# Some Normality Criteria of Meromorphic Functions

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## Abstract

This paper studies some normality criteria for a family of meromorphic functions, which improve some results of Lahiri, Lu and Gu, as well as Charak and Rieppo.

### Keywords

Rational Function Complex Number Compact Subset Meromorphic Function Nonnegative Integer## 1. Introduction and Main Results

Let Open image in new window be a nonconstant meromorphic function in the complex plane Open image in new window . We shall use the standard notations in Nevanlinna's value distribution theory of meromorphic functions such as Open image in new window , Open image in new window , and Open image in new window (see, e.g., [1, 2]). The notation Open image in new window is defined to be any quantity satisfying Open image in new window as Open image in new window possibly outside a set of Open image in new window of finite linear measure.

Let Open image in new window be a family of meromorphic functions on a domain Open image in new window . We say that Open image in new window is normal in Open image in new window if every sequence of functions Open image in new window contains either a subsequence which converges to a meromorphic function Open image in new window uniformly on each compact subset of Open image in new window or a subsequence which converges to Open image in new window uniformly on each compact subset of Open image in new window . (See [1, 3].)

The Bloch principle [3] is the hypothesis that a family of analytic (meromorphic) functions which have a common property Open image in new window in a domain Open image in new window will in general be a normal family if Open image in new window reduces an analytic (meromorphic) function in the open complex plane Open image in new window to a constant. Unfortunately the Bloch principle is not universally true. But it is also very difficult to find some counterexamples about the converse of the Bloch principle, and hence it is interesting to study the problem.

In 2005, Lahiri [4] proved the following criterion for the normality, and gave a counterexample to the converse of the Bloch principle by using the result.

Theorem A

If there exists a positive number Open image in new window such that for every Open image in new window , one has Open image in new window whenever Open image in new window , then Open image in new window is normal.

In this direction, Lahiri and Dewan [5] as well as Xu and Zhang [6] proved the following result.

Theorem B.

where Open image in new window and Open image in new window are positive integers.

If for every Open image in new window

(i)all zeros of Open image in new window have multiplicity at least Open image in new window ,

(ii)there exists a positive number Open image in new window such that for every Open image in new window one has Open image in new window whenever Open image in new window ,

then Open image in new window is normal in Open image in new window so long as Open image in new window Open image in new window ; or Open image in new window Open image in new window and Open image in new window .

Here, we also give a counterexample to the converse of the Bloch principle by considering Theorem B, which is similar to an example in [7].

Example 1.1.

but Theorem B is true especially when Open image in new window is an empty set for every Open image in new window in the family.

In the following, we continue to study the normal family when Open image in new window and Open image in new window in Theorem B.

Theorem 1.2.

where Open image in new window is a positive integer.

If for every Open image in new window

(i)all zeros of Open image in new window have multiplicity at least Open image in new window ,

(ii)there exists a positive number Open image in new window such that for every Open image in new window , one has Open image in new window whenever Open image in new window , then Open image in new window is normal in Open image in new window .

Corollary 1.3.

Let Open image in new window be a family of meromorphic functions in a domain Open image in new window , all of whose zeros have multiplicity at least Open image in new window , and let Open image in new window , Open image in new window be two finite constants. Suppose that Open image in new window , where Open image in new window is a positive integer. Then Open image in new window is normal in Open image in new window .

Recently, Lu and Gu [8] considered two related normal families.

Theorem C.

Let Open image in new window be a family of meromorphic functions in a domain Open image in new window ; all of whose zeros have multiplicity at least Open image in new window . Suppose that, for each Open image in new window , Open image in new window for Open image in new window , then Open image in new window is a normal family on Open image in new window , where Open image in new window is a nonzero finite complex number and Open image in new window is an integer number.

Theorem D.

Let Open image in new window be a family of meromorphic functions in a domain Open image in new window ; all of whose zeros have multiplicity at least Open image in new window , and all of whose poles are multiple. Suppose that, for each Open image in new window , Open image in new window for Open image in new window , then Open image in new window is a normal family on Open image in new window , where Open image in new window is a nonzero finite complex number and Open image in new window is an integer number.

In this paper, we give a simple proof and improve the above results.

Theorem 1.4.

Let Open image in new window be a family of meromorphic functions in a domain Open image in new window ; all of whose zeros have multiplicity at least Open image in new window . Suppose that, for each Open image in new window , Open image in new window for Open image in new window , then Open image in new window is a normal family on Open image in new window , where Open image in new window is a nonzero finite complex number and Open image in new window is an integer number.

