On the Meromorphic Solutions of Fermat-Type Differential Equations ()
1. Introduction and Main Results
Throughout this paper, we concentrate on such meromorphic functions that are nonconstant and meromorphic in the whole complex plane
. Then it is assumed that the reader is familiar with the fundamental notation and terminology of Nevanlinna’s value distribution theory (see [1] [2] [3] [4]) and in particular with the most usual of symbols:
,
,
,
and the order
and so on. Meanwhile, we denote by
the family of all meromorphic functions
such that
, where
outside of a possible exceptional set of finite logarithmic measure. Moreover, we also include all constant functions in
.
The following equation
(1.1)
can be regarded as the Fermat diophantine equations
over function fields, where n is a positive integer. Montel [5] obtained that Equation (1.1) has no nonconstant entire solutions when
. Gross [6] proved that Equation (1.1) has no nonconstant meromorphic solutions when
. For
, Gross [7] showed that all meromorphic solutions of Equation (1.1) of the form
and
, where
is a nonconstant meromorphic function. For
, Baker [8] proved that the only nonconstant meromorphic solutions of Equation (1.1) are the functions
and
for a nonconstant entire function u and a cubic root
of unity, where
denotes the Weierstrass
function. Further, Yang [9] investigated a generalization of the Fermat-type equation
(1.2)
and obtained that Equation (1.2) has no nonconstant entire solutions when
. For more detail, we refer the reader to the work of Hu, Li and Yang [10].
As we known, Halburd-Korhonen [11] and Chiang-Feng [12] independently proved the difference analogue of the logarithmic derivative lemma in 2006. Afterwards, a number of papers have focused on entire solutions of complex difference equations and differential-difference equations. For some works related to partial differential equations of Fermat type, see [13] - [18].
In 2012, Liu et al. [13] investigated the entire solutions of the Fermat-type differential equation of the
(1.3)
and obtained the following results.
Theorem A ( [13]) Equation (1.3) has no transcendental entire solutions with finite order, provided that
, where
are positive integers,
is a constant.
Theorem B ( [13]) The transcendental entire solutions with finite order of the equation
must satisfy
, where B is a constant and
or
, k is an integer.
For
, Liu [13] gave examples to illustrate the existence of the solutions of the Equation (1.3).
Example A
is a solution of the equation
, where
.
Example B
is a solution of the equation
, where
.
Since no attempts, till now, have so far been made by any researchers investigating the form of the solution of Equation (1.3) When
. Naturally, we pose the following questions.
Question 1. Can we get the forms of the solutions of Equation (1.3) when
.
In addition, we recall the definition of exponential polynomial, an exponential polynomial of order
, which is an entire function of the form:
(1.4)
where
and
are polynomials in z for
, such that
. Following Steinmetz [19], such a function can be written in the form:
(1.5)
where
are m distinct finite nonzero complex numbers, while
is either an exponential polynomial of degree less than q or an ordinary polynomial in z for
.
In this paper, we pay our attention to the above question and prove the following some theorems that improve and extend Theorem B.
Theorem 1.1. Let
be the exponential polynomial solutions of the equation
(1.6)
where
, one of the following conclusions hold:
1) If
, then
, where
,
satisfy
.
2) If
, then
, where
,
satisfy
.
Example 1.1. If
in (1.6), then the exponential polynomial solutions of the following equation
must satisfy
, where
,
. It is easy to obtain that the above equation has a solution
.
Example 1.2. If
in (1.6), then the exponential polynomial solutions of the following equation
must satisfy
, where
,
. It is easy to obtain that the above equation has a solution
, where
are constants.
Further, we study the solutions of Fermat-type differential equation
(1.7)
where
are positive integers,
,
. First of all, one fact needs to be clear. For
, the general trivial solutions of Equation (1.7) is
, where
. In this paper, we study the solution of nontrivial solutions and prove the following results.
Theorem 1.2. Let
be the exponential polynomial solutions of the equation
(1.8)
where
and
, one of the following conclusions hold:
1) If
and
, then
, where
,
satisfy
. If
and
, then
, where
,
satisfy
.
2) If
,
and
(namely
), then
, where
,
satisfy
. If
,
and
, then
, where
,
satisfy
.
3) If
,
and
(namely
), then
, where
,
satisfy
.
Example 1.3. If
,
,
in (1.8), then the exponential polynomial solutions of the following equation
must satisfy
, where
,
. It is easy to obtain that the above equation has a solution
.
Example 1.4. If
,
,
in (1.8), then the exponential polynomial solutions of the following equation
must satisfy
, where
,
. It is easy to obtain that the above equation has a solution
.
Example 1.5. If
,
,
in (1.8), then the exponential polynomial solutions of the following equation
must satisfy
, where
,
. It is easy to obtain that the above equation has a solution
.
Example 1.6. If
,
,
in (1.8), then the exponential polynomial solutions of the following equation
must satisfy
, where
,
. It is easy to obtain that the above equation has a solution
.
Theorem 1.3. The transcendental entire solutions of
(1.9)
must satisfy
where
,
and
.
Remark 1. Since for a particular choice of
in Theorem 1.3, then
,
, namely
. If
, then
,
, if
, then
,
, k is an integer. The result obtained is the same as Theorem B, and Theorem 1.3 omit the condition of the finite order. Therefore, Theorem 1.3 improves and extends Theorem B.
Theorem 1.4. The following equation
(1.10)
has no nontrivial meromorphic solutions, where
is positive integers and
.
Theorem 1.5 Let
be positive integers satisfying
, then Equation (1.7) has no nontrivial meromorphic solutions with
.
2. Some Lemmas
Lemma 2.1. (see [2]) Let
be meromorphic functions,
be entire functions satisfying following conditions:
1)
;
2) for
,
are not constants;
3) for
,
where
, E is a possible exceptional set of finite logarithmic measure, then
.
Lemma 2.2. (see [2]) Let
,
,
be meromorphic functions and satisfying
,
be nonconstant and
where
,
, and I has infinite linear measure, then
or
.
Lemma 2.3. (see [2]) Let
be a nonconstant meromorphic function in the complex plane and
, where
and
are two mutually prime polynomials in f. If the coefficients
,
are small functions of f and
,
, then
Lemma 2.4. (see [2]) Let
be a nonconstant entire function and
, then
.
Lemma 2.5. (see [9]) Let
,
,
,
be meromorphic functions and satisfying
,
. If
,
, then the following equation
has no meromorphic solution.
Lemma 2.6. (see [11]) Let
be a nonconstant meromorphic function with
,
, then for
we have
for all r outside of a set of finite logarithmic measure.
Lemma 2.7. (see [12]) Let
be a nonconstant meromorphic function with
,
, then for
, we have
for all r outside of a set of finite logarithmic measure.
Lemma 2.8. (see [20]) Let
be a nonconstant meromorphic function, then
and
.
Lemma 2.9 (see [21]) Let
be a nonconstant meromorphic function and
,
, then
.
3. Proofs of Theorems
Proof of Theorem 1.1
Assume that
is the exponential polynomial solutions of (1.6), substituting (1.5) into (1.6) yields
(3.1)
Let
(3.2)
Further, we get
(3.3)
From (3.2) and (3.3), for
,
, we have
Combining (3.1) with Lemma 2.1, we know that
(3.4)
We deduce from (3.4) that
. If not, suppose that
, we have
Namely
(3.5)
Using logarithmic derivative Lemma and Lemma 2.6 in (3.5), then for
, we have
which implies that
. Thus, we have
where
are polynomials.
Assume that
Further, we have
(3.6)
Substituting (3.6) into
yields
(3.7)
where
.
Assume that
Further, we get
(3.8)
Substituting (3.8) into
yields
(3.9)
From (3.7), we can deduce that
and
, which implies that
.
From (3.9), we can conclude the following
1) If
, we obtain
, then
, which contradicts that
is nonconstant.
2) If
and
, we obtain
, then
, where
.
3) If
and
, we obtain
, then
, where
.
From the above discussion, we can get that the case of the exponential polynomial solutions of Equation (1.6) is as follows.
Case 1. If
, then the exponential polynomial solutions of Equation (1.6) must satisfy
, where
,
satisfy
.
Case 2. If
, then the exponential polynomial solutions of Equation (1.6) must satisfy
, where
,
satisfy
.
This completes the proof of Theorem 1.1.
Proof of Theorem 1.2
We rewrite (1.8) as follows
(3.10)
Assume that
is the exponential polynomial solutions of (3.10) and let
Thus, we have
(3.11)
Substituting (3.11) into (3.10) yields
(3.12)
Let
(3.13)
Further, we have
(3.14)
From (3.13) and (3.14), for
,
, we have
Combining Lemma 2.1 with (3.12), we know that
(3.15)
We deduce from (3.4) that
. If not, suppose that
, we have
Namely
(3.16)
Using logarithmic derivative Lemma and Lemma 2.6 in (3.16), then for
, we have
which implies that
. Thus, we have
where
are polynomials.
