|∇(lnu−χv−ξw)|2=ddt∫R2(ulnu−χuv−ξuw)+χ∫R2(vt−Δv+v)vt+ξ∫R2uvw=ddt∫R2(ulnu−χuv−ξuw)−χ∫R2v2t+χ2ddt∫R2(v2+|∇v|2)+ξ∫R2uvw, | (3.5) |
which, upon being integrated from 0 to t, yields simply that (3.3). We give some lemmas to deal with the term ∫R2ulnu in (1.7).
Lemma 3.3. ([1,Lemma 2.1]) Let ψ be any function such that eψ∈L1(R2) and denote ˉu=Meψ(∫R2eψdx)−1 with M a positive arbitrary constant. Let E:L1+(R2)→R∪{∞} be the entropy functional
E(u;ψ)=∫R2(ulnu−uψ)dx |
and let RE:L1+(R2)→R∪{∞} defined by
RE(u∣ˉu)=∫R2uln(uˉu)dx |
be the relative (to ˉu) entropy.
Then E(u;ψ) and RE(u∣ˉu) are finite or infinite in the same time and for all u in the set U={u∈L1+(R2),∫R2u(x)dx=M} and it holds true that
E(u;ψ)−E(ˉu;ψ)=RE(u∣ˉu)≥0. |
Next, we give a Moser-Trudinger-Onofri inequality.
Lemma 3.4. ([1,Lemma 3.1]) Let H be defined as H(x)=1π1(1+|x|2)2. Then
∫R2eφ(x)H(x)dx≤exp{∫R2φ(x)H(x)dx+116π∫R2|∇φ(x)|2dx}, | (3.6) |
for all functions φ∈L1(R2,H(x)dx) such that |∇φ(x)|∈L2(R2,dx).
Lemma 3.5. ([1,Lemma 2.4]) Let ψ be any function such that eψ∈L1(R2), and let f be a non-negative function such that (f1{f≤1})∈L1(R2)∩L1(R2,|ψ(x)|dx). Then there exists a constant C such that
∫R2f(x)(lnf(x))−dx≤C−∫{f≤1}f(x)ψ(x)dx. |
With the help of Lemma 3.2–3.5, we now use the subcritical mass condition (1.8) to derive a Gronwall-type inequality and to get a time-dependent bound for ‖(ulnu)(t)‖L1.
Lemma 3.6. Under the subcritical mass condition (1.8) and (1.5), there exists C=C(u0,v0,w0)>0 such that
‖(ulnu)(t)‖L1+‖v(t)‖2H1≤CeξKγt,∀t∈(0,Tmax), | (3.7) |
where K>0 and γ are defined by (3.8) and (3.10) below, respectively.
Proof. According to the third equation of (1.3), we have for all t∈(0,T)
‖w‖L∞≤‖w0‖L∞:=K, | (3.8) |
then we apply the estimate of (3.3) to find that
F(t)+∫t0∫R2u|∇(lnu−χv−ξw)|2≤F(0)+ξK∫t0∫R2uv,∀t∈(0,Tmax). | (3.9) |
For our later purpose, since M<8πχ, we first choose some positive constant γ>0 small enough to satisfy
χ−M(χ+γ)28π>0, | (3.10) |
then by the definition of F(t) in (1.7), we use (3.1) and (3.8) to deduce that
F(t)=∫R2ulnu−χ∫R2uv−ξ∫R2uw+χ2∫R2(v2+|∇v|2)≥∫R2ulnu−(χ+γ)∫R2uv−ξKM+χ2∫R2(v2+|∇v|2)+γ∫R2uv. | (3.11) |
Similar as the calculation shown in [1], we set ˉu(x,t)=Me(χ+γ)v(x,t)H(x)(∫R2e(χ+γ)v(x,t)H(x)dx)−1, where H(x) is defined in Lemma 3.4. Then, we can apply the Entropy Lemma 3.3 with ψ=(χ+γ)v+lnH to obtain
E(u;(χ+γ)v+lnH)≥E(ˉu;(χ+γ)v+lnH)=MlnM−Mln(∫R2e(χ+γ)v(x,t)H(x)dx). | (3.12) |
Furthermore, applying Lemma 3.4 with φ=(χ+γ)v to the last term in the right hand side of (3.12), we have that
