http://jipam.vu.edu.au/
Volume 3, Issue 5, Article 76, 2002
EXTENSIONS OF HIONG’S INEQUALITY
MING-LIANG FANG AND DEGUI YANG DEPARTMENT OFMATHEMATICS,
NANJINGNORMALUNIVERSITY, NANJING, 210097, PEOPLE’SREPUBLIC OFCHINA
mlfang@pine.njnu.edu.cn COLLEGE OFSCIENCES,
SOUTHCHINAAGRICULTURALUNIVERSITY, GUANGZHOU, 510642,
PEOPLE’SREPUBLIC OFCHINA
yangde@macs.biu.ac.il
Received 15 July, 2002; accepted 25 July, 2002 Communicated by H.M. Srivastava
ABSTRACT. In this paper, we treat the value distribution ofφfn−1f(k), wheref is a transcen- dental meromorphic function,φis a meromorphic function satisfyingT(r, φ) =S(r, f), nand kare positive integers. We generalize some results of Hiong and Yu.
Key words and phrases: Inequality, Value distribution, Meromorphic function.
2000 Mathematics Subject Classification. Primary 30D35, 30A10.
1. INTRODUCTION
Let f be a nonconstant meromorphic function in the whole complex plane. We use the following standard notation of value distribution theory,
T(r, f), m(r, f), N(r, f), N(r, f), . . .
(see Hayman [1], Yang [4]). We denote byS(r, f)any function satisfying S(r, f) = o{T(r, f)},
asr →+∞, possibly outside of a set with finite measure.
In 1956, Hiong [3] proved the following inequality.
ISSN (electronic): