We investigate the existence of analytic solutions of a class of second-order differential equations involving iterates of the unknown function x^′′(z) + cx^′(z) = x(az + bx(z)) in the complex field ℂ. By reducing the equation with the Schröder transformation to the another functional differential equation without iteration of the unknown function λ²g^′′(λz)g^′(z) − λg^′(λz)g^′′(z) + = , we get its local invertible analytic solutions.

1. Introduction

Functional differential equations with state dependent delay have attracted the attentions of many authors in the last years because of their extensive applications (e.g., [1–4] ). However, there are only a few papers dealing with functional differential equation with state derivative dependent delay. In [5] Eder studied the functional differential equation x^′(t) = x(x(t)). V. R. Petahov [6] proved the existence of solutions of equation

()

In [7–9], the authors studied the existence of analytic solutions of the following second-order iterative functional differential equations:

()

respectively. Since such equations are quite different from the usual differential equations, the standard existence and uniqueness theorems cannot be applied directly. It is therefore of interest to find some or all of their solutions. In this paper, we will discuss the existence of analytic solutions for another second-order functional differential equation with a state derivative dependent delay:

()

where a, b, and c are complex numbers. When c = 0, (1.3) change into functional differential equation x^′′(z) = x(az + bx(z)), it has been studied in [9]. For the general equation (1.3) the same idea, however, cannot be applied. Therefore, in Section 3 we first reduce (1.3) to an iterative functional differential equation. Then, as in [9], the author reduces again this iterative functional differential equation with the Shchröder transformation, that is, x(z) = (1/b)(g(λg⁻¹(z)) − az) to a functional differential equation with proportional delay (which is called the auxiliary equation). Lastly, according to the position of an indeterminate constant λ in complex plane, we discuss the existence of analytic solutions.

In the next section, we will seek explicit analytic solutions of (1.3) in the form of power functions, in the case b = 0.

2. Explicit Analytic Solutions

In case b = 0, (1.3) changes into functional differential equation

()

For the above equation we have the following proposition.

Proposition 2.1. Suppose 0 < |a| ≤ 1. Then (2.1) has an analytic solution x(z) in a neighborhood of the origin, satisfying x(0) = μ and x^′(0) = η ∈ ℂ.

Proof. Let

()

be the expansion of formal solution x(z) of (2.1). Substituting (2.2) into (2.1), we get

()

By comparing the coefficients, we have

()

In view of |a| ≤ 1 and c is complex number, thus there exists a positive number m, so that

()

Thus if we define recursively a sequence

by B₀ = |μ|, B₁ = |η|,

()

then one can show that by induction

()

Now if we define

()

then

()

that is,

()

Let

()

for (z, w) from a neighborhood of (0, |μ|). Since

, there exists a unique function w(z), analytic on a neighborhood of zero, such that w(0) = |μ|, w^′(0) = |η| and satisfying the equality Q(z, w(z)) = 0. According to (2.8) and (2.10), we have M(z) = w(z). It follows that the power series (2.8) converges on a neighborhood of the origin, which implies that the power series (2.2) also converges in a neighborhood of the origin. The proof is complete.

Thus, the desired solution of (2.1) is

()

3. Analytic Solutions of the Auxiliary Equation

A distinctive feature of the (1.3) when b ≠ 0 is that the argument of the unknown function is dependent on the state x(z), and this is the case we will emphasize in this paper. We now discuss the existence of analytic solution of (1.3) by locally reducing the equation to another functional differential equation with proportional delays. Let

()

Then for any number z, we have

()

and so x(y(z)) = (1/b)(y(y(z)) − ay(z)). Therefore, in view of (1.3) and x^′′(z) = (1/b)y^′′(z), we have

()

To find analytic solution of (3.3), we first seek an analytic solution g(z) of the auxiliary equation

()

satisfying the initial value conditions

()

where μ and η are complex numbers, and λ satisfies one of the following conditions:

(H1)
0 < |λ| < 1;
(H2)
λ = e^2πiθ, where θ ∈ ℝ∖ℚ is a Brjuno number, that is, where {p_k/q_k} denotes the sequence of partial fraction of the continued fraction expansion of θ;
(H3)
λ = e^2πiq/p for some integers p ∈ ℕ with p ≥ 2 and q ∈ ℤ∖{0}, and α ≠ e^2πil/k for all 1 ≤ k ≤ p − 1 and l ∈ ℤ∖{0}.

