30 Global Attraction toStationaryStates
Then the system (1.3.1), (1.3.2) is formally Hamiltonian with the Hilbert
phase spaceEand the Hamiltonian functional
H(ψ,π)=
1
2
ψ
R
[|π(x)|
2
+|ψ
ω
(x)|
2
]dx+
N
→
k=1
Vk(ψ(xk)), (ψ,π)∈E.
(1.3.7)
We consider solutionsY(t)=(ψ(·,t),˙ψ(·,t))∈C(R,E), and we write the
system (1.3.1), (1.3.2) in the form
Y(t)=F(Y (t)), t∈R. (1.3.8)
Let us discuss the definition of the Cauchy problem for the functionsY(t)∈
C(R,E). The first equation of (1.3.2) makes sense and holds automatically
becauseψ∈C(R
2
,R
d
)by the Sobolev embedding theorem due toY(t)∈
C(R,E).Theequation (1.3.1)is understood in the sense of distributions of
(x,t)∈[R\Q]×R. Hence, this equation is equivalent to the d’Alembert
decompositions for everyk=1,...,N+1,
ψ(x,t)=f
k(xt)+g k(x+t), x∈→ k:=(xk1,xk), t∈R,(1.3.9)
wheref
k,gk∈C(R,R
d
)due toψ∈C(R
2
,R
d
), and we denotex 0:= ∞
andx
N+1=+∞. Hence, for allk=1,...,Nand(x,t)∈→ k×R,
ψ
ω
(x,t)=f
ω
k
(xt)+g
ω
k
(x+t),ψ(x,t)=f
ω
k
(xt)+g
ω
k
(x+t),
(1.3.10)
where all derivatives are understood in the sense of distributions. The assump-
tionY(t)∈C(R,E)implies
f
ω
k
(·),g
ω
k
(·)∈L
2
loc
(R,R
d
),∀k=1,...,N+1. (1.3.11)
We now explain the second equation of (1.3.2).
Definition 1.3.2In the second equation of (1.3.2), for everyk=1,...,N,
∈
ψ
ω
(xk0,t):=f
ω
k
(xkt)+g
ω
k
(xk+t)∈L
2
loc
(R,R
d
)
ψ
ω
(xk+0,t):=f
ω
k+1
(xkt)+g
ω
k+1
(xk+t)∈L
2
loc
(R,R
d
)
∇
∇
∇
∇
∇
. (1.3.12)
Note that the functionsf
kandg kin (1.3.9) are unique up to an additive
constant, so the definition (1.3.12) is unambiguous.