#### Abstract

We study the uniqueness of meromorphic functions that share one small function with more general differential polynomial . As corollaries, we obtain results which answer open questions posed by Yu (2003).

#### 1. Introduction and Main Results

In this paper, a meromorphic functions mean meromorphic in the whole complex plane. We use the standard notations of Nevanlinna theory (see [1]). A meromorphic function is called a small function with respect to if , that is, as possibly outside a set of finite linear measure. If and have the same zeros with same multiplicities (ignoring multiplicities), then we say that and share CM (IM).

For any constant , we denote by the counting function for zeros of with multiplicity no more than and the corresponding for which multiplicity is not counted. Let be the counting function for zeros of with multiplicity at least and the corresponding for which the multiplicity is not counted.

Let and be two nonconstant meromorphic functions sharing value 1 IM. Let be common one point of and with multiplicity and , respectively. We denote by the counting (reduced) function of those 1 points of where ; by the counting function of those 1-points of where ; by the counting function of those 1-points of where . In the same way, we can define , and (see [2]).

In 1996, Brück [3] posed the following conjecture.

Conjecture 1. *Let be a nonconstant entire function such that the hyper-order of is not a positive integer and . If and share a finite value CM, then , where is a nonzero constant.*

In [3], under an additional hypothesis, Brück proved that the conjecture holds when .

Theorem A. *Let be a nonconstant entire function. If and share the value 1 CM and if , then , for some constant .**Many people extended this theorem and obtained many results. In 2003, Yu [4] proved the following theorem.*

Theorem B. *Let . Let be a nonconstant meromorphic function and a meromorphic function such that , and do not have any common pole and as . If and share the value 0 CM and
**
then .*

Theorem C. *Let . Let be a nonconstant entire function and be a meromorphic function such that and as . If and share the value 0 CM and
**
then .*

In the same paper, the author posed the following questions.

*Question 1. *Can CM shared value be replaced by an IM shared value in Theorem C?

*Question 2. *Is the condition sharp in Theorem C?

*Question 3. *Is the condition sharp in Theorem B?

In 2004, Liu and Gu [5] applied different method and obtained the following theorem which answers some questions posed in [4].

Theorem D. *Let . Let be a nonconstant meromorphic function and a meromorphic function such that and as . If and share the value 0 CM and and do not have any common poles of same multiplicity and
**
then .*

Theorem E. *Let . Let be a nonconstant entire function and a meromorphic function such that and as . If and share the value 0 CM and
**
then .*

Recently, Zhang and Lü [6] considered the problem of meromorphic functions sharing one small function with its th derivative and proved the following theorem.

Theorem F. *Let , be integers and a nonconstant meromorphic function. Also let be a small meromorphic function with respect to . If and share the value IM and
**
or and share the value CM and
**
then .*

Regarding these results, a natural question is what can be said when a nonconstant meromorphic function shares one nonzero small meromorphic function with , where is a differential polynomial in .

*Definition 2. *Any expression of the type
is called differential polynomial in of degree , lower degree , and weight , where are nonnegative integers, are meromorphic functions satisfying and

Further, if , then the differential polynomial is called a homogeneous differential polynomial in of degree .

Correspond to the above question, we obtain the following results, which extend and improve Theorems A–F and give answers to the questions possed by Yu [4] for more general differential polynomial.

Theorem 3. *Let be a nonconstant meromorphic function and be a small meromorphic function such that . P[f] be a nonconstant differential polynomial in as defined in (7). If and share the value IM and
**
then .*

*Remark 4. *Taking , that is, , in (9), we get , which improves (5) and extends the theorem to more general differential polynomial as defined in (7).

Theorem 5. *Let be a nonconstant meromorphic function and be a small meromorphic function such that . Let a nonconstant differential polynomial in as defined in (7). If and share the value CM and
**
then .*

*Remark 6. *Taking , that is, , in (10), we get , which improves (6) and extends the theorem to more general differential polynomial as defined in (7).

Remark 6 gives answer to Question 3 of [4].

