Citation: Igor B. Krasnyuk, Roman M. Taranets, Marina Chugunova. Dynamic boundary conditions in the interface modeling of binary alloys[J]. AIMS Mathematics, 2018, 3(3): 409-425. doi: 10.3934/Math.2018.3.409
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We investigate equations of the form
ut−uxx=fT(t,u), t∈R, x∈R, | (1) |
where
fT(t,u)=g(u)−mT(t)u, | (2) |
and
g>0 on (0,1), g(0)=g(1)=0, g′(0)>0, g′(1)<0, | (3) |
and
u↦g(u)u decreasing on (0,+∞). | (4) |
The previous hypotheses imply in particular that
g(u)≤g′(0)u, ∀u∈[0,+∞), | (5) |
and that
g<0 on (1,+∞). | (6) |
In Sections 2 and 4, the function
fT(t,u)<0, ∀(t,u)∈R×(1,+∞). | (7) |
In Section 3, the function
Equations of the type
ut−uxx=g(u)−mT(t)u, t∈R, x∈R, | (8) |
are proposed to model the spatial evolution over time of a cancerous tumor in the presence of chemotherapy. The quantity
{mT=φ on [0,1),mT=0 on [1,T). | (9) |
In the absence of treatment, cancer cells reproduce and spread in space. This reproduction is modeled by the reaction term of KPP type
g′(0)−∫10φ(t)dt<0. | (10) |
This inequality is not really restricting. Indeed, we shall see after that this hypothesis is in fact a condition so that the patient is cured in the case where there is no rest period in the cycles of chemotherapy (that is
We now refine the previous modelling. In fact, the concentration of drug in the patient's body is not a datum. We only know the concentration of drug injected to the patient. We denote
DT(t)={1, ∀t∈[0,1],0, ∀t∈(1,T). | (11) |
The concentration of drug
{m′(t)=DT(t)−m(t)τ, ∀t∈R+,m(0)=m0≥0. | (12) |
The real number
ut−uxx=g(u)−m(t)u+εp(t,u), t∈R, x∈R, |
where
The mathematical study of reaction-diffusion equations began in the 1930's. Fisher [12] and Kolmogorov, Petrovsky and Piskunov [17] were interested in wave propagation in population genetics modeled by the homogeneous equation
ut−uxx=f(u), t∈R, x∈R. | (13) |
In the
Freidlin and Gärtner in [13] were the first to study heterogeneous equations. More precisely, they generalized spreading properties for KPP type equations with periodic in space coefficients. Since this work, numerous papers have been devoted to the study of heterogeneous equations with KPP or other reaction terms. We can cite e.g. [2,3,4,5,6,8,10,16,19,27,28,29] in the case of periodic in space environment, [14,18,19,24,25] in the case of periodic in time environment and [21,22,23] in the case of periodic in time and in space environment. The works of Nadin [21,22] and Liang and Zhao [19] are the closest of our paper. We will compare later the contributions of our work with these references. We now give the main results of the paper.
When the nonlinearity is not homogeneous, there are no planar front solutions of (8) anymore. For equations with coefficients depending periodically on the space variable, Shigesada, Kawasaki and Teramoto [26] defined in 1986 a notion more general than the planar fronts, namely the pulsating fronts. This notion can be extended for time dependent periodic equations as follows.
Definition 1.1. For equation (1), assume that
{u(t,x)=U(t,x−ct), ∀t∈R, ∀x∈R,U(⋅,−∞)=θ, U(⋅,+∞)=0, uniformly on R,U(t+T,x)=U(t,x), ∀t∈R, ∀x∈R. |
So, a pulsating front connecting
{Ut−cUξ−Uξξ−fT(t,U)=0, ∀(t,ξ)∈R×R,U(⋅,−∞)=θ, U(⋅,+∞)=0, uniformly on R,U(t+T,ξ)=U(t,ξ), ∀(t,ξ)∈R×R. |
In this definition, by standard parabolic estimates, the limiting state
{y′=fT(t,y) on R,y(0)=y(T), | (14) |
whose solutions are called equilibrium states of the equation (1).
If
{(Φθ,fT)′=(fTu(t,θ)+λθ,fT)Φθ,fT on R,Φθ,fT>0 on R,Φθ,fT is T−periodic. | (15) |
These quantities are called respectively principal eigenvalue and principal eigenfunction associated with
λθ,fT=−1T∫T0fTu(s,θ(s))ds. |
We now recall the definition of the Poincaré map
{y′=fT(t,y) on R,y(0)=α. | (16) |
Definition 1.2. The
PT(α)=yα(T). |
We conclude, with the fact that each nonnegative solution of (14) is associated with a fixed point of
(PT)′(αT)=e−TλyαT,fT. | (17) |
We can find these results concerning the notions of principal eigenvalue and Poincaré map in [7], [9], [15] and [20].
Let
λ0,fT{>0 if T<T∗,<0 if T>T∗,=0 if T=T∗. | (18) |
This is indeed the case if
λ0,fT=−g′(0)+1T∫T0mT(s)ds=−g′(0)+1T∫10φ(s)ds. |
Furthermore, for this type of functions, hypothesis (10) implies that
Proposition 1. We consider the real number
(Ⅰ) If
(Ⅱ) If
(i) For any
1T∫T0fTu(s,wT(s))ds≤0. |
(ii) If
(iii) If
(iv) If
limT→+∞1T∫T0wT(t)dt=1. |
The same result of existence and uniqueness (result of the type
Let us now summarize a result in [22], which deals with the evolution of
Proposition 2. [22] Let
{ut−uxx=fT(t,u) on (0,+∞)×R,u(0,⋅)=u0 on R. | (19) |
If
0≤u(t,x)≤MΦ0,fT(t)e−λ0,fTt, ∀(t,x)∈R+×R. | (20) |
If
supx∈R|u(t,x)|t→+∞→0. |
If
supx∈K|u(t,x)−wT(t)|t→+∞→0. |
A similar result was proved for KPP nonlinearities depending periodically on space by Berestycki, Hamel and Roques in [5].
In the biological context with
We now study in more detail the case where the treatment is not effective, that is, the case where
Theorem 1.3. [18], [21] Let
(Ⅱ) We denote
{ut−uxx=fT(t,u) on (0,+∞)×R,u(0,⋅)=u0 on R. |
If
∀c∈(0,c∗T), limt→+∞sup|x|<ct|u(t,x)−wT(t)|=0. |
If
∀c>c∗T, limt→+∞sup|x|>ctu(t,x)=0. |
In his paper [21], Nadin considers in the first assertion of the spreading properties in Theorem 1.3 initial conditions which are more general. He assumes that
We can now characterize the critical speed
Proposition 3. For every
c∗T=2√−λ0,fT. | (21) |
Hence, if
limT→(T∗)+c∗T=0, |
and, if
limT→+∞c∗T=2√g′(0). |
In the case where the treatment is not effective, the invasion of space by the equilibrium state
We end this section by stating the existence of pulsating fronts in the case of nonlinearities which are not of KPP type (that is hypotheses (4) and (5) are not necessarily verified, but we still assume (3), (6) and (18)). For these nonlinearities, there is still a positive solution to problem (14), but it may not be unique. According to Cauchy-Lipschitz theorem, solutions of (14) are ordered on
Proposition 4. Let
We are interested in the case of nonlinearities which are no more periodic in time, but which are the sum of a function which converges as
ut−uxx=g(u)−m(t)u+εp(t,u), t∈R, x∈R, | (22) |
where
|p(t,u)u|≤C, ∀(t,u)∈R+×(0,+∞). | (23) |
The function
limt→+∞|m(t)−mT∞(t)|=0. | (24) |
Indeed, an elementary calculation implies that for any
m(t)={τ[1+((e1τ−1)(enTτ−1)eTτ−1+m0T−enTτ)e−tτ], ∀t∈[nT,nT+1),τ[(e1τ−1)(e(n+1)Tτ−1)eTτ−1+m0T]e−tτ, ∀t∈[nT+1,(n+1)T). |
Consequently, if we define the positive
mT∞(t)={τ[1+(e1τ−1eTτ−1−1)e−tτ], ∀ t∈[0,1],τe1τ−1eTτ−1eT−tτ, ∀ t∈[1,T), |
then the convergence result (24) holds. Furthermore, we have
f(t,u)=g(u)−m(t)u, and fε(t,u)=f(t,u)+εp(t,u). |
According to (24), we have
supu∈(0,+∞)|f(t,u)−fT(t,u)u|t→+∞→0. | (25) |
The function
The aim of this section is to show that Proposition 2 and the spreading results of Theorem 1.3 hold true when we replace
Theorem 1.4. Let
{ut−uxx=fε(t,u) on (0,+∞)×R,u(0,⋅)=u0 on R. | (26) |
If
limt→+∞supx∈R|uε(t,x)|=0. |
If
lim supt→+∞ supx∈K|uε(t,x)−wT(t)|≤MTε. |
We saw in Proposition 1 that
F(x)=−1T∫T0fT(s,xwT(s))wT(s)ds, |
then we have
Let us give a sketch of the proof. For
Note that the case
Proposition 5. Let
(Ⅰ) If
(Ⅱ) If
(i) If
(ii) If
Concerning the spreading results of Theorem 1.3, they remain true if we replace
Theorem 1.5. Let
{ut−uxx=fε(t,u) on (0,+∞)×R,u(0,⋅)=u0 on R. |
If
lim supt→+∞sup|x|<ct|uε(t,x)−wT(t)|≤MTε, |
where
If
limt→+∞sup|x|>ctuε(t,x)=0. |
The proof of this theorem uses the same ideas as the proof of Theorem 1.4. For
As in Section 1.1, we consider a
{mT=φ on [0,1),mT=0 on [1,T), |
where
ut−uxx=g(u)−mTτ(t)u, t∈R, x∈R, | (27) |
where
{mTτ(t)=1Tφ(tT), ∀t∈[0,τ),mTτ(t)=0, ∀t∈[τ,T), |
where the function
∫T0mTτ(t) dt=1T∫τ0φ(tT)dt=∫10φ(t)dt. | (28) |
So, it is clear that the quantity of drug administered during a cycle of chemotherapy is independent of the treatment duration
fT(t,u)=g(u)−mT(t)u and fTτ(t,u)=g(u)−mTτ(t)u. |
The first proposition deals with the principal eigenvalue associated with
Proposition 6. Let
λ0,fTτ=λ0,fT=−g′(0)+∫10φ(s)dsT. |
Consequently, if
{y′=fTτ(t,y) on R,y(0)=y(T). | (29) |
The same proof as in Proposition 1 implies that for any
Let us now study the case where the treatment is not efficient, that is,
Finally, we are interested in the influence of the parameter
Proposition 7. Let
{(0,T)→(0,+∞)τ↦wTτ(0) |
is continuous and decreasing.
Consequently, in the case where the treatment is not efficient, the shorter the duration of the chemotherapy cycle, the larger the value of the equilibrium state
Outline. Section 2 is devoted to the proof of Propositions 1, 3 and 4. Section 3 gathers the proof of Theorem 1.4, Proposition 5 and Theorem 1.5. Finally, we prove in Section 4 Propositions 6 and 7.
We first investigate solutions of (14), showing Proposition 1. We begin with the case where
(w∗)′(t)w∗(t)=g(w∗(t))w∗(t)−mT(t), ∀t∈[0,T]. |
We integrate this equation between
∫T0(g(w∗(s))w∗(s)−mT(s))ds=0. | (30) |
Yet, as
1T∫T0(g(w∗(s))w∗(s)−mT(s))ds<−λ0,fT≤0, |
which contradicts (30).
We now consider the case where
Lemma 2.1. There exists
Proof. Indeed, according to the fact that
Lemma 2.2. For all
Proof. Let
Lemma 2.1 and Lemma 2.2 imply that there exists
ρ∗=inf{ρ≥1 | w1(t)≤ρw2(t), ∀t∈[0,T]}. |
We have
w1(t)≤ρ∗w2(t), ∀t∈[0,T]. | (31) |
Moreover there exists
w1(t∗)=ρ∗w2(t∗). | (32) |
We are going to show that
w′1(t)=fT(t,w1(t)), ∀t∈[0,T]. | (33) |
Furthermore
(ρ∗w2)′(t)>fT(t,ρ∗w2(t)), ∀t∈[0,T]. | (34) |
Indeed, for all
(ρ∗w2)′(t)=ρ∗w′2(t)=ρ∗w2(t)(g(w2(t))w2(t)−mT(t))>ρ∗w2(t)(g(ρ∗w2(t))ρ∗w2(t)−mT(t)) (according to (4) since ρ∗>1)=fT(t,ρ∗w2(t)). |
According to (31), (32), (33), (34) and the
w1(t)=ρ∗w2(t), ∀t∈[0,T]. |
It is a contradiction because
We denote
We now study the function
PTi(α)=yTiα(Ti), ∀α≥0, |
where
{y′=fTi(t,y) on R,y(0)=α. | (35) |
We saw in (Ⅱ) that the function
y′=fT1(t,y). |
Consequently, since
yT1α≡yT2α on [0,T1]. |
Furthermore, from (3), (7) and the fact that
0<yT1α(T1)=yT2α(T1)<1. |
On
yT1α(T1)=yT2α(T1)<yT2α(T2). |
In other terms
PT1(α)<PT2(α). |
Finally, we have necessarily
We show now the continuity property. Let
T∗<T−<Tn<T+, ∀n≥n−. | (36) |
We will demonstrate that
{wTn(t)=wTn(0)+∫t0fTn(s,wTn(s))ds, ∀t∈[0,T+],wTn(0)=wTn(Tn). |
Passing to the limit as
{˜w(t)=˜w(0)+∫t0f˜T(s,˜w(s))ds, ∀t∈[0,˜T]⊂[0,T+],˜w(0)=˜w(˜T). |
The function
{˜w′=f˜T(t,˜w) on [0,˜T],˜w(0)=˜w(˜T), |
and
(wTn)′(t)wTn(t)=fTn(t,wTn(t))wTn(t), ∀t∈[0,Tn]. |
We integrate the previous equation over
We study now the behavior of the equilibrium state
{wTn(t)=wτn(0)+∫t0fTn(s,wTn(s))ds, ∀t∈[0,T+],wTn(0)=wTn(Tn). |
Passing to the limit as
{w∗(t)=w∗(0)+∫t0fT∗(s,w∗(s))ds, ∀t∈[0,T∗]⊂[0,T+],w∗(0)=w∗(T∗). |
The function
{(w∗)′=fT∗(t,w∗) on [0,T∗],w∗(0)=w∗(T∗), |
and
supR|wTn|=sup[0,Tn]|wTn|≤sup[0,T+]|wTn|n→+∞→0, |
which completes the proof of this point.
We study now the case where
Lemma 2.3. Under assumptions (9) and (10), the real number
δ:=inf{wT(1) | T≥T∗+1} |
is positive. Furthermore,
Proof. We argue by way of contradiction. Let us suppose there exists a sequence
0<wT+(0)<wTn(0), ∀n≥n+. |
Up to extraction of a subsequence,
0<wT+(0)≤w∗(0). | (37) |
The same reasoning as previously implies that the function
{(w∗)′=g(w∗)−φ(t)w∗ on [0,1],w∗(1)=0. |
By uniqueness, we have necessarily
We return to the proof of the last point of Proposition 1. We consider
{y′=g(y) on (1,+∞),y(1)=δ, |
where
{y′=g(y) on (1,T),y(1)=wT(1). |
Since
1−ε2≤wT(t)<1, ∀t∈(lε,T). |
Furthermore
|1T∫T0wT(t)dt−1|≤1T(∫lε0|wT(t)−1|dt+∫Tlε|wT(t)−1|dt). |
Yet,
1T∫lε0|wT(t)−1|dt≤2lεT≤2lεTε=ε2. |
and
1T∫Tlε|wT(t)−1|dt≤T−lεTε2≤ε2. |
So
We begin by showing the characterization of
(Φμ)t=(μ2+fTu(t,0)+λμ)Φμ on R. |
We divide the previous equation by
c∗T=inf{c∈R | there exists μ>0 such that λμ+μc=0}. |
Consequently, we have
c∗T=inf{c∈R | there exists μ>0 such that μ2−μc−λ0,fT=0}. |
We thus look for the smallest real number
c∗T=2√g′(0)−1T∫T0mT(t)dt. |
Hence the function
limT→+∞c∗T=2√g′(0) if 1T∫T0mT(t)dtT→+∞→0, and limT→(T∗)+c∗T=0, |
which concludes the proof of Proposition 3.
Let
{y′=fT(t,y) on R,y(0)=α, |
then we denote
α0=inf{α∈(0,1] | PT(α)=α}. |
To simplify the notations, we denote
Lemma 2.4. We have
Proof. We assume that
∫T0fT(s,yαn(s))−fT(s,0)yαn(s)ds=0. |
Passing to the limit as
∫T0fTu(s,0)ds=0, |
which contradicts the fact that
Since
|fT(t,εΦ0,fT(t))−εΦ0,fT(t)fTu(t,0)|≤|λ0,fT|2εΦ0,fT(t), | (38) |
where
{(U0)t−(U0)ξξ−c0(U0)ξ=fT(t,U0) on R×R,U0(⋅,⋅)=U0(⋅+T,⋅) on R×R,U0(⋅,−∞)=yT , U0(⋅,+∞)=0 uniformly on R. | (39) |
Necessarily
∂ξU0(t,ξ)<0, ∀(t,ξ)∈R×R. |
Let
C={c∈R | there exists a pulsating front U of speed c such that ∂ξU<0 on R×R} |
is closed and included in
Given
εa,r=min{min[0,T]×[−a,a]U1(⋅,⋅+r)2Φ0,fT(⋅),ε0,yT(0)Φ0,fT(0)}. |
We consider the problem
{Ut−Uξξ−c2Uξ=fT(t,U) on (0,T)×(−a,a),U(0,⋅)=U(T,⋅) on [−a,a].U(⋅,−a)=U1(⋅,−a+r) , U(⋅,a)=εa,rΦ0,fT on [0,T]. | (40) |
We begin by showing that the previous problem has a solution.
Proposition 8. There exists a solution to problem (40).
Proof. We consider the problem
{Ut−Uξξ−c2Uξ=fT(t,U) on (0,+∞)×(−a,a),U(⋅,−a)=U1(⋅,−a+r) , U(⋅,a)=εa,rΦ0,fT on [0,+∞),U(0,⋅)=ψ on [−a,a], |
where
C={ψ∈C0([−a,a],[0,1]) | εa,rΦ0,fT(0)≤ψ≤U1(0,⋅+r) on [−a,a]}. |
Note that this set is not empty since
Lemma 2.5. Let
εa,rΦ0,fT(t)<Uψ(t,ξ)<U1(t,ξ+r) ∀ (t,ξ)∈(0,+∞)×(−a,a). | (41) |
Proof. Since
(U1(⋅+r))t−(U1(⋅+r))ξξ−c2(U1(⋅+r))ξ−fT(t,U1(⋅+r))=(c1−c2)(U1(⋅+r))ξ>0. |
Moreover, since
Uψ(t,ξ)≤U1(t,ξ+r) ∀(t,ξ)∈[0,+∞)×[−a,a]. |
In the same way, since
(εa,rΦ0,fT)t−(εa,rΦ0,fT)ξξ−c2(εa,rΦ0,fT)ξ−fT(t,εa,rΦ0,fT)=εa,rΦ0,fT(λ0,fT+fTu(t,0))−fT(t,εa,rΦ0,fT)=εa,rλ0,fTΦ0,fT−(fT(t,εa,rΦ0,fT)−εa,rΦ0,fTfTu(t,0))≤εa,rλ0,fTΦ0,fT−εa,rλ0,fT2Φ0,fT<0. |
Furthermore since
εa,rΦ0,fT(t)≤Uψ(t,ξ) ∀ (t,ξ)∈[0,T]×[−a,a], |
The fact that the inequalities in (41) are strict is a consequence of the strong maximum principle.
We return to the proof of Proposition 8. We consider
T:C→Cψ↦Uψ(T,⋅) |
Owing to (41) and the
(Uψ−Uφ)t−(Uψ−Uφ)ξξ−c2(Uψ−Uφ)ξ=β(t,ξ)(Uψ−Uφ), |
where
β(t,ξ)={fT(t,Uψ(t,ξ))−fT(t,Uφ(t,ξ))Uψ(t,ξ)−Uφ(t,ξ), if Uψ(t,ξ)≠Uφ(t,ξ),fTu(t,Uψ(t,ξ)),if Uψ(t,ξ)=Uφ(t,ξ). |
Since
|Uψ(t,ξ)−Uφ(t,ξ)|≤‖ψ−φ‖L∞([−a,a])e‖fTu‖L∞([0,T]×[0,1])t. |
If we take
‖Uψ(T,⋅)−Uφ(T,⋅)‖L∞([−a,a])≤e‖fTu‖L∞([0,T]×[0,1])T‖ψ−φ‖L∞([−a,a]). |
So
We prove now that
So, according to Shauder's fixed point theorem, there exists
To simplify the notations, we denote now
εa,rΦ0,fT(t)<Ua,r(t,ξ)<U1(t,ξ+r) ∀ (t,ξ)∈[0,T]×(−a,a). | (42) |
We are now going to use a sliding method and we first give a comparison lemma.
Lemma 2.6. Let
Vh≤U on [0,T]×[−a,a−h]. |
Proof. We denote
εa,rΦ0,fT≤U1(⋅,−a+r)2<U1(⋅,−a+r) on [0,T], |
it occurs that
h∗=inf{h_≥0 | ∀h∈[h_,2a], Vh≤U on [0,T]×Ih}. |
We have
Vh∗≤U on R×Ih∗. | (43) |
Furthermore, if we define the bounded function
η(t,ξ)={fT(t,U(t,ξ))−fT(t,Vh∗(t,ξ))U(t,ξ)−Vh∗(t,ξ), if U(t,ξ)≠Vh∗(t,ξ),fTu(t,U(t,ξ)),if U(t,ξ)=Vh∗(t,ξ), |
then, we have on
(U−Vh∗)t−c2(U−Vh∗)ξ−(U−Vh∗)ξξ=η(t,ξ)(U−Vh∗). | (44) |
Consequently, according to (43) and (44), if there exists
Vh∗=U on R×Ih∗. | (45) |
Yet, according to (42) (which is automatically fulfilled from the arguments used in Lemma 2.5), and since
Vh∗(t,−a)=V(t,−a+h∗)<U1(t,−a+h∗+r)<U1(t,−a+r)=U(t,−a). |
Consequently,
Vh∗(t,a−h∗)=V(t,a)=εa,rΦ0,fT(t)<U(t,a−h∗). |
So, it occurs that
Vh∗<U on R×Ih∗. |
Since
Corollary 1. There exists a unique function
Proof. We apply the conclusion of Lemma 2.6 with
Corollary 2. The function
Proof. Let
{(U∗)t−(U∗)ξξ−c2(U∗)ξ=fT(t,U∗) on R×(−a,a),U∗(0,⋅)=U∗(T,⋅) on [−a,a],U∗(⋅,−a)=U1(⋅,−a+r∗) , U∗(⋅,a)=εa,r∗Φ0,fT on [0,T]. |
The uniqueness of the solution of the previous problem (Corollary 1) implies that we have
Corollary 3. For any
∂ξUa,r(t,ξ)<0. |
Proof. We apply Lemma 2.6 with
Proposition 9. There exist
Proof. There exists
εa,r=min{U1(ta,r,ξa,r+r)2Φ0,fT(ta,r),ε0,yT(0)Φ0,fT(0)} |
Let
εa,rnn→+∞→εa:=min{yT(ta)2Φ0,fT(ta),ε0,,yT(0)Φ0,fT(0)} |
We thus have
Ua,rn0(0,0)≥34εaΦ0,fT(0). |
Let now
Ua,˜rn1(0,0)≤14εaΦ0,fT(0). |
According to Corollary 2, there exists
Ua,ra(0,0)=12εaΦ0,fT(0), |
which completes the proof.
Proposition 10. There exists a sequence
Proof. Since
εann→+∞→ε∗:=min{yT(t∗)2Φ0,fT(t∗),ε0,,yT(0)Φ0,fT(0)}>0. |
According the standard parabolic estimates, up to extraction of a subsequence,
{(U2)t−(U2)ξξ−c2(U2)ξ=fT(t,U2) on [0,T]×R,U2(0,⋅)=U2(T,⋅) on R,U2(0,0)=12ε∗Φ0,fT(0),(U2)ξ≤0 on [0,T]×R. |
Since
U2(0,0)∈(0,yT(0)2]. |
The functions
Let
εT=1C+1min{|λ0,fT|,−g(2)2}>0, |
where
fT(t,u)−(C+1)εu≤fε(t,u)≤fT(t,u)+(C+1)εu. | (46) |
We define the
fT−ε(t,u)=fT(t,u)−(C+1)εu, and fTe(t,u)=fT(t,u)+(C+1)εu. | (47) |
According to (7), it occurs that
{fT−ε(t,u)≤0, ∀(t,u)∈R×[2,+∞),fT(t,u)≤0, ∀(t,u)∈R×[2,+∞). | (48) |
Furthermore, according to (4) and (6), for any
fTe(t,u)≤0, ∀(t,u)∈R×[2,+∞), | (49) |
Concerning the principal eigenvalues associated with the equilibrium
{λ0,fTε=λ0,fT−(C+1)ε,λ0,fT−ε=λ0,fT+(C+1)ε. | (50) |
We begin by handling the case where
λ0,fTe>0. | (51) |
We consider
{(vε)t−(vε)xx=fTε(t,vε) on (0,+∞)×R,vε(0,⋅)=uε(nεT,⋅) on R. |
Owing to (46) and the
(uε(⋅+nεT,⋅))t−(uε(⋅+nεT,⋅))xx=fε(t+nεT,uε(⋅+nεT,⋅))≤fTε(t,uε(⋅+nεT,⋅)). |
So, applying a comparison principle, we obtain
0≤uε(t+nεT,x)≤vε(t,x), ∀(t,x)∈R+×R. | (52) |
According to (51), Proposition 2 applied with the
limt→+∞supx∈Rvε(t,x)=0. |
Hence, owing to (52),
limt→+∞supx∈Ruε(t,x)=0, |
which concludes the proof of the first part of Theorem 1.4.
