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On Decay Properties of Solutions for

the Vlasov–Poisson System

By Kosuke Ono

Department of Mathematical Sciences The University of Tokushima Tokushima 770-8502, JAPAN e-mail : ono@ias.tokushima-u.ac.jp

(Received September 30, 2009)

Abstract

We study decay properties of solutions to the Cauchy problem for the collision-less Vlasov–Poisson system which appears Vlasov plasma physics and stems from Liouville’s equation coupled with Poisson’s equation for the determining the self-consistent electro-statics or gravitational forces.

2000 Mathematics Subject Classification. 82D10, 82C40, 35B40

1

Introduction

We consider the Cauchy problem for the following kinetic system

∂tf + v· ∇xf + E· ∇vf = 0 inRN× RN × (0, ∞) (1.1) E(x, t) =−∇xU (x, t) inRN× (0, ∞) (1.2)

f (x, v, 0) = ϕ(x, v)≥ 0 , (1.3)

where U = U (x, t) is a potential which generates the force field E = E(x, t). Then, the system (1.1)–(1.3) describes the evolution of a microscopic density

f = f (x, v, t)≥ 0 of particles subject to the action of the force field E. We

will be mainly interested in the Vlasov–Poisson system where the force field is self-consistent and given by

− ∆xU (x, t) = γρ(x, t) , U (x, t)→ 0 as |x| → ∞ , (1.4) ρ(x, t) =

f (x, v, t) dv .

This work was in part supported by Grant-in-Aid for Scientific Research (C) of JSPS

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where ∇x = (∂x1,· · · , ∂xN), ∇v = (∂v1,· · · , ∂vN), ∆x is the Laplacian in the

x variable, and γ is a constant. The sign γ = +1 represents to electrostatic

(repulsive) interaction between the particles of the same species, while γ =−1 represents gravitational (attractive) interaction (see Risken [11], Glassy [5] for physical interpretations).

From (1.2) and (1.4), we have

E(x, t) = γ SN−1

x

|x|N ∗ ρ(x, t) , (1.5)

where SN−1 is (N− 1)-dimensional volume of the N-dimensional unit sphere,

and the symbol∗ is the convolution in the x variable.

The existence of local solutions of the system is known for every N ∈ N (e.g. [3], [4], [6], [8]). The Global existence problem has been studied by several authors under suitable restrictions (see [1], [2], [6], [7], [12], [14]).

In this paper we study decay properties of solutions to the Cauchy problem for the Vlasov–Poisson system.

Let f = f (x, v, t) ≥ 0 be a strong solution of the Vlasov–Poisson system with non-negative initial datum ϕ(x, v)∈ C1

0(RN× RN), where C01(RN× RN) denotes the space of compactly supported, continuously differentiable functions (see [9], [10]).

Our main result is as follows.

Theorem 1.1 Let N ≥ 4 and γ > 0. Then the solution f = f(x, v, t) ≥ 0 of the Vlasov–Poisson system satisfies that

∥|x/t − v|2f L1 x,v ≤ C1t −2, t > 0 , (1.6) and for 1≤ q ≤ 1 + 2/N, ∥ρ(t)∥Lqx ≤ C1t −N(1−1/q), t > 0 , (1.7) and for N/(N− 1) < p ≤ N(N + 2)/(N2− N − 2), ∥E(t)∥Lpx≤ C1t −N(1−1/N−1/p), t > 0 , (1.8) where C1 = C1(∥(1 + |x|2)ϕ∥L1 x,v,∥ϕ∥L∞x,v) is a constant depending on ∥(1 + |x|2 L1 x,v and∥ϕ∥L∞x,v.

Finally we fix some notation. The function spaces Lpx,v and Lpx mean Lp(RN×RN) and Lp(RN) with usual norms∥·∥

Lpx,v and∥·∥Lpx for 1≤ p ≤ ∞,

respectively. Positive constants will be denoted by C and will change from line to line.

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2

Proof

We first state the well-known convolution inequality (see for instance [13]).

Lemma 2.1 (Hardy–Littlewood–Sobolev inequality) Let 0 < λ < N and 1 < q < p <∞. Then

∥|x|−λ∗ f(x)∥

Lpx ≤ C∥f∥Lqx for f ∈ L

q x with 1 + 1/p = λ/N + 1/q.

The following proposition plays an important role in the proof of Theorem 1.1. Proposition 2.2 (1) d dt∥E(t)∥ 2 L2 x=−2γE· j dx , j =vf dv (2) N− 2 ∥E(t)∥ 2 L2 x= ∫ x· Eρ dx , ρ =f dv

Proof. (1) Using (1.2) and integrating by parts, we observe that

d dt∥E(t)∥ 2 L2 x = d dt|∇xU|2dx =−2U ∆Utdx =−2γU ∂tρ dx = 2γU∇x· j dx = 2γ∇xU· j dx = −2γE· j dx ,

where we used the fact ∂tρ +∇x·j = 0, indeed, ∂tρ =

∂tf dv =−(v·∇xf + E· ∇vf ) dv =−∇ ·vf dv =−∇x· j.

(2) Using (1.2) and (1.4) and integrating by parts, we observe that ∫ x· Eρ dv = 1 γx· ∇xU ∆xU dx = 1 γk,jxkUxkUxjxjdx =1 γk,j∂xj(xkUxk)Uxjdx =1 γ  ∫ |∇xU|2dx + 1 2 ∑ k,jxk∂xk(U 2 xj) dx   =1 γ∥E(t)∥2 L2 x− 1 2 ∑ k,jUx2 jdx   =1 γ ( ∥E(t)∥2 L2 x− N 2 ∫ |∇xU|2dx ) = N− 2 ∥E(t)∥ 2 L2 x.

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Proof of Theorem 1.1 Using the Vlasov–Poisson system and integrating by

parts, we observe that

d dt∥|x − tv| 2f L1 x,v =−2 ∫∫ (x− tv) · vf dvdx − ∫∫ |x − tv|2(v· ∇ xf + E· ∇vf ) dvdx = ∫∫ |x − tv|2E· ∇ vf dvdx =−2t ∫∫ (x− tv) · Ef dvdx =−2tx· Eρ dx + 2t2 ∫ E· j dx , where ρ =f dv and j =vf dv.

From Proposition 2.2, we have

d dt∥|x − tv| 2f L1 x,v = N− 2 γ t∥E(t)∥ 2 L2 x− 1 γt 2d dt∥E(t)∥ 2 L2 x or d dt { ∥|x − tv|2f L1 x,v+ 1 γt 2d dt∥E(t)∥ 2 L2 x } =−N− 4 γ t∥E(t)∥ 2 L2 x.

When γ > 0 and N≥ 4, we see

∥|x − tv|2f L1 x,v+ 1 γt 2d dt∥E(t)∥ 2 L2 x ≤ ∥|x| 2ϕ L1 x,v or ∥|x/t − v|2f L1 x,v+ 1 γ d dt∥E(t)∥ 2 L2 x≤ ∥|x| 2ϕ L1 x,vt −2, t > 0 , (2.1)

which gives the estimate (1.6). For a≥ 1 and R > 0, we observe

f dv |x/t−v|≤R f dv +|x/t−v|≥R (R−2|x/t − v|2f )1/af1−1/adv ≤ CRN∥f∥ L∞x,v+ R−2/a (∫ |x/t − v|2f dv )1/a(∫ f dv )1−1/a .

Optimizing the above estimate in R, that is, taking

RN +2/a= ( C∥f∥L∞x,v )−1(∫ |x/t − v|2f dv )1/a(∫ f dv )1−1/a ,

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we have that ∫ f dv ≤ C ( ∥f∥−1 L∞x,v (∫ |x/t − v|2f dv )1/a(∫ f dv )1−1/a)aN/(aN +2) ∥f∥L∞x,v,

and from the H¨older inequality,

f dv∥L(aN +2)/(aN ) x ≤ C∥f∥2/(aN +2) L∞x,v (∫ (∫ |x/t − v|2f dv )1/a(∫ f dv )1−1/a dx )aN/(aN +2) ≤ C(∥f∥2/(aN ) L∞x,v ∥|x/t − v| 2f1/a L1 x,v∥f∥ 1−1/a L1 x,v )aN/(aN +2) .

