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under the constraint on the total impulse of control u:
Z
T
0
h f(t),u(t)dti ≤ 1, (2)
where x(t) ∈ V = R
n
, u(t) ∈ W = R
m
, A(t), B(t) are
matrices of appropriate dimensions, U(t) is a given
central symmetric convex body in W, f is an arbitrary
continuous function such that f(t) ∈ U
◦
(t), andU
◦
(t)
is the polar of set U(t).
Assume that the Kalman type condition of com-
plete controllability holds, namely, for any vector
u ∈ W and a time moment T, function Φ(T,t)B(t)u
does not vanish identically in any interval of time t.
Under these assumptions the reachable sets D (T) to
system (1), (2) are central symmetric convex bodies.
The problem addressed is to study the limit be-
havior of the reachable sets D (T) as T → ∞. The
reachable sets are regarded as elements of the metric
space B of central symmetric convex bodies with the
Banach-Mazur distance ρ:
ρ(Ω
1
,Ω
2
) = log(t(Ω
1
,Ω
2
)t(Ω
2
,Ω
1
)),
where t(Ω
1
,Ω
2
) = inf{t ≥ 1 : tΩ
1
⊃ Ω
2
}.
The general linear group GL(V) naturally acts on the
space B by isometries. The factorspace S is called the
space of shapes of central symmetric convex bodies,
where the shape ShΩ ∈ S of a convex body Ω ∈ B
is the orbit ShΩ = {CΩ : detC 6= 0} of the point Ω
with respect to the action of GL(V). The Banach-
Mazur factormetric makes S into a compact metric
space. The convergence of the reachable sets D (T)
and their shapes is understood in the sense of the
Banach-Mazur metric. For two asymptotically equal
functions with values in the space of convex bodies
or the space of their shapes, the following notations
are used: Ω
1
(T) ∼ Ω
2
(T), if ρ(Ω
1
(T),Ω
2
(T)) → 0
as T → ∞, and similarly ShΩ
1
(T) ∼ ShΩ
2
(T), if
ρ(ShΩ
1
(T),ShΩ
2
(T)) → 0 as T → ∞. The conver-
gence of convex bodies may be also understood in the
sense of convergence of their support functions. Re-
mind that the support function of a convex compact
set is given by formula: H
Ω
(ξ) = sup
x∈Ω
hx,ξi, where
ξ ∈ V
∗
, and uniquely defines the set Ω. The equiva-
lence of the two definitions of convergence of convex
bodies — in the terms of convergence of their support
functions and in the sense of the Banach-Mazur met-
ric — is established by the following lemma (Figurina
and Ovseevich, 1999):
Lemma 1. A sequence Ω
i
∈ B converges to Ω ∈ B
in the sense of the Banach-Mazur metric if and only
if the corresponding sequence of the support func-
tions H
i
(ξ) = H
Ω
i
(ξ) converges to the support func-
tion H
Ω
(ξ) pointwise and is uniformly bounded on the
unit sphere in the dual space V
∗
.
We address the periodic case, when the con-
stituents A, B, and U of control system (1), (2) are
supposed to be continuous and periodic in t. To fix
ideas, the period is assumed to be 1. It would be in-
teresting to understand the limit behavior of reach-
able sets for a general linear system. This prob-
lem, however, seems rather difficult, since already the
time-invariant case is nontrivial, and, say, for quasi-
periodic systems it is not clear how to prove the cor-
responding natural conjectures.
3 ASYMPTOTIC BEHAVIOR OF
SHAPES OF THE REACHABLE
SETS
We study the limit behavior as T → +∞ of the curve
T 7→ ShD (T) under different assumptions on the
spectrum of the monodromy matrix. At the heart of
the considerations below there is an explicit formula
for the support function of the reachable set:
Lemma 2. The support function of the reachable set
D (T) to system (1), (2) is given by
H
D (T)
(ξ) = sup
t∈[0,T]
H
U(t)
(B(t)
∗
Φ(T,t)
∗
ξ), (3)
where Φ(t,s) is the fundamental matrix of linear sys-
tem ˙x = A(t)x.
Stable Case. Let the system (1) be asymptotically
stable, i.e.
Φ(T,t) = o(1) as T − t → +∞,
and o(1) is uniformly small. It is easy to estab-
lish the stability criterion: system (1) is asymptoti-
cally stable iff the spectrum of the monodromy matrix
M = Φ(1,0) is contained in the open unit disk of the
complex plane.
Let us show that the curve T 7→ D (T) is asymptot-
ically periodic as T → ∞. In other words, there exists
such a continuous periodic curve f : R/Z → B that
D (T) ∼ f(T) as T → +∞. Informally speaking, the
curve T 7→ D (T) is reeled on a limit cycle.
The curve f can be given by an explicit formula.
Define function
F (T) = F (T,ξ) = sup
t∈(−∞,T]
H
U
(B
∗
Φ(T,t)
∗
ξ), (4)
where the argument t of periodic functions B and U
is omitted. Due to the stability condition, F (T) is a
continuous periodic function of T. The periodicity of
F follows from the equality
Φ(T + 1,t + 1) = Φ(T,t)
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