We now consider the case where
|u−v|≤μ⇒|fT(t,v)−fT(t,u)−fTu(t,u)(v−u)|≤λwT,fT2|v−u|. | (53) |
We define the two positive real numbers
˜MT=8(C+1)λwT,fTsup[0,T]wTinf[0,T]ΦwT,fT>0, |
and
˜εT=min{εT,λwT,fT4(C+1),inf[0,T]wT2˜MTsup[0,T]ΦwT,fT,min{μT,1}˜MTsup[0,T]ΦwT,fT}>0, | (54) |
where
λ0,fT−ε<0, λ0,fT<0, and λ0,fTe<0. | (55) |
Owing to (48), (49) and (55), the same proof as in Proposition 1 implies that there exists a unique
Lemma 3.1. There exists
{supt∈[0,T]|wTε(t)−wT(t)|≤MTε,supt∈[0,T]|wT−ε(t)−wT(t)|≤MTε. | (56) |
Proof. We begin by proving the first inequality. We define the function
ˉvε(t)=wT(t)+˜MTεΦwT,fT(t). |
We are interested in the problem
{y′=fTε(t,y) on R,y(0)=y(T). | (57) |
We will show that
(ˉvε)′(t)−fT(t,ˉvε(t))−(C+1)εˉvε(t)=fT(t,wT(t))+˜MTεΦwT,fT(t)fTu(t,wT(t))−fT(t,ˉvε(t))+˜MTεΦwT,fT(t)λwT,fT−(C+1)εˉvε(t). |
Since
fT(t,wT(t))+˜MTεΦwT,fT(t)fTu(t,wT(t))−fT(t,ˉvε(t))≥−λwT,fT2˜MTεΦwT,fT(t). |
Consequently,
(ˉvε)′(t)−fT(t,ˉvε(t))−(C+1)εˉvε(t)≥λwT,fT2˜MTεΦwT,fT(t)−(C+1)εˉvε(t)=˜MTεΦwT,fT(t)(λwT,fT2−(C+1)ε)−(C+1)εwT(t). |
Yet
λwT,fT2−(C+1)ε≥λwT,fT4. |
Hence
(ˉvε)′(t)−fT(t,ˉvε(t))−(C+1)εˉvε(t)≥˜MTεΦwT,fT(t)λwT,fT4−(C+1)εwT(t)=ε(λwT,fT4˜MTΦwT,fT(t)−(C+1)wT(t)). |
Consequently, according to the definition of
λwT,fT4˜MTΦwT,fT(t)−(C+1)wT(t)=(2ΦwT,fT(t)inf[0,T]ΦwT,fTsup[0,T]wT−wT(t))(C+1)>0. |
Finally,
We now show that
(wT)′(t)−fT(t,wT(t))−(C+1)εwT(t)=−(C+1)εwT(t)<0. |
According to Lemma
wT(t)<˜wε(t)<wT(t)+˜MTεΦwT,fT(t), ∀t∈R. | (58) |
In particular,
supt∈[0,T]|wT(t)−wTε(t)|≤εMT, |
where
We now give a sketch of the proof of the second inequality of Lemma 3.1. We define the function
v_ε(t)=wT(t)−˜MTεΦwT,fT. |
We are interested in the problem
{y′=fT−ε(t,y) on R,y(0)=y(T). | (59) |
We can show in the same way as previously that
wT(t)−˜MTεΦwT,fT(t)<ˆwε(t)<wT(t), ∀t∈R. | (60) |
Yet
wT(t)−˜MTεΦwT,fT(t)≥wT(t)−12ΦwT,fTsup[0,T]ΦwT,fTinf[0,T]wT>0. |
Consequently
supt∈[0,T]|wT(t)−wT−ε(t)|≤εMT, |
which completes the proof of Lemma 3.1.
Let us now complete the proof of Theorem Theorem 1.4. We recall that
{(˜uε)t−(˜uε)xx=fTε(t,˜uε) on (0,+∞)×R,˜uε(0,⋅)=uε(nεT,⋅) on R, |
and
{(˜u−ε)t−(˜u−ε)xx=fT−ε(t,˜u−ε) on (0,+∞)×R,˜u−ε(0,⋅)=uε(nεT,⋅) on R, |
where
{(vε)t−(vε)xx=fε(t+nεT,vε) on R+×R,vε(0,⋅)=uε(nεT,⋅) on R. |
Owing to (46) and the
(vε)t−(vε)xx=fε(t+nεT,vε)≤fTε(t+nεT,vε)=fTε(t,vε) |
Consequently, since
vε(t,x)≤˜uε(t,x), ∀(t,x)∈R+×R. |
In other words
uε(t+nεT,x)≤˜uε(t,x), ∀(t,x)∈R+×R. |
Actually, we can show in the same way that
˜u−ε(t,x)≤uε(t+nεT,x)≤˜uε(t,x), ∀(t,x)∈R+×R. |
According to the
˜u−ε(t,x)−wT(t)≤uε(t+nεT,x)−wT(t+nεT)≤˜uε(t,x)−wT(t). | (61) |
Therefore, for any
{˜u−ε(t,x)−wT(t)≥−supx∈K|˜u−ε(t,x)−wT−ε(t)|−supt∈[0,T]|wT−ε(t)−wT(t)|,˜uε(t,x)−wT(t)≤supx∈K|˜uε(t,x)−wTε(t)|+supt∈[0,T]|wTε(t)−wT(t)|. |
On the other hand, owing to Proposition 2, there exists
supx∈K|˜u−ε(t,x)−wT−ε(t)|+supx∈K|˜ue(t,x)−wTe(t)|≤η. | (62) |
According to Lemma 3.1, (61) and (62), we thus have, for any
|uε(t+neT,x)−wT(t+neT)|≤η+MTε. |
In other words, for any
supx∈K|uε(t,x)−wT(t)|≤η+MTε, |
That is
lim supt→+∞supx∈K|uε(t,x)−wT(t)|≤MTε, |
which completes the proof of Theorem 1.4.