Putting q = (aN + 2)/(aN ) (i.e. a = 2/(N (q− 1))), we obtain that for

1≤ q ≤ 1 + 2/N, f dv∥Lq x ≤ C ( ∥f∥q−1 L∞x,v∥|x/t − v| 2fN2(q−1) L1 x,v ∥f∥ 1−N 2(q−1) L1 x,v )1/q .

Here, we note that∥f∥L1

x,v =∥ϕ∥L1x,v, indeed,

d

dt∥f∥L1x,v =

∫∫

∂tf dvdx = ∫∫(v· ∇xf + E· ∇vf ) dvdx = 0. And, f is a constant along characteristics,

indeed, since f is an integral of the system of ordinary differential equations ˙

X = V , V = E(X, t) ,˙ t≥ 0 , f satisfies that

f (X(t), V (t), t) = f (X(0), V (0), 0) = ϕ(X(0), V (0)) , t≥ 0 ,

and hence,∥f∥L∞x,v ≤ ∥ϕ∥L∞x,v (see [9], [10]).

Thus, we have that for 1≤ q ≤ 1 + 2/N,

f dv∥Lqx ≤ C1∥|x/t − v| 2 f∥ N 2(1−1/q) L1 x,v , and from (2.1), ∥ρ(t)∥Lqx=f dv∥Lqx ≤ C1t −N(1−1/q), t > 0 ,

which implies the estimate (1.7), where C1= C1(∥(1 + |x|2)ϕ∥L1

x,v,∥ϕ∥L∞x,v) is

a constant depending on∥(1 + |x|2 L1

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Moreover, using Lemma 2.1 with λ = N− 1, we obtain ∥E(t)∥Lpx ≤ C∥ x |x|N ∗ ρ(t)∥Lpx ≤ C∥ρ(t)∥Lqx ≤ C1t−N(1−1/N−1/p), t > 0 with 1/p = 1/q− 1/N, 1 < q ≤ (N + 2)/N, i.e. N/(N − 1) < p ≤ N(N + 2)/(N2− N − 2), which implies the estimate (1.8). 

References

[1] C. Bardos and P. Degond, Global existence for the Vlasov–Poisson equation in 3 space variables with small initial data, Ann. Inst. H. Poincare Anal. Non Lineaire 2 (1985) 101–118.

[2] J. Batt, Global symmetric solutions of the initial value problem of stellar dynamics, J. Differential Equations 25 (1977) 342–364.

[3] R. DiPerna and P.L. Lions, Global weak solutions of kinetic equations, Rend. Sem. Mat. Univ. Politec. Torino 46 (1988) 259–288.

[4] R. Kurth, Das Anfangswertproblem der Stellardynamik, Z. Astrophys. 30 (1952) 213– 229.

[5] R. Glassey, The Cauchy Problem in Kinetic Theory, SIAM, 1996.

[6] E. Horst, On the classical solutions of the initial value problem for the unmodified nonlinear Vlasov equation. I. General theory, Math. Methods Appl. Sci. 3 (1981) 229– 248.

[7] E. Horst, On the classical solutions of the initial value problem for the unmodified nonlinear Vlasov equation. II., Math. Methods Appl. Sci. 4 (1982) 19–32.

[8] E. Horst and R. Hunze, Weak solutions of the initial value problem for the unmodified nonlinear Vlasov equation, Math. Methods Appl. Sci. 6 (1984) 262–279.

[9] R. Illner and G. Rein, Time decay of the solutions of the Vlasov–Poisson system in the plasma physical case, Math. Methods Appl. Sci. 19 (1996) 1409–1413.

[10] K. Pfaffelmoser, Global classical solutions of the Vlasov–Poisson system in three di-mensions for general initial data, J. Differential Equations 95 (1992) 281–303. [11] H. Risken, The Fokker-Planck equation, Springer series in synergetics Vol. 18, Springer–

Verlag, Berlin 1989.

[12] J. Schaeffer, Global existence for the Poisson–Vlasov system with nearly symmetric data, J. Differential Equations 69 (1987) 111–148.

[13] C. Sogge, Fourier integrals in classical analysis, Cambridge 1993.

[14] S. Ukai and T. Okabe, On classical solutions in the large in time of two dimensional Vlasov’s equation, Osaka J. Math. 15 (1978) 245–261.

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