We observe that λ is inside the unit circle S¹ in case (H1) but on S¹ in the rest cases. More difficulties are encountered for λ on S¹ since the small divisor λⁿ⁺² − λ is involved in the latter (3.10). Under Diophantine condition: “λ = e^2πiθ, where θ ∈ ℝ∖ℚ and there exist constants ζ > 0 and δ > 0 such that |λⁿ − 1| ≥ ζ⁻¹n^−δ for all n ≥ 1, ” the number λ ∈ S¹ is “far” from all roots of the unity and was considered in different settings [7–12]. Since then, we have been striving to give a result of analytic solutions for those λ “near” a root of the unity, that is, neither being roots of the unity nor satisfying the Diophantine condition. The Brjuno condition in (H2) provides such a chance for us. Moreover, we also discuss the so-called the resonance case, that is, the case of (H3).

Theorem 3.1. If (H1) holds. Then, for the initial value conditions (3.5), (3.4) has an analytic solution of the form

()

in a neighborhood of the origin.

Proof. Rewrite (3.4) in the form

()

Therefore, in view of g^′(0) = η ≠ 0, we obtain

()

We now seek a solution of (3.4) in the form of a power series (3.6). By defining b₀ = μ and b₁ = η and then substituting (3.6) into (3.9), we see that the sequence

is successively determined by the condition

()

in a unique manner. Now we show that the resulting power series (3.6) converges in a neighborhood of the origin. First of all, we have lim _n→∞|1/(λⁿ⁺² − λ)| = 1/|λ|, thus there exists a positive number M, such that

()

If we define a sequence

by D₀ = |μ|, D₁ = |η| and

()

then in view of (3.10), we can show by induction that

()

Now if we define

()

then

()

We get immediately

()

That is,

()

Let

()

for (z, w) from a neighborhood of (0, |μ|). Since R(0, |μ|) = 0 and

, and the implicit function theorem, there exists a unique function w(z), analytic in a neighborhood of zero, such that w(0) = |μ|, w^′(0) = |η| and R(z, w) = 0. According to (3.14) and (3.17), we have G(z) = w(z). It follows that the power series (3.14) converges in a neighborhood of the origin. So does (3.6). The proof is complete.

Now, we discuss local invertible analytic solutions of auxiliary equation (3.4) in cases (H2). In order to study the existence of analytic solutions of (3.6) under the Brjuno condition, we first recall briefly the definition of Brjuno numbers and some basic facts. As stated in [13], for a real number θ, we let [θ] denote its integer part and {θ} = θ − [θ] its fractional part. Then every irrational number θ has a unique expression of the Gauss’ continued fraction

()

denoted simply by θ = [d₀, d₁, …, d_n, …], where d_j’s and θ_j’s are calculated by the algorithm: (a) d₀ = [θ], θ₀ = {θ} and (b) d_n = [1/θ_n−1], θ_n = {1/θ_n−1} for all n ≥ 1. Define the sequences {p_n} _n∈ℕ and {q_n} _n∈ℕ as follows:

()

It is easy to show that p_n/q_n = [d₀, d₁, …, d_n]. Thus, for every θ ∈ ℝ∖ℚ we associate, using its convergence, an arithmetical function B(θ) = ∑_n≥0 log q_n+1/q_n. We say that θ is a Brjuno number or that it satisfies Brjuno condition if B(θ)<+∞. The Brjuno condition is weaker than the Diophantine condition. For example, if

for all n ≥ 0, where c > 0 is a constant, then θ = [d₀, d₁, …, d_n, …] is a Brjuno number but is not a Diophantine number. So, the case (H2) contains both Diophantine condition and a part of α “near” resonance. Let θ ∈ ℝ∖ℚ and {q_n} _n∈ℕ be the sequence of partial denominators of the Gauss’s continued fraction for θ. As in [13], let

