#### Abstract

We are proving the new oscillation theorems for the solutions of third-order linear nonautonomous differential equation with complex coefficients. In the case of real coefficients we derive the oscillation criterion that is invariant with respect to the adjoint transformation. Our main tool is a new version of Levinson's asymptotic theorem.

#### 1. Introduction

Consider an ordinary nonautonomous differential equation of the third order with complex valued variable coefficients , , and .

A solution of (1.1) is said to be oscillatory if it has an infinite sequence of zeros in , and nonoscillatory, otherwise. Equation (1.1) is said to be *non-oscillatory* if all solutions are non-oscillatory and is said to be *oscillatory* if there exists at least one oscillatory solution.

Oscillation theorems for ordinary differential equation of the third order in the case of real variable coefficients have been studied in [1–7]. To the best of the author's knowledge, the oscillations of the solutions of nonautonomous third order equations with complex coefficients have not been studied yet.

Let be the set of times differentiable functions on . By substitution equation (1.1) with turns to the following equation: where the functions and are given by

If or then the solutions of (1.1) and (1.2) have the same oscillation properties; that is, (1.1) is oscillatory if and only if (1.2) is oscillatory.

Define characteristic (Weierstrass) function of (1.2) depending on a phase function By direct calculations To consider the case of complex coefficients, we are using asymptotic solutions of (1.2) in Euler form with phase functions , , that are approximate solutions of the characteristic equation Char.

Theorem 1.1 (see [4]). *If Mammana's condition
**
is satisfied except at isolated points at which may vanish, then (1.2) is oscillatory if and only if its adjoint is oscillatory.*

We will show that Mammana's condition (1.7) is connected with the dichotomy condition of Levinson, and it has a topological character (see condition (2.24) below).

Theorem 1.2 (see Lazer [5]). *Assume that conditions
**
are satisfied. Then (1.2) with the real coefficients is oscillatory.*

The adjoint transformation , or transforms (1.2) to its adjoint equation . Note that condition (1.9) is not invariant with respect to the adjoint transformation . In the case of real coefficients under some restrictions, we will give the criterion of oscillations of solutions of (1.2) that is invariant with respect to the adjoint transformation (see Theorem 2.9 below).

#### 2. Main Theorems

Let be the Wronskian of two differentiable functions and . The following asymptotic theorem is proved by using Levinson's asymptotic theorem [8].

Theorem 2.1. *Assume that there exists complex-valued phase functions , , such that expressions , do not change a sign, that is,
**
where
**Then solutions of (1.2) may be represented in the form
**
where , are defined in (1.5), (1.6), and
*

Note that conditions (2.2) and (2.3) are given in terms of characteristic functions.

We will say that (1.2) has asymptotic solutions corresponding to the phase functions , if (2.1)–(2.3) are satisfied.

Theorem 2.2. *The solution of (1.2) corresponding to the asymptotic solution with the phase is oscillatory if and only if
*

The following theorem we deduce from Theorem 2.2 by choosing

Theorem 2.3. *Assume that there exists a complex-valued function such that , and
**
Then (1.2) with complex coefficients has one nonoscillatory solution and two linearly independent oscillatory solutions if and only if
*

By taking from Theorem 2.3, we get the following corollary.

Corollary 2.4. *Assume that for some complex number **
Then (1.2) with complex coefficients has one non-oscillatory solution and two linearly independent oscillatory solutions if and only if
*

By taking from Corollary 2.4, we obtain well-known result [4].

Corollary 2.5. *Assume that conditions
**
are satisfied. Then (1.2) with complex coefficients is non-oscillatory.*

By taking from Theorem 2.3 we get another corollary.

Corollary 2.6. *Assume that conditions
**
are satisfied. Then (1.2) with complex coefficients is non-oscillatory.*

*Example 2.7. *From Corollary 2.6, (1.2) with
is non-oscillatory. Note that Corollary 2.5 is not applicable for this example since condition (2.15) fails.

In the case by taking , from Theorem 2.3, we deduce the following theorem.

Theorem 2.8. *Assume that , , and
**
Then (1.2) with the real coefficients has one non-oscillatory solution and two linearly independent oscillatory solutions if and only if
*

Another result may be proved by the different choice of the phase functions as follows: where is defined in (2.4), and

Define 3 auxiliary regions on the real plane

Theorem 2.9. *Assume that , is simply connected region for some , and conditions (2.2) and (2.3),
**
are satisfied. Then (1.2) has one non-oscillatory solution and two linearly independent oscillatory solutions if and only if
**
for at least one of cubic roots.*

Note that condition (2.25) is invariant with respect to the adjoint transformation .

