Relative asymptotic equivalence of dynamic equations on time scales

This paper aims to study the relative equivalence of the solutions of the following dynamic equations y (cid:2) ( t ) = A ( t ) y ( t ) and x (cid:2) ( t ) = A ( t ) x ( t ) + f ( t , x ( t )) in the sense that if y ( t ) is a given solution of the unperturbed system, we provide suﬃcient conditions to prove that there exists a family of solutions x ( t ) for the perturbed system such that (cid:2) y ( t ) – x ( t ) (cid:2) = o ( (cid:2) y ( t ) (cid:2) ), as t → ∞ , and conversely, given a solution x ( t ) of the perturbed system, we give suﬃcient conditions for the existence of a family of solutions y ( t ) for the unperturbed system, and such that (cid:2) y ( t ) – x ( t ) (cid:2) = o ( (cid:2) x ( t ) (cid:2) ), as t → ∞ ; and in doing so, we have to extend Rodrigues inequality, the Lyapunov exponents, and the polynomial exponential trichotomy on time scales.


Introduction
In the study of nature, in particular, in physics, engineering, and economics, among other fields, there are phenomena that vary continuously or discretely, where each phenomenon can be modeled by a differential or difference equation. However, there exists also the possibility that these phenomena can vary both continuously and discretely. The theory of time scales calculus made it possible to create models to study such mixed phenomena, and it also turns out to be a powerful tool in continuous and discrete analysis from a unified point of view (see, for instance, [2,4,5,7,8] and the reference therein). A time scale, which is denoted by T, is any closed nonempty subset of the real numbers, for instance, N, Z, and q Z for q > 0 are times scales, and was introduced by S. Hilger [12,14] in order to create the theory of dynamic equations that allows unifying differential and difference equations, as well as their extensions, from the same perspective (see [4,5]). For example, if f represents the derivative for a function f defined on T, then it turns out that f = f , the usual derivative, if T = R, and f = f , the usual forward difference operator, if T = Z. In the last years the qualitative study of the solutions of dynamic equations on time scales has been attracting the interest and effort of many mathematicians; in particular, the study of stability, existence of dichotomies, existence of bounded solutions, the existence of peri-odic and almost periodic solutions, have been treated by several researchers (for instance, see [3,9,11,17,18,[20][21][22][25][26][27] and the references therein), however, to the best to our knowledge, the study of asymptotic equivalence of solutions of dynamic equations on time scales has not been carried out. In the case of ordinary differential equations, functional differential equations, and difference equations, we can find works in this direction, see, for instance, [6,15,16,23,24] and the reference therein.
Motivated by this fact, in this paper we shall investigate the relative asymptotic equivalence of the solutions of the following two dynamic equations on time scales: where (X, · ) is a Banach space, the operator A : T − → L(X) is rd-continuous, f : T × X − → X is an rd-continuous function which is a small perturbation in some sense with f (t, 0) = 0 and for h : [t 0 , ∞) T − → R + an rd-continuous function. We will understand this equivalence in the following sense: given a solution y(t) of system (1), there exists a family of solutions x(t) of system (2) such that y(t)x(t) = o( y(t) ), as t → ∞, and conversely, if x(t) is a solution of (2), then there exists a family of solutions y(t) for the unperturbed system (1) such that y(t) - It is important to mention that, if T = R, we get the equations studied in [16], while if T = N ∪{0} then we have the equations treated in [15]. In both papers, the main tools used were a concept of polynomial exponential dichotomy and the so-called Rodrigues inequality (see also [24] and [19]), which is a generalization of Gronwall's inequality. Concretely, we will treat the following problems: The direct problem Let y(t) be a solution of (1), with y(t) = 0, for sufficiently large t. Then, does there exist a solution x(t) of (2) such that the relative error satisfies Converse problem Let x(t) be a solution of (2), with x(t) = 0, for sufficiently large t. Then, does there exist a solution y(t) of (1) such that the relative error satisfies To solve these problems, we will extend the Rodrigues inequality, the definition of Lyapunov exponents, and we introduce the definition of polynomial exponential trichotomy on time scales.
The paper is organized as follows. In the next section, we present some fundamentals results about time scales. In Sect. 3 we analyze the direct problem, and in Sect. 4 we study the converse problem. Section 5 is devoted to an example to illustrate our results. We end this work with a remark.