()

Let

be the set of integers j ≥ 0 such that either j ∈ A_k or for some j₁ and j₂ in A_k, with j₂ − j₁ < E_k, one has j₁ < j < j₂ and q_k divides j − j₁. For any integer n ≥ 0, define

()

where

We then define function h_k : ℕ → ℝ₊ as follows:

()

Let g_k(n): = max{h_k(n), [n/q_k]}, and define k(n) by the condition q_k(n) ≤ n ≤ q_k(n)+1. Clearly, k(n) is nondecreasing. Now we are able to state the following result.

Lemma 3.2 (Davie’s lemma [14]). Let Then

(a)
there is a universal constant γ > 0 (independent of n and θ) such that
()
(b)
K(n₁) + K(n₂) ≤ K(n₁ + n₂) for all n₁ and n₂;
(c)
−log |αⁿ − 1| ≤ K(n) − K(n − 1).

Theorem 3.3. Suppose (H2) holds. Then (3.4) has an analytic solution of the form (3.6) in a neighborhood of the origin such that g(0) = μ, g^′(0) = η.

Proof. As in the proof of Theorem 3.1, we seek a power series solution of the form (3.6). Set b₀ = μ and b₁ = η, (3.10) holds again. From (3.10) we get

()

To construct a governing series of (3.6), we consider the implicit functional equation

()

where R is defined in (3.18). Similarly to the proof of Theorem 3.1, using the implicit function theorem we can prove that (3.26) has a unique analytic solution φ(z, μ, η, M) in a neighborhood of the origin such that φ(0, μ, η, M) = |μ| and

. Thus φ(z, μ, η, M) in (3.26) can be expanded into a convergent series

()

in a neighborhood of the origin. Replacing (3.27) into (3.26) and comparing coefficients, we obtain that C₀ = |μ|, C₁ = |η| and

()

Note the power series (3.27) converges in a neighborhood of zero. Hence there is a positive constant T such that

()

Now by induction, we prove that

()

where K : ℕ → ℝ is defined in Lemma 3.2. In fact, |b₁| = C₁. For inductive proof, we assume that |b_m| ≤ C_me^K(m−1). From (3.10) and Lemma 3.2 ,we know

()

Note that

()

Hence

()

as desired. In view of (3.29) and Lemma 3.2, we know that K(n) ≤ n(B(θ) + γ) for some universal constant γ > 0. Then

()

That is,

()

This implies that the convergence radius of (3.6) is at least

. This concludes the proof.

In case (H3), the constant λ is not only the unit circle in ℂ, but also a root of unity. In such a case, the resonant case, both the Diophantine and the Brjuno conditions are not satisfied. Let

be a sequence define by D₀ = |μ|, D₁ = |η| and

()

where

and M is defined in Theorem 3.1.

Theorem 3.4. Suppose that (H3) holds, let be determined by b₀ = μ, b₁ = η and

()

where

()

If Θ(vp − 1, λ) = 0, v = 1,2, …, then (3.4) has an analytic solution g(z) in a neighborhood of the origin such that g(0) = μ, g^′(0) = η, and g^(vp−1)(0) = (vp − 1)! η_vp−1, where all η_vp−1’s are arbitrary constants satisfying the inequality |η_vp−1| ≤ D_vp−1, and sequence

is defined in (3.36). Otherwise, if Θ(vp − 1, λ) ≠ 0, v = 1,2, …, then (3.4) has no any analytic solution g(z) in a neighborhood of the origin.