E(u;(χ+γ)v+lnH)=∫R2ulnu−(χ+γ)∫R2uv−∫R2ulnH≥MlnM−Mln(∫R2e(χ+γ)v(x,t)H(x)dx)≥MlnM−M(χ+γ)∫R2vH−M(χ+γ)216π∫R2|∇v|2. | (3.13) |
Then by Young's inequality, we have M(χ+γ)∫R2vH≤M(χ+γ)216π∫R2v2+4Mπ∫R2H2. Together with (3.13) and the fact ∫R2H2(x)dx=13π, we can easily obtain
∫R2ulnu−(χ+γ)∫R2uv−∫R2ulnH≥MlnM−M(χ+γ)∫R2vH−M(χ+γ)216π∫R2|∇v|2≥MlnM−M(χ+γ)216π∫R2v2−M(χ+γ)216π∫R2|∇v|2−43M. | (3.14) |
Substituting (3.14) into (3.11), we have
F(t)≥∫R2ulnu−(χ+γ)∫R2uv+χ2∫R2(v2+|∇v|2)+γ∫R2uv−ξKM≥MlnM+∫R2ulnH+(χ2−M(χ+γ)216π)∫R2(v2+|∇v|2)+γ∫R2uv−(ξK+43)M≥MlnM+∫R2ulnH+γ∫R2uv−(ξK+43)M | (3.15) |
by (3.10). Now we turn to estimate the second term on the right side of (3.15). We set ϕ(x)=ln(1+|x|2), then we can obtain by Young's inequality
ddt∫R2uϕ=∫R2utϕ=∫R2u∇ϕ⋅∇(lnu−χv−ξw)≤∫R2u|∇ϕ|2+14∫R2u|∇(lnu−χv−ξw)|2,for all t∈(0,Tmax). |
By the fact |∇ϕ(x)|=|2x1+|x|2|≤1, we have
ddt∫R2uϕ≤∫R2u+14∫R2u|∇(lnu−χv−ξw)|2≤M+14∫R2u|∇(lnu−χv−ξw)|2, |
upon being integrated from 0 to t, which yields simply that for all t∈(0,Tmax)
∫R2uln(1+|x|2)≤∫R2u0ln(1+|x|2)+Mt+14∫t0∫R2u|∇(lnu−χv−ξw)|2. | (3.16) |
By the definition of H(x), we have for all t∈(0,Tmax)
∫R2ulnH=−2∫R2uln(1+|x|2)−Mlnπ≥−2∫R2u0ln(1+|x|2)−2Mt−12∫t0∫R2u|∇(lnu−χv−ξw)|2−Mlnπ. | (3.17) |
Substituting (3.15) and (3.17) into (3.9), we have for all t∈(0,Tmax)
γ∫R2uv≤ξK∫t0∫R2uv+2Mt+F(0)+2∫R2u0ln(1+|x|2)+(lnπ+ξK+43−lnM)M. | (3.18) |
From (1.4), we have assumed for convenience that u0lnu0 and u0ln(1+|x|2) belongs to L1(R2) for convenience. Then we conclude an integral-type Gronwall inequality as follows
γ∫R2uv≤ξK∫t0∫R2uv+2Mt+C1,∀t∈(0,Tmax), | (3.19) |
where C1=F(0)+2∫R2u0ln(1+|x|2)+(lnπ+ξK+43−lnM)M is a finite number. Solving the integral-type Gronwall inequality (3.19) via integrating factor method, we infer that for some C2>0
∫R2uv+∫t0∫R2uv≤C2eξKγt,∀t∈(0,Tmax). |
Then by (3.9), one can simply deduce that F(t) grows no great than exponentially as well:
F(t)≤C3eξKγt,∀t∈(0,Tmax). | (3.20) |
Similarly, this along with (1.7) shows that for some C4>0
∫R2ulnu+∫R2v2+∫R2|∇v|2≤C4eξKγt,∀t∈(0,Tmax). | (3.21) |
According to Lemma 3.5 with ψ=−(1+δ)ln(1+|x|2), for arbitrary δ>0 in order to have e−(1+δ)ln(1+|x|2)∈L1(R2), we have for all t∈(0,Tmax)
∫R2u(lnu)−dx≤(1+δ)∫R2uln(1+|x|2)dx+C5≤(1+δ){∫R2u0ln(1+|x|2)+Mt+14∫t0∫R2u|∇(lnu−χv−ξw)|2}+C5≤1+δ4{F(0)−F(t)+ξK∫t0∫R2uv}+M(1+δ)t+C6≤C7eξKγt | (3.22) |
for some Ci>0 (i=5,6,7). Finally, the identity
∫R2|ulnu|dx=∫R2ulnudx+2∫R2u(lnu)−dx | (3.23) |
gives that ‖(ulnu)(t)‖L1≤C8eξKγt for some C8>0. Together with (3.21), this easily yield (3.7).