Preliminaries
A time scale T is an arbitrary nonempty closed subset of R. We will assume that T has the topology inherited from the standard topology of the real numbers. The time scale interval [a, b] T is defined as [a, b] T = {t ∈ T : a ≤ t ≤ b}, with a, b ∈ T, and open intervals and neighborhoods are defined similarly. The forward jump operator σ : T − → T is defined by σ (t) := inf{s ∈ T : s > t} and the backward jump operator ρ : T − → T is defined by ρ(t) := sup{s ∈ T : s < t}. We put inf ∅ = sup T (i.e., σ (t) = t if T has a maximum t) and We define the set T κ by We shall say that a function f : T − → X is right dense continuous, or just rd-continuous, if (a) f is continuous at every right-dense point t ∈ T, is rd-continuous for any continuous function x : T − → X.
A function f : T − → X is called delta differentiable (or simply differentiable) at t ∈ T κ provided there exists a number f (t) with the property that given any ε > 0, there is a neighborhood U = (tδ, t + δ) T for some δ > 0 such that f (σ (t))f (s)f (t)(σ (t)s) ≤ ε|σ (t) -s| for all s ∈ U, in this case the number f (t) will be call the delta derivative of f at t.
If there exists a function F : T − → X such that F (t) = f (t), t ∈ T κ , then F is called an antiderivative of f and the Cauchy integral is defined by From Theorem 1.74 in [4], we have that every rd-continuous function has an antiderivative, and if F(t) = t s f (τ ) τ , then F (t) = f (t), t ∈ T κ , i.e., F is an antiderivative of f . A function p : T − → R is said to be regressive (resp. positively regressive) if 1 + μ(t)p(t) = 0 (resp. 1 + μ(t)p(t) > 0), t ∈ T and the equation has a unique solution The function e p (t, s) satisfies the following properties (see [4,Theorem 2.36]): (1) e p (t, s)e q (t, s) = e p⊕q (t, s), (2) e p (t,s) e q (t,s) = e p q (t, s), 1+μ(t)q(t) . We will denote by R (resp. R + ) the set of all regressive and rd-continuous functions (resp. positively regressive and rd-continuous).
A mapping A : Henceforth we will suppose that sup{T} = ∞. Since A is rd-continuous, we have that A generates an evolution operator family T(t, s) = e A (t, s) with t ≥ s, and the only solution of (1) with y(s) = y s is given by y(t) = T(t, s)y s (see [10] and [25]). On the other hand, since f is rd-continuous and satisfies condition (3), we have, by Theorem 5.7 in [13], that equation (2), with x(s) = x s , has a unique solution defined on T, which is given by

The direct problem
In this section, we just treat the direct problem through Theorem 3.1, but before we will present the definition of polynomial exponential trichotomy on time scales. It is important to mention that, in the case of dichotomies on time scales, the initial contributions are by Pötzsche in [20,21].
Definition 3. 1 We shall say that equation (1) has a polynomial exponential trichotomy on time scales with respect to α ∈ C rd (T,

s, P(t)T(t, s) = T(t, s)P(s), Q(t)T(t, s) = T(t, s)Q(s), and S(t)T(t, s) = T(t, s)S(s),
For each l, 0 ≤ l ≤ N -1, the following conditions hold: where β > 0. If l = 0 then P(s) = 0. In this case the projections onto the stable, unstable, and center spaces are contained in L(X).
Remark 1 Conditions (2) and (3) allow for the evolution operator T(t, s) to be invertible between the ranges of U and Q, respectively, for details see Proposition 2.3 in [20].

Definition 3.2
If α > 0 and l is a nonnegative integer, we shall say that a function y : The following theorem is our main result in this section (3) is satisfied and that equation (1) has a polynomial exponential trichotomy on time scales with respect to α and the function h(t) satisfies Then there exists a solution x(t) of (2) such that Proof Let us consider the change of variable then we obtain the following equation for z: where . A straightforward computation shows that the evolution operator T α (t, s) generated by A α (t) is given by Hence, we obtain the following conditions for T α (t, s): Therefore, the problem is reduced to proving the existence of a solution of (14) such that If we suppose for a moment that z(t) is a solution of (14) such that (16) holds. Then using the variation of constant formula for equation (14) (Theorem 5.8 in [4]), we get that If we let τ − → ∞, then we get In fact, since As it will become clear in the next calculation, condition (16) is satisfied if we require So, we get that for t ≥ η. Conversely, if z(t), with t -l z(t) bounded, satisfies the integral equation (17), then z satisfies equation (14). Now, to complete the proof, we shall prove that the integral equation (17) has a solution in the Banach space We define the operator : Next, we shall prove that maps Z l into Z l . In fact, for z ∈ Z l we have the following estimate: and then where y α,l := sup t≥η t -l e α (t, t 0 ) y(t) . Therefore, lim t→∞ ( z)(t) t l = 0, and so z ∈ Z l . Now, we shall prove that the operator is a contraction mapping. In fact, let z 1 , z 2 ∈ Z l and consider We choose η large enough such that ∞ η σ N-1 (s)h(s) s < 1/M. Therefore, we get that is a contraction mapping from Z l to Z l . By applying the contraction mapping principle, it follows that this operator has a unique fixed point which depends on ω ∈ X. The solution x(t) of (2) that solves the Direct Problem, is given by x(t) := e α (t, t 0 )z(t) + y(t), where z(·) is the fixed point of . The above estimates also imply that t -l z(t) − → 0, as t − → ∞.