In 2009, Charak and Rieppo [7] generalized Theorem A and obtained two normality criteria of Lahiri's type.

Theorem E.

If there exists a positive constant Open image in new window such that Open image in new window for all Open image in new window whenever Open image in new window , then Open image in new window is a normal family.

Theorem F.

If there exists a positive constant Open image in new window such that Open image in new window for all Open image in new window whenever Open image in new window , then Open image in new window is a normal family.

Naturally, we ask whether the above results are still true when Open image in new window is replaced by Open image in new window in Theorems E and F. We obtain the following results.

Theorem 1.5.

If there exists a positive constant Open image in new window such that Open image in new window for all Open image in new window whenever Open image in new window , then Open image in new window is a normal family.

Theorem 1.6.

## 2. Some Lemmas

Lemma 2.1 (see [9]).

Let Open image in new window be a family of functions meromorphic on the unit disc, all of whose zeros have multiplicity at least Open image in new window , then if Open image in new window is not normal, there exist, for each Open image in new window

(a)a number Open image in new window

(b)points Open image in new window Open image in new window

(c)functions Open image in new window

(d)positive number Open image in new window such that Open image in new window locally uniformly, where Open image in new window is a nonconstant meromorphic on Open image in new window , all of whose zeros have multiplicity at least Open image in new window , such that Open image in new window

Here, as usual, Open image in new window is the spherical derivative.

Lemma 2.2.

Let Open image in new window be rational in the complex plane and Open image in new window positive integers. If Open image in new window has only zero with multiplicity at least Open image in new window , then Open image in new window takes on each nonzero value Open image in new window .

Proof.

In Lemma 6 of [7], the case of Open image in new window is proved. We just consider the case of Open image in new window by a different way which comes from [10].

where Open image in new window is a polynomial with Open image in new window .

where Open image in new window is a nonzero constant.

where Open image in new window is a polynomial of the form Open image in new window , in which Open image in new window , Open image in new window , Open image in new window are constants.

It is a contradiction with Open image in new window and Open image in new window . This proves the lemma.

Lemma 2.3 (see [11]).

Let Open image in new window be a transcendental meromorphic function all of whose zeros have multiplicity at least Open image in new window , then Open image in new window assumes every finite nonzero value infinitely often, where Open image in new window if Open image in new window , and Open image in new window if Open image in new window .

Remark 2.4.

The lemma was first proved by Wang as Open image in new window if Open image in new window and Open image in new window if Open image in new window in [12]. Recently, the result is improved by [11].

Lemma 2.5.

Let Open image in new window be a meromorphic function all of whose zeros have multiplicity with at least Open image in new window in the complex plane, then Open image in new window must have zeros for any constant Open image in new window .

Proof.

If Open image in new window is rational, then by Lemma 2.2 the conclusion holds.

If Open image in new window is transcendental, supposing that Open image in new window has no zeros, then by Lemma 2.3, we can get a contradiction. This completes the proof of the lemma.

Lemma 2.6.

Let Open image in new window be meromorphic in the complex plane, and let Open image in new window be a constant, for any positive integer Open image in new window ; if Open image in new window , then Open image in new window is a constant.

Proof.

and Open image in new window with Open image in new window being a set of Open image in new window values of finite linear measure. It is a contradiction.

Lemma 2.7 (see [13]).

Let Open image in new window be a transcendental meromorphic function, and let Open image in new window , Open image in new window be two integers. Then for any nonzero value Open image in new window , the function Open image in new window has infinitely many zeros.

Lemma 2.8 (see [14]).

Let Open image in new window be a transcendental meromorphic function, and let Open image in new window be an integer. Then for any nonzero value Open image in new window , the function Open image in new window has infinitely many zeros.

Lemma 2.9.

has a finite zero.

Proof.

has always a nonzero solution; say Open image in new window . By [14, Corollary Open image in new window ] or [15], Lemmas 2.2, 2.7, and 2.8, the meromorphic function Open image in new window cannot avoid it and thus there exists Open image in new window such that Open image in new window .

and we complete the proof of the lemma.

Remark 2.10.

If Open image in new window , we need Open image in new window when Open image in new window by Lemma 2.3. We can get a similar result.

## 3. Proof of Theorems

Proof of Theorem 1.2.