Next, we wish to raise the regularity of u based on the local L1-boundedness of ulnu. In particular, for subcritical mass M, we have ∫R2(u(x,t)−k)+dx≤M for any k>0, while for k>1 we have for all t∈(0,Tmax)
∫R2(u(x,t)−k)+dx≤1lnk∫R2(u(x,t)−k)+lnu(x,t)dx≤1lnk∫R2u(x,t)(lnu(x,t))+dx≤CeξKγtlnk. | (3.24) |
Lemma 3.7. Under the condition (1.5) and (1.8), for any T∈(0,Tmax), there exists C(T)>0 such that the local solution (u,v,w) of (1.1) verifies that for any p≥2
∫R2up(x,t)dx≤C(T), ∀t∈(0,T], | (3.25) |
where C(T)=2pˉC(T)+(2k)p−1M with k and ˉC(T) respectively given by (3.37) and (3.40) below, which are finite for any T>0.
Proof. Let k>0, to be chosen later. We derive a non-linear differential inequality for the quantity Yp(t):=∫R2(u(x,t)−k)p+dx, which guarantees that the Lp-norm of u remains finite.
Multiplying the equation of u in (1.3) by p(u−k)p−1+ yields, using integration by parts,
ddt∫R2(u−k)p+dx=−4(p−1)p∫R2|∇(u−k)p2+|2dx−(p−1)χ∫R2(u−k)p+Δvdx−pkχ∫R2(u−k)p−1+Δvdx−(p−1)ξ∫R2(u−k)p+Δwdx−pkξ∫R2(u−k)p−1+Δwdx=I1+I2+I3+I4+I5. | (3.26) |
Now using the equation of v in (1.3) and the nonnegativity of v, one obtains
I2=−(p−1)χ∫R2(u−k)p+Δvdx=(p−1)χ∫R2(u−k)p+(−vt−v+u)dx≤−(p−1)χ∫R2(u−k)p+vt+(p−1)χ∫R2(u−k)p+1+dx+(p−1)kχ∫R2(u−k)p+dx | (3.27) |
and
I3=−pkχ∫R2(u−k)p−1+Δvdx=pkχ∫R2(u−k)p−1+(−vt−v+u)dx≤−pkχ∫R2(u−k)p−1+vtdx+pkχ∫R2(u−k)p+dx+pk2χ∫R2(u−k)p−1+dx. | (3.28) |
Using Gagliardo-Nirenberg inequality ∫R2f4(x)dx≤C∫R2f2(x)dx∫R2|∇f(x)|2dx with f=(u−k)p2+ and Hölder inequality, we obtain for ε>0
|∫R2(u−k)p+vtdx|≤(∫R2(u−k)2p+dx)12‖vt‖L2≤C(∫R2(u−k)p+dx)12(∫R2|∇(u−k)p2+|2dx)1/2‖vt‖L2≤C(p)ε‖vt‖2L2∫R2(u−k)p+dx+2εp∫R2|∇(u−k)p2+|2dx. | (3.29) |
Similarly, we have, for p≥32
|∫R2(u−k)p−1+vtdx|≤(∫R2(u−k)2(p−1)+dx)12‖vt‖L2≤(C(M,p)+C(p)∫R2(u−k)2p+dx)12‖vt‖L2≤C(M,p)‖vt‖L2+C(p)ε‖vt‖2L2∫R2(u−k)p+dx+p−1εp2k∫R2|∇(u−k)p2+|2dx. | (3.30) |
Then we can see that
I2+I3≤(p−1)∫R2(u−k)p+1+dx+(p−1)p∫R2|∇(u−k)p2+|2dx +C(p,χ)(k+1)‖vt‖2L2∫R2(u−k)p+dx+C(M,p,χ)k‖vt‖L2 +(2p−1)kχ∫R2(u−k)p+dx+pk2χ∫R2(u−k)p−1+dx | (3.31) |