The converse problem
In the present section we will study the converse problem, as a special case. First, we will present the Rodrigues inequality on time scales. We need the following lemma.

If u(t) ≥ 0 is a bounded rd-continuous and decreasing function defined for t ∈ [t 0 , ∞) T and satisfies
For τ > M, suppose first that β(t) = c is constant, then

Lemma 4.2 (Rodrigues inequality) Let f and g be nonnegative rd-continuous functions
Let γ (t) > 0 be a decreasing rd-continuous function, for t ≥ η and η sufficiently large, in such a way that

Suppose that u is a nonnegative continuous function such that γ u is bounded and
Then v(t) is an increasing continuous function such that Combining the above inequalities, we get Then, By using Lemma 4.1, it follows that

Definition 4.1
Let y(t) be a solution of (1). We say that λ > 0 is a Lyapunov exponent of y(t) if, given ε > 0, then there exist T ∈ [t 0 , ∞) T and L > 0 such that Remark 2 It is easy to see that (21) implies that This γ (λ) was used in [22] to characterize stability of linear systems on time scales.

Lemma 4.3 Let T(t, s) be the evolution operator generated by A(t).
Assume that there exist complementary projections P(s), Q(s), α < β, and K > 0, as in the first part of Definition 3.1, such that T(t, s)P(s) ≤ Ke α (t, s), t ≥ s,

Now, applying the Rodrigues inequality with
we get that is bounded for t ≥ η and this contradicts the hypothesis that the Lyapunov exponent of x(t) is λ.

Lemma 4.4 Suppose that T(t, s) is the evolution operator generated by A(t). Assume that there exist complementary projections P(t), Q(t)
, α < β, K > 0, and a positive integer n, as in the first part of Definition 3.1, such that and ∞ t 0 If x(t) is a solution of (2) with Lyapunov exponent α, then x(t) t n e α (t,t 0 ) is bounded for t ≥ η.
Therefore, x(t) t n e α (t,t 0 ) is bounded.
The following theorem gives answer to the converse problem.
which implies that the conditions from Lemma 4.3 are satisfied and therefore Let us define l = min m ∈ {0, . . . , N -1} : is bounded for t ≥ η .
As in Lemma 4.3, if we put where A = 1 0 1 1 and f : T × R 2 − → R 2 satisfying condition (3). We compare system (27) with the unperturbed system The eigenvalues of the matrix A are λ 1 = λ 2 = 1, therefore 1 + μ(t) = 0, so A is regressive for any time scale T. Hence, is easy to see that the evolution operator associated to (28) is given by . Example 5.2 Let T = {2 n : n ∈ N 0 }, where N 0 is the set of nonnegative integers. Consider the following system: where A = -3 -2 3 4 and f (t, x) = 0 sin(x(σ (t))) σ (t)t . The eigenvalues of A are λ 1 = -2 and λ = 3. Since 1 -2μ(t) = 1 -2t = 0 for all t ∈ T, we get that A is regressive.

Concluding remarks
In this paper we studied the asymptotic relative equivalence between the solutions of two dynamic equations on time scales, one linear and the other formed by its nonlinear perturbation, unifying and extending the results presented in the references. To accomplish this goal, we introduced the concept of a polynomial exponential trichotomy on time scales which is a generalization of the concept of polynomial exponential dichotomy presented by Leiva and Rodrigues in [16]. It is important to mention that this concept is more general than the simple concept of polynomial dichotomy, as it involves several projections that appear in a natural way when one breaks the Jordan decomposition block in order to study a relative behavior of solutions of differential equations. In a forthcoming paper we are going to show the existence and roughness of this type of trichotomy.