Let
(3.17)
Further, we have
(3.18)
Substituting (3.17), (3.18) into
yields
(3.19)
where
.
Let
(3.20)
Further, we have
(3.21)
Substituting (3.20), (3.21) into
yields
(3.22)
From (3.19), we can conclude the following
(i) If
, we obtain
,
, then
.
(ii) If
and
, we obtain
,
, then
, where
.
(iii) If
and
, we obtain
,
, then
, where
.
From (3.22), we can conclude the following
(i) If
, we obtain
, then
, which contradicts that
is nonconstant.
(ii) If
and
, we obtain
, then
, where
.
(iii) If
and
, we obtain
, then
, where
.
From the above discussion, we can get that the case of the exponential polynomial solutions of Equation (1.8) is as follows.
Case 1. If
and
, then
, where
,
satisfy
. If
and
, then
, where
,
satisfy
.
Case 2. If
,
and
(namely
), then
, where
,
satisfy
. If
,
and
, then
, where
,
satisfy
.
Case 3. If
,
and
(namely
), then
, where
,
satisfy
.
Thus, we complete the proof of Theorem 1.2.
Proof of Theorem 1.3
Assume that
is a transcendental entire solution of (1.9), we rewrite (1.9) as follows
(3.23)
then
It then follows that
,
have no zeros. With Welerstrass factorization theorem for entire functions, we have
(3.24)
where
is a entire function. From (3.24), we get
(3.25)
From (3.25), on the one hand, we obtain
, on the other hand, we have
(3.26)
Next we divide our discussion into two cases.
Case 1. If
is a constant, then
from the above identity, we can deduce that
.
Case 2. If
is a non-constant entire function, then from (3.26), we get
(3.27)
Denote
Obviously,
is a nonconstant and by Lemma 2.4 and 2.9, we can obtain that
. Thus, we obtain
(3.28)
Combining (3.28) and Lemma 2.2, we get
or
.
Now two subcases will be considered in the following.
Subcase 2.1. If
, then from (3.28), we have
Further, we get
which implies that
must be a polynomial and
. Note that
is a transcendental entire function and
is a constant, which is a contradiction.
Subcase 2.2. If
, then from (3.28), we have
From the above identity, we can get that
(3.29)
which implies that
must be a polynomial and
. Assume that
, from (3.29), we have
and
.
This completes the proof of Theorem 1.3.
Proof of Theorem 1.4
Now we divide our discussion into two cases.
Case 1. Assume that
is a nonconstant entire solutions of (1.10), we rewrite (1.10) as follows
(3.30)
Denote
,
, from the references [ [5], Theorem3], we get the equation
has no nonconstant entire function solution when
. Thus, Equation (3.30) has no nontrivial entire function solution when
.
Case 2. Assume that
is a meromorphic solutions with at least one pole of (1.10), we rewrite (1.10) as follows
(3.31)
Suppose that
is p multiplicity pole of
. From (3.31), we get
is p multiplicity pole of
, which implies that
is
multiplicity pole of
. Thus we get
is
multiplicity pole of
, which implies that
is
multiplicity pole of
. Sequential recurrence, we can get that
is 1 multiplicity pole of
, this contradiction with
is a meromorphic function with at least one pole.
This completes the proof of Theorem 1.4.
Proof of Theorem 1.5
Assume that
is a meromorphic solutions of (1.7), we rewrite (1.7) as follows
(3.32)
Next we discuss the following two cases.
Case 1. If
, then by lemma 2.8 and lemma 2.9, we can obtain that
. This means that
and
. This, combining with lemma 2.5, we can get that the Equation (3.32) has no nontrivial meromorphic solutions when
.
Case 2. If
, then by lemma 2.8 and lemma 2.9, we get
. This means that
. Now, comparing the characteristic functions of both side of (1.7), by lemma 2.3, we have
, and we know that
, which is a contradiction. Thus Equation (1.7) has no nontrivial meromorphic solutions with
.
This completes the proof of Theorem 1.5.
Funding
This work is supported by the National Natural Science Foundation of China (Grant No. 11801291) and the Natural Science Foundation of Fujian Province, China (Grant Nos. 2020R0039; 2019J05047; 2019J01672).
NOTES
*Corresponding Author.