Proof. We seek a power series solution of (3.4) of the form (3.6), as in the proof of Theorem 3.1, where the equality in (3.10) or (3.29) is indispensable. If Θ(vp − 1, λ) ≠ 0, v = 1,2, …, then the equality in (3.37) does not hold for n = vp − 1, sinceλ^vp − 1 = 0. In such a circumstance (3.4) has no formal solutions.

If Θ(vp − 1, λ) = 0 for all natural numbers v, the corresponding b_vp−1 in (3.29) has infinitely many choices in ℂ; this is, the formal series solution (3.6) defines a family of solutions with infinitely many parameters. Choose b_vp−1 = η_vp−1 arbitrarily such that

()

where D_vp−1 is defined by (3.36). In what follows, we prove that the formal series solution (3.6) converges in a neighborhood of the origin. Observe that

for n ≠ vp − 1. It follows from (3.29) that

()

for n ≠ vp − 1, v = 1,2, …. Further, we can prove that

()

Set

()

It is easy to check that (3.42) satisfies the implicit functional equation

()

where R is defined in (3.18). Moreover, similarly to the proof of Theorem 3.1, we can prove that (3.43) has a unique analytic solution ψ(z, μ, η, ΓM) in a neighborhood of the origin such that ψ(0, μ, η, ΓM) = |μ|, and

. Thus (3.42) converges in a neighborhood of the origin. Therefore, the series (3.6) converges in a neighborhood of the origin. The proof is complete.

Theorem 3.5. Suppose the conditions of Theorem 3.1 or Theorem 3.3 or Theorem 3.4 are satisfied. Then (1.3) has an analytic solution x(z) = (1/b)(g(λg⁻¹(z)) − az) in a neighborhood of the number μ, where g(z) is an analytic solution of (3.4).

Proof. In view of Theorem 3.1 or Theorem 3.3 or Theorem 3.4, we may find a sequence such that the function g(z) of the form (3.6) is an analytic solution of (3.4) in a neighborhood of the origin. Since g^′(0) = η ≠ 0, the function g⁻¹(z) is analytic in a neighborhood of g(0) = μ. If we now define x(z) = (1/b)(g(λg⁻¹(z)) − az) then

()

then

()

as required.

4. Analytic Solution of (1.3)

In the previous section, we have shown that under the conditions of Theorem 3.1 or Theorem 3.3 or Theorem 3.4, (3.3) has an analytic solution y(z) = g(λg⁻¹(z)) in a neighborhood of the number μ, where g(z) is an solution of (3.4). Since the function y(z) = g(λg⁻¹(z)) can be determined by (3.10), it is possible to calculate, at last in theory, the explicit form of y(z), an analytic solution of (1.3), in a neighborhood of the fixed point μ of y(z) by means of (3.2). However, knowing that an analytic solution of (1.3) exists, we can take an alternative route as follows. Assume that x(z) is of the form

()

we need to determine the derivatives x⁽ⁿ⁾(μ), n = 0,1, 2, …. First of all, in view of (1.3) and (3.2), we have

()

respectively. Furthermore,

()

Next by calculating the derivatives of both sides of (1.3), we obtain successively

()

so that

()

Recall the formula for the higher derivatives of composition. Namely, for n ≥ 1,

()

where h(z) = az + bx(z), and P_i,k is a polynomial with nonnegative coefficients.

We have

()

Thus the desired solutions is

()