We begin by proving
f(t,u)≤fT∗(t,u)−g(2)2u, ∀t∈[t0,+∞),∀u∈[0,+∞), | (63) |
where we recall that
f(t,u)≤0, ∀t∈[t0,+∞),∀u∈[2,+∞), | (64) |
We define
M=max{2,supRu0}. |
The real number
0≤u(t,x)≤M, ∀t∈R, ∀x∈R. | (65) |
We denote
{v′=f(t,v) on R+,v(0)=M. |
Owing to (65), we have
0≤u(t+t0,x)≤v(t), ∀t≥0, ∀x∈R. |
Furthemore, since
v(t)≤M, ∀t≥0. |
To summarize
0≤u(t,+t0,x)≤v(t)≤M, ∀t≥0, ∀x∈R. | (66) |
We will show that
v(tn)>δ0, ∀n∈N. |
For any
{v′n(t)=f(t+knT∗,vn(t)) ∀t∈[−knT∗,+∞),vn(˜tn)=v(tn)>δ0. |
Up to extraction of a subsequence,
{(v∗)′=fT∗(t,v∗) on R,v∗(t∗)≥δ0. | (67) |
Furthermore, owing to (66), we have
0≤v∗(t)≤M, ∀t∈R. | (68) |
We consider
{σ′=fT∗(t,σ) on R+,σ(0)=M. |
Owing to (7) and the fact that
{(σ∗)′=fT∗(t,σ∗) on [0,T∗],σ∗(0)=σ∗(T∗)=l. |
According to Proposition 1, we have necessarily
v∗(−nT∗+t)≤σ(t), ∀t∈R+, ∀n∈N. |
In particular
v∗(t∗)≤σn(t∗), ∀n∈N. |
Passing to the limit as
v∗(t∗)≤σ∗(t∗)=0, |
which is a contradiction with (67). Consequently
We now prove (Ⅱ). We begin by considering the case where
fε(t,u)=fT∗(t,u)+εu, ∀(t,u)∈R×R+. |
Let
fε(t,u)≤0, ∀t∈R, ∀u∈[2,+∞). |
Furthermore
supx∈K|uε(t,x)−wTε(t)|t→+∞→0. |
We now consider the case where
fε(t,u)≤fT∗(t,u), ∀(t,u)∈R+×R+. |
We denote
{ut−uxx=fT∗(t,u) on (0,+∞)×R,u(0,⋅)=u0 on R. |
From the comparison principle, it occurs that
0≤uε(t,x)≤u(t,x) ∀(t,x)∈R+×R. | (69) |
According to (Ⅰ), we have
Proof. Let
c∗T,ε=2√|λ0,fTε|=2√−λ0,fT+(C+1)ε, |
and
c∗T,−ε=2√|λ0,fT−ε|=2√−λ0,fT−(C+1)ε. |
In particular, since
c∈(0,c∗T,−ε)∩(0,c∗T,ε). | (70) |
We define
ˆεc,T=min{˜εT,εc,T}>0. | (71) |
We consider
uε(nεT,x)≥˜uε,0, ∀x∈R. | (72) |
Let
{(˜uε)t−(˜uε)xx=fT−ε(t,˜uε) on (0,+∞)×R,˜uε(0,⋅)=˜u0,ε on R. |
Owing to (46), (72) and the fact that
˜uε(t,x)≤uε(t+nεT,x), ∀(t,x)∈R+×R. | (73) |
According to (49), we have
˜C=max{2,supRu0}, |
then according to the maximum principle, we have
uε(nεT,x)≤˜C, ∀x∈R. | (74) |
Let
{(vε)t=fTε(t,vε) on R+,vε(0)=˜C. | (75) |
Owing to (46) and (74), we can still apply a comparison principle to get that
uε(t+nεT,x)≤vε(t), ∀(t,x)∈R+×R. | (76) |
According to (49) and the fact that
{(v∗ε)′=fTε(t,v∗ε) on [0,T],v∗ε(0)=v∗ε(T)=l. |
So
|fTε(t,κεΦ0,fTε(t))−(fTε)u(t,0)κεΦ0,fTε(t)|≤−λ0,fTε2κεΦ0,fTε(t), ∀t∈[0,T]. |
Consequently, we have on
(κεΦ0,fTε)′−fTε(t,κεΦ0,fTε)≤κεΦ0,fTε(λ0,fTε+(fTε)u(t,0))−(κεΦ0,fTε(fTε)u(t,0)+λ0,fTε2κεΦ0,fTε)≤λ0,fTε2κεΦ0,fTε≤0. |
Hence, the function
0<κεΦ0,fTε(t)≤vε(t), ∀t∈R+. |
Using the
0<κεΦ0,fTε(t)≤v∗ε(t), ∀t∈R+. |
Consequently, we have necessarily
n≥nη,ε⇒supt∈[0,T]|vε(t+nT)−wTε(t)|≤η. | (77) |
On the other hand, according to (70), the spreading properties in periodic case (Proposition 1.3) give the existence of
t≥tc,η,ε⇒sup|x|<ct|wT−ε(t)−˜uε(t,x)|≤η. | (78) |
Let
˜uε(t,x)≤uε(t+nεT,x)≤vε(t) |
The fact that
˜uε(t,x)−wT(t)≤uε(t+nεT,x)−wT(t+nεT)≤vε(ntT+˜t)−wT(˜t) |
Hence, according to (77) and Lemma 3.1
uε(t+nεT,x)−wT(t+nεT)≤|vε(ntT+˜t)−wTε(˜t)|+|wTε(˜t)−wT(˜t)|≤η+MTε, |
and on the other hand, owing to (78) and Lemma 3.1, it occurs that
uε(t+nεT,x)−wT(t+nεT)≥−sup|y|<ct|wT−ε(t)−˜uε(t,y)|−sup[0,T]|wT−ε−wT|≥−η−MTε. |
To conclude, for any
sup|x|<ct|uε(t,x)−wT(t)|≤η+MTε, |
which concludes the proof of the first assertion of Theorem 1.5.