For the case of the real constant coefficients , from Theorem 2.9, one can deduce the obvious result that (1.2) is oscillatory if and only if . Indeed in this case condition (2.25) turns to , and conditions (2.2), (2.3), and (2.24) could be dropped.

*Remark 2.10. *Levinson's dichotomy condition (2.24) is satisfied if the modified Mammana's condition is satisfied as follows:
If , then under condition (2.24) may change the sign.

Theorem 2.9 does not exclude the case , but Theorem 1.2 does. In the case conditions of Theorem 2.9 are simplified.

Theorem 2.11. *Assume that is real, it does not change the sign, and conditions
**
are satisfied. Then equation
**
has one non-oscillatory solution and two linearly independent oscillatory solutions if and only if
*

In the case (self-adjoint equation (1.2)), condition (2.25) turns to (2.20) (see Theorem 2.8 above), which is Leighton's (see [9]) necessary condition of oscillations for solutions of the second-order equation .

*Example 2.12. *Equation
where is a real number and is oscillatory by Corollary 2.4 since conditions (2.13) and (2.14) are satisfied with . Note that for this example Theorem 1.2 is not applicable since both conditions (1.8) and (1.9) fail even when .

#### 3. Proofs

Our main tool is Levinson's asymptotic theorem.

Theorem 3.1 (see [8]). *Let be an diagonal matrix function which satisfies dichotomy condition.**For each pair of integers and in exist constants such that for all x and t, **
Let the matrix satisfy or
**
by which we mean that each entry in has an absolutely convergent infinite integral. Then the system
**
has a vector solution with the asymptotic form
**
where is the identity matrix, is the error matrix-function, and is a constant column vector.*

* Proof of Theorem 2.1. *Rewrite (1.2) as a system
By transformation
where matrix function is defined via phase functions as follows:
we get the following:
where

Choosing specific auxiliary functions
we have
Here and further we suppress the time variable for the simplicity.

From Liouville's formula applied to (3.7) with the assumption that are solutions of , we get the following
The Liouville's formula may be written in the form
We always are choosing the phase functions such that (3.13) is satisfied (see (2.4)). From (3.13), we get that
To apply Theorem 3.1 to system (3.8) note that from (2.1) it follows dichotomy condition (3.1) of Theorem 3.1:
Condition (3.2) of Theorem 3.1 turns to , and it is followed from

One can drop condition since from (3.15) we have
Assuming that
condition may be dropped as well since
So condition (3.2) of Theorem 3.1 turns to
or (2.2) and (2.3). From Theorem 3.1 applied to system (3.8) and we get that
or representation (2.5).

*Proof of Theorem 2.2. * Theorem 2.2 is followed from Theorem 2.1 since in representation (2.5) one may choose asymptotic solutions as follows:
which are oscillating if and only if condition (2.7) is satisfied.

*Proof of Theorem 2.3. *Theorem 2.3 is deduced from Theorem 2.2 by choosing phase functions as in (2.8). From
condition (2.1) turns to condition (2.9). From (2.4), we get that
Since in conditions (2.2)-(2.3) the function appears in denominator we should assume that , or . By direct calculations, we get that
In view of
conditions (2.2) and (2.3) of Theorem 2.2 turn to (2.10) and (2.11).

Further the asymptotic solution
is oscillating if and only if (2.12) is satisfied. Indeed, the solution corresponding to the asymptotic solution is non-oscillatory ( does not have zeros; otherwise is undefined at some points).

* Proof of Corollaries 2.4 and 2.6. *We deduce Corollaries 2.4 and 2.6 from Theorem 2.3 by the special choice of function as follows:
From (2.10) and (2.11) we get that
or in the case
which is equivalent to (2.13).

Further from (2.12) we get condition (2.14) in the case :
The proof of Corollary 2.6 is followed from (3.30) to (3.27) by choosing .

*Proof of Theorem 2.8. * Theorem 2.8 is followed from Theorem 2.3. Indeed from , we have . Condition (2.10) turns to (2.18) as follows:
and condition (2.11) turns to (2.19) since
where

*Proof of Theorem 2.9. *Let choose the phase functions as in (2.21). We deduce Theorem 2.9 from Theorem 2.2. By calculations
to deduce dichotomy conditions (2.1) from (2.24), it is enough to show that

*Case 1 (). *In this case and are real, are complex conjugate and from (3.37)
If , then , .

If , then , .

Otherwise if , then , and

Further condition (2.7) turns to (2.25) since .

*Case 2 (). *In this case is complex valued, are real. In view of
we get that
Condition (2.25) fails in this case: and (1.2) is non-oscillatory. Further
If then , , , and we get the following:
If then , , , and we get the following:

*Proof of Theorem 2.11. * By taking , we get the following:
and Theorem 2.9 turns to Theorem 2.11.