References

1 Hale J., Theory of Functional Differential Equations, 1977, 2nd edition, Springer, New York, NY, USA, MR0508721, ZBL0461.05016.
10.1007/978-1-4612-9892-2
Web of Science® Google Scholar
2 Bellman R. and Cooke K. L., Differential-Difference Equations, 1963, Academic Press, New York, NY, USA, MR0147745, ZBL0226.76030.
10.1063/1.3050672
Google Scholar
3 Brjuno A. D., Analytic form of differential equations, Transactions of the Moscow Mathematical Society. (1971) 25, 131–288.
Google Scholar
4 Driver R. D., Existence theory for a delay-differential system, Contributions to Differential Equations. (1963) 1, 317–336, MR0150421, ZBL0126.10102.
Google Scholar
5 Eder E., The functional-differential equation x^′(t) = x(x(t)), Journal of Differential Equations. (1984) 54, no. 3, 390–400, MR760378, https://doi.org/10.1016/0022-0396(84)90150-5, ZBL0497.34050.
10.1016/0022-0396(84)90150-5
Web of Science® Google Scholar
6 Petuhov V. R., On a boundary value problem, Trudy Seminara po Teorii Differencial′nyh Uravneniĭ s Otklonjajuščimsja Argumentom. Universitet Družby Narodov im. Patrisa Lumumby. (1965) 3, 252–255, MR0208109.
Google Scholar
7 Si J.-G. and Wang X.-P., Analytic solutions of a second-order functional-differential equation with a state derivative dependent delay, Colloquium Mathematicum. (1999) 79, no. 2, 273–281, MR1670209, ZBL0927.34077.
10.4064/cm-79-2-273-281
Google Scholar
8 Si J.-G. and Wang X.-P., Analytic solutions of a second-order iterative functional differential equation, Journal of Computational and Applied Mathematics. (2000) 126, no. 1-2, 277–285, MR1806761, https://doi.org/10.1016/S0377-0427(99)00359-3, ZBL0983.34056.
10.1016/S0377-0427(99)00359-3
Web of Science® Google Scholar
9 Si J.-G. and Wang X.-P., Analytic solutions of a second-order functional differential equation with a state dependent delay, Results in Mathematics. (2001) 39, no. 3-4, 345–352, MR1834580, ZBL1017.34074.
10.1007/BF03322694
Google Scholar
10 Si J.-G. and Zhang W., Analytic solutions of a second-order nonautonomous iterative functional differential equation, Journal of Mathematical Analysis and Applications. (2005) 306, no. 2, 398–412, MR2135843, https://doi.org/10.1016/j.jmaa.2005.01.005, ZBL1083.34060.
10.1016/j.jmaa.2005.01.005
Web of Science® Google Scholar
11 Si J.-G. and Zhang W., Analytic solutions of a q-difference equation and applications to iterative equations, Journal of Difference Equations and Applications. (2004) 10, no. 11, 955–962, MR2082681, https://doi.org/10.1080/10236190412331272607, ZBL1067.39035.
10.1080/10236190412331272607
Web of Science® Google Scholar
12 Liu T. and Li H., Local analytic solution of a second-order functional differential equation with a state derivative dependent delay, Applied Mathematics and Computation. (2008) 197, no. 1, 158–166, MR2396300, https://doi.org/10.1016/j.amc.2007.07.057, ZBL1143.34042.
10.1016/j.amc.2007.07.057
Web of Science® Google Scholar
13 Carletti T. and Marmi S., Linearization of analytic and non-analytic germs of diffeomorphisms of (ℂ, 0), Bulletin de la Société Mathématique de France. (2000) 128, no. 1, 69–85, MR1765828, ZBL0997.37017.
10.24033/bsmf.2363
Web of Science® Google Scholar
14 Davie A. M., The critical function for the semistandard map, Nonlinearity. (1994) 7, no. 1, 219–229, MR1260138, https://doi.org/10.1088/0951-7715/7/1/009, ZBL0997.37500.
10.1088/0951-7715/7/1/009
Web of Science® Google Scholar

Citing Literature

All articles

Analytic Solutions of an Iterative Functional Differential Equation near Resonance

Abstract

1. Introduction

2. Explicit Analytic Solutions

3. Analytic Solutions of the Auxiliary Equation

4. Analytic Solution of (1.3)

References

Citing Literature

References

Information

About Wiley Online Library

Help & Support

Opportunities

Connect with Wiley

Analytic Solutions of an Iterative Functional Differential Equation near Resonance

Abstract

1. Introduction

2. Explicit Analytic Solutions

3. Analytic Solutions of the Auxiliary Equation

4. Analytic Solution of (1.3)

References

Citing Literature

References

Related

Information