We now show the second part of the theorem. We consider
c′>min{c∗T,−ε,c∗T,ε}. | (79) |
Furthermore, according to (4), (23) and (25), there exists
fε(t,u)≤Du, ∀t∈R+, ∀u∈R+. | (80) |
We define
{Ht−Hxx=0 on (0,+∞)×R,H(0,⋅)=u0 on R. |
The function
H(t,x)=12√πt∫Supp(u0)e−(x−y)24tu0(y)dy, ∀t∈(0,+∞),∀x∈R, | (81) |
where
uε(t,x)≤H(t,x)eDt, ∀t∈R+,∀x∈R. |
In particular, owing to (81), it occurs that
uε(nεT,x)≤eDnεT2√πnεT∫Supp(u0)e−(x−y)24nεTu0(y)dy, ∀x∈R. | (82) |
We define the real number
γc′,ε=c′+√(c′)2+4λ0,fTe2. |
Let us note that
uε(nεT,x)≤Mc′,εΦ0,fTe(0)e−γc′,εx, ∀x∈R. | (83) |
We also define the function
vc′,ε(t,x)=Mc′,εΦ0,fTe(t)e−γc′,ε(x−c′t), |
We have on
(vc′,ε)t−(vc′,ε)xx=(−γ2c′,ε+γc′,εc′+λ0,fTe)Mc′,εΦ0,fTee−γc′,ε(x−c′t)+(fTe)u(t,0)vc′,ε. |
Hence according to (5) and the fact that
(vc′,ε)t−(vc′,ε)≥fTe(t,vc′,ε) |
Furthermore, owing to (46), (47) and the
(uε)t−(uε)xx=fε(t+nεT,uε)≤fTε(t+nεT,uε)=fTε(t,uε) |
Consequently, since (83) implies that
0≤uε(t+nεT,x)≤vc′,ε(t,x), ∀(t,x)∈R+×R. |
For all
0≤supx>ctuε(t,x)≤supx>ctvc′,ε(t,x)≤vc′,ε(t,ct)=Mc′,εΦ0,fTe(t)e−γc′,ε(c−c′)tt→+∞→0. |
In the same way, we can show that
0≤supx<−ctuε(t+nεT,x)t→+∞→0. |
To summarize
limt→+∞sup|x|>ctuε(t,x)=0, |
which concludes the proof of the second assertion of Theorem 1.5.
We begin by proving Proposition 6.
Proof. Owing to (28), the principal eigenvalue associated with
λ0,fTτ=−g′(0)+∫T0mTτ(t) dt=−g′(0)+∫10φ(t)dt=λ0,fT. |
We now demonstrate Proposition 7.
Proof. Let
PTτ(α)=yτ,α(T), |
where
{(yτ,α)′=fTT(t,yτ,α) on R+,yτ,α(0)=α. | (84) |
In the same way as in the proof of Proposition 1, we show that the function
We begin by showing the continuity property. Let
\begin{cases} w_{\tau _n}^T(t) = w_{\tau _n}^T(0)+\int_0^t f_{\tau _n}^T(s,w_{\tau _n}^T(s))ds,~~\forall t \in [0,T],\\ w_{\tau _n}^T(0) = w_{\tau _n}^T(T). \end{cases} |
Passing to the limit as
\begin{cases} w^*(t) = w^*(0)+\int_0^t f_{\tau ^*}^T(s,w^*(s))ds,~~\forall t \in [0,T],\\ w^*(0) = w^*(T). \end{cases} |
The function
\begin{cases} (w^*)' = f^T_\tau (t,w^*)~~\text{on}~[0,T],\\ w^*(0) = w^*(T). \end{cases} |
Owing to Proposition 1, it follows that
\frac{(w^T_{\tau_n})'(t)}{w^T_{\tau_n}(t)} = \frac{f^{T}(t,w^T_{\tau_n}(t))}{w^T_{\tau_n}(t)},~~\forall t\in[0,T]. |
We integrate the previous equation over
We now study the monotonicity of this function. We consider two real numbers
P^T_{\tau_i}(\alpha) = y_{\tau _i,\alpha}(T), |
where
z_{\tau _i,\alpha}(t) = y_{\tau _i,\alpha}(t) e^{ \int_0^t m^T_{\tau _i}(s)ds}. | (85) |
This function solves on
(z_{\tau _i,\alpha})' = \frac{g\Big(z_{\tau _i,\alpha} e^{-\int_0^t m^T_{\tau _i}(s)ds}\Big)}{ e^{- \int_0^t m^T_{\tau _i}(s)ds}} |
For any
e^{-\int_0^t m^T_{\tau _1}(s)ds} \leq e^{-\int_0^t m^T_{\tau _2}(s)ds}. | (86) |
According to (4) and the fact that
\frac{g\Big(z_{\tau _1,\alpha} e^{-\int_0^t m^T_{\tau _1}(s)ds}\Big)}{ z_{\tau _1,\alpha} e^{- \int_0^t m^T_{\tau _1}(s)ds}} \geq \frac{g\Big(z_{\tau _1,\alpha} e^{-\int_0^t m^T_{\tau _2}(s)ds}\Big)}{ z_{\tau _1,\alpha} e^{- \int_0^t m^T_{\tau _2}(s)ds}}. | (87) |
In other terms,
z_{\tau _1,\alpha}(t) \geq z_{\tau _2,\alpha}(t),~~~\forall t\in [0,T]. |
Actually, the previous inequality is strict with
y_{\tau _1,\alpha}(T) e^{ \int_0^T m^T_{\tau _1}(s)ds} > y_{\tau _2,\alpha}(T) e^{ \int_0^Tm^T_{\tau _2}(s)ds} |
According to (28), it occurs that
\int_0^T m^T_{\tau _1}(s)ds = \int_0^T m^T_{\tau _2}(s)ds = \int_0^1 \varphi (s)ds. |
Consequently
y_{\tau _1,\alpha}(T) > y_{\tau _2,\alpha}(T). |
In other words,
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