Theorem 7. *Let be a nonconstant entire function and a small meromorphic function such that . Let be a nonconstant differential polynomial in as defined in (7) If and share the value IM and
**
then .*

*Remark 8. *Taking , that is, , in (11), we get
Remark 8 gives answer to Question 1 of Yu [4].

Theorem 9. *Let be a nonconstant entire function and be a small meromorphic function such that . be a nonconstant differential polynomial in as defined in (7). If and share the value CM and
**
then .*

*Remark 10. *Taking , that is, , in (13), we get , which improves Theorem C and extends the theorem to more general differential polynomial as defined in (7).

Remark 10 gives answer to Question 2 of Yu [4].

*Remark 11. *By proving Remarks 6, 8, and 10 we have answered Questions 3, 1, and 2 (of [4]), respectively, for the case . Theorems 3–9 improve and generalize Theorems A–F for more general differential polynomial .

#### 2. Lemma

Lemma 12 (see [7]). *Let ba a meromorphic function and be a differential polynomial in . Then
**
where .*

Lemma 13 (see [8]). *Let be a nonconstant meromorphic function, then
*

Lemma 14 (see [9]). *Let
**
where and are two nonconstant meromorphic functions. If and share 1 IM and , then
*

Lemma 15. *Let be a transcendental meromorphic function. Let be defined as in (7). If , we have
*

*Proof. *By the first fundamental theorem, we have
We have
or
By (21), (23) and Lemma 12, we obtain (19).

Since
we get
Substituting (25) in (19), we obtain (20).

Lemma 16 (see [10]). *Let be a transcendental meromorphic function, a differential polynomial in of degree and weight . Then , .*

#### 3. Proof of Theorems

*Proof of Theorem 3. *Let
From the conditions of Theorem 3, we know that and share 1 IM. From (26), we have

Let be defined by (17). Suppose that . By Lemma 14, (18) holds.

From (17) and (28), we have
where denotes the counting function corresponding to the zeros of which are not the zeros of and . Similarly, is defined.

From the second fundamental theorem, we have
Since and share 1 IM, we get from (33):

From this, (18), and (34), we have
It is clear that
Combining (37), and (38), we obtain
Substituting (39) in (35) and using (28), we obtain
Using (26) and (19), we get

From (16), (20), and (26) we have
From (41) and (42), we get
Therefore, we have
which is a contradiction to our hypothesis (9).

Thus . By integration, we get from (17) that
where and are constants. Thus
We discuss the following three cases.*Case 1.* Suppose that . From (46), we have
From this and second fundamental theorem, we have
Therefore, we have
which is a contradiction to our hypothesis (9).*Case 2.* Suppose that , From (46), we get
we claim .

If from (50), we obtain
From this, second fundamental theorem, and (20), we have
Hence, we have
which is a contradiction to our hypothesis (9)

Thus, .

From (50) we have .

Therefore, we have .*Case 3.* Suppose that , from (46) we have
If , we obtain from (54) that
By the same argument as in Case 2, we obtain a contradiction. Hence, .

From (54), we get
that is,
From (57), we have
Using (54), (57), Lemma 12, and first fundamental theorem, we get
From this, we have
which is a contradiction. This completes the proof of Theorem 3.

*Proof of Theorem 5. *Let and be given by (26). From the assumption of Theorem 5, we know that and share 1 CM:
Proceeding as in Theorem 3, we obtain (41).

Using (61) in (41), we get
We have
which contradicts (10).

Thus, . Proceeding as in Theorem 3, we prove Theorem 5.

*Proof of Theorem 7. * is a nonconstant entire function. Taking in proof of Theorem 3, we obtain Theorem 7.

*Proof of Theorem 9. * is a nonconstant entire function. Taking in proof of Theorem 5, we obtain Theorem 9.

#### Acknowledgments

The authors thank the referee for his/her valuable suggestions. This research work is supported by the Department of Science and Technology Government of India, Ministry of Science and Technology, Technology Bhavan, New Delhi, India, under the sanction Letter no. (SR/S4/MS: 520/08).