Therefore for all large values of Open image in new window , it follows from the given condition that Open image in new window .

Since Open image in new window is not a pole of Open image in new window , there exists a positive number Open image in new window such that in some neighborhood of Open image in new window we get Open image in new window .

which is a contradiction. This proves the theorem.

Proof of Theorem 1.4.

It follows that Open image in new window or Open image in new window by Hurwitz's theorem. From Lemma 2.6, we obtain that Open image in new window . By Lemma 2.5, we get a contradiction. This completes the proof of the theorem.

Proof of Theorem 1.5.

where Open image in new window is replaced by Open image in new window and Open image in new window , Open image in new window .

In the following, we can get a contradiction in a similar way with the proof of the last part of Theorem 1.2. This completes the proof of the theorem.

Proof of Theorem 1.6.

for some Open image in new window .

In the following, we can get a contradiction in a similar way with the proof of the last part of Theorem 1.5. This completes the proof of the theorem.

## Notes

### Acknowledgments

The authors would like to thank Professor Lahiri for supplying the electronic file of the paper [4]. The authors were supported by NSF of China (No. 10771121, No. 10801107), NSF of Guangdong Province (No. 9452902001003278, No. 8452902001000043), and Department of Education of Guangdong (No. LYM08097).

### References

- 1.Yang L:
*Value Distribution Theory*. Springer, Berlin, Germany; 1993:xii+269.MATHGoogle Scholar - 2.Yang C-C, Yi H-X:
*Uniqueness Theory of Meromorphic Functions, Mathematics and Its Applications*.*Volume 557*. Kluwer Academic Publishers, Dordrecht, The Netherlands; 2003:viii+569.CrossRefMATHGoogle Scholar - 3.Schiff JF:
*Normal Families, Universitext*. Springer, New York, NY, USA; 1993:xii+236.CrossRefMATHGoogle Scholar - 4.Lahiri I: A simple normality criterion leading to a counterexample to the converse of the Bloch principle.
*New Zealand Journal of Mathematics*2005, 34(1):61–65.MathSciNetMATHGoogle Scholar - 5.Lahiri I, Dewan S: Some normality criteria.
*Journal of Inequalities in Pure and Applied Mathematics*2004., 5(2, article 35):MathSciNetMATHGoogle Scholar - 6.Xu JF, Zhang ZL: Note on the normal family.
*Journal of Inequalities in Pure and Applied Mathematics*2006., 7(4, article 133):MathSciNetMATHGoogle Scholar - 7.Charak KS, Rieppo J: Two normality criteria and the converse of the Bloch principle.
*Journal of Mathematical Analysis and Applications*2009, 353(1):43–48. 10.1016/j.jmaa.2008.11.066MathSciNetCrossRefMATHGoogle Scholar - 8.Lu Q, Gu Y: Zeros of differential polynomial and its normality.
*Chinese Quarterly Journal of Mathematics*2009, 24(1):75–80.MathSciNetMATHGoogle Scholar - 9.Pang XC, Zalcman L: Normal families and shared values.
*The Bulletin of the London Mathematical Society*2000, 32(3):325–331. 10.1112/S002460939900644XMathSciNetCrossRefMATHGoogle Scholar - 10.Hu P-C, Meng D-W: Normality criteria of meromorphic functions with multiple zeros.
*Journal of Mathematical Analysis and Applications*2009, 357(2):323–329. 10.1016/j.jmaa.2009.04.016MathSciNetCrossRefMATHGoogle Scholar - 11.Zou WL, Zhang QD: On the zero of .
*Journal of Sichuan Normal University*2008, 31(6):662–666.MATHGoogle Scholar - 12.Wang J-P: On the value distribution of .
*Kyungpook Mathematical Journal*2006, 46(2):169–180.MathSciNetMATHGoogle Scholar - 13.Yang CC, Hu PC: On the value distribution of .
*Kodai Mathematical Journal*1996, 19(2):157–167. 10.2996/kmj/1138043595MathSciNetCrossRefMATHGoogle Scholar - 14.Alotaibi A: On the zeros of for .
*Computational Methods and Function Theory*2004, 4(1):227–235.MathSciNetCrossRefMATHGoogle Scholar - 15.Zhang ZF, Song GD: On the zeros of .
*Chinese Annals of Mathematics, Series A*1998, 19(2):275–282.MathSciNetMATHGoogle Scholar

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