by setting ε=4χ. According to the equation of w and v and (3.8), one obtains for all t∈(0,T)
−Δw(x,t)=−Δw0(x)e−∫t0v(x,s)ds+2e−∫t0v(x,s)ds∇w0(x)⋅∫t0∇v(x,s)ds−w0(x)e−∫t0v(x,s)ds|∫t0∇v(x,s)ds|2+w0(x)e−∫t0v(x,s)ds∫t0Δv(x,s)ds≤‖Δw0‖L∞−e−∫t0v(x,s)ds(√w0∫t0∇v(x,s)ds−∇w0√w0)2+e−∫t0v(x,s)ds|∇w0|2w0+w0(x)e−∫t0v(x,s)ds∫t0(vs(x,s)+v−u)ds. | (3.32) |
Here to estimate the last integral of the right side of (3.32) we first note (1.7) guarantees that
w0(x)e−∫t0v(x,s)ds∫t0(vs(x,s)+v−u)ds≤‖w0‖L∞e−∫t0v(x,s)ds[v(x,t)−v0+∫t0v(x,s)ds]≤‖w0‖L∞v+‖w0‖L∞e,∀t∈(0,T) |
by the nonnegativity of w0 and v0 and the fact e−xx≤1e for all x>0. Substituting (3.8) and (3.32) into (3.26), we have
I4+I5=−(p−1)ξ∫R2(u−k)p+Δwdx−pkξ∫R2(u−k)p−1+Δwdx≤(p−1)Kξ∫R2(u−k)p+vdx+(p−1)K1ξ∫R2(u−k)p+dx+pkKξ∫R2(u−k)p−1+vdx+pkK1ξ∫R2(u−k)p−1+dx, |
where K1=‖Δw0‖L∞+4‖∇√w0‖2L∞+Ke. Applying similar procedure as (3.29) and (3.30) to ∫R2(u−k)p+vdx and ∫R2(u−k)p−1+vdx, this yields
I4+I5≤(p−1)p∫R2|∇(u−k)p2+|2dx+C(p,K,ξ)k‖v‖2L2∫R2(u−k)p+dx+C(M,p,K,ξ)k‖v‖L2+(p−1)K1ξ∫R2(u−k)p+dx+pkK1ξ∫R2(u−k)p−1+dx | (3.33) |
by setting ε=2Kξ. Combining (3.26), (3.31) and (3.33), we have for all t∈(0,T)
ddt∫R2(u−k)p+dx≤−2(p−1)p∫R2|∇(u−k)p2+|2dx+(p−1)∫R2(u−k)p+1+dx+[(2p−1)kχ+(p−1)K1ξ]∫R2(u−k)p+dx+(pk2χ+pkK1ξ)∫R2(u−k)p−1+dx+C(p,K,χ,ξ)(k+1)(‖∂tv‖2L2+‖v‖2L2)∫R2(u−k)p+dx+C(M,p,K,χ,ξ)k(‖∂tv‖L2+‖v‖L2). | (3.34) |
Next, we estimate the nonlinear and negative contribution −2(p−1)p∫R2|∇(u−k)p2+|2dx in terms of ∫R2(u−k)p+1+dx, with the help of the Sobolev's inequality ‖f‖2L2≤c1‖∇f‖2L1. Indeed, by (3.24),
∫R2(u−k)p+1+dx=∫R2((u−k)(p+1)2+)2dx≤c1(∫R2|∇(u−k)(p+1)2+|dx)2=C(p)(∫R2(u−k)12+|∇(u−k)p2+|dx)2≤C(p)∫R2(u−k)+dx∫R2|∇(u−k)p/2+|2dx≤C(p)CeξKγTlnk∫R2|∇(u−k)p/2+|2dx,∀0<t≤T. | (3.35) |
Moreover, since for p≥2 it holds true that
∫R2(u−k)p−1+dx≤∫R2(u−k)+dx+∫R2(u−k)p+dx. | (3.36) |
Inserting (3.35) and (3.36) into (3.34) gives for p≥2 and 0<t≤T that
ddt∫R2(u−k)p+dx≤(p−1)(1−2lnkpC(p)CeξKγT)∫R2(u−k)p+1+dx+C(p,K,χ,ξ)k(1+‖∂tv‖2L2+‖v‖2L2)∫R2(u−k)p+dx+C(M,p,K,χ,ξ)k(‖∂tv‖L2+‖v‖L2+1). |
For any fixed p we can choose k=k(p,T) sufficiently large such that
δ:=2lnkpC(p)CeξKγT−1>0, | (3.37) |
namely, k=exp((1+δ)pC(p)CeξKγT2). For such a k, using the interpolation
∫R2(u−k)p+dx≤(∫R2(u−k)+dx)1p(∫R2(u−k)p+1+dx)(1−1p)≤M1p(∫R2(u−k)p+1+dx)(1−1p), |
we end up with the following differential inequality for Yp(t), p≥2 fixed and 0<t≤T
ddtYp(t)≤−(p−1)M−1p−1δYβp(t)+c2(p,K,χ,ξ)k(1+‖∂tv‖2L2+‖v‖2L2)Yp(t)+c3(M,p,K,χ,ξ)k(1+‖∂tv‖2L2+‖v‖2L2), | (3.38) |
where β=pp−1>1. Let us write the differential inequality (3.38) as follows for simplicity:
ddtYp(t)≤−˜CYβp(t)+g(t)Yp(t)+g(t),0<t≤T, | (3.39) |
where g(t)=ˉC(M,p,K,χ,ξ)k(1+‖∂tv‖2L2+‖v‖2L2) and ˜C=(p−1)M−1p−1δ>0. According to (3.7), (3.9) and (3.20), we can see that g(t)≤ˉC(M,p,K,χ,ξ)keξKtγ. Then by comparison inequality, we show that there exists a constant ˉC(T) such that for all t∈(0,T)
Yp(t)≤Yp(0)exp(∫t0g(s)ds)+∫t0g(τ)exp(∫tτg(s)ds)dτ≤Yp(0)ˉC(M,p,K,χ,ξ)keξKTγT+ˉC(M,p,K,χ,ξ)keξKTγeˉC(M,p,K,χ,ξ)keξKTγT:=ˉC(T). | (3.40) |
It is sufficient to observe that for any k>0
∫R2up(x,t)dx=∫{u≤2k}up(x,t)dx+∫{u>2k}up(x,t)dx≤(2k)p−1M+2p∫{u>2k}(u(x,t)−k)pdx≤(2k)p−1M+2p∫R2(u(x,t)−k)p+dx, | (3.41) |
where the inequality xp≤2p(x−k)p, for x≥2k, has been used. Therefore, (3.25) follows for any p≥2 by (3.40) and (3.41) choosing k=k(p,T) sufficiently large such that (3.37) holds true.
Proof of Theorem 1.1. According to the local Lp−boundedness of Lemma 3.7 and Lemma 2.2 we must have the local H2×H3×H3-boundedness of (u,v,w), which contracts the extensibility criteria in (2.35). Then we must obtain that Tmax=∞, that is, the strong solution (u,v,w) of (1.3) exists globally in time and is locally bounded as in (2.2).
The authors convey sincere gratitude to the anonymous referees for their careful reading of this manuscript and valuable comments which greatly improved the exposition of the paper. The authors are supported in part by National Natural Science Foundation of China (No. 12271092, No. 11671079).
The authors declare there is no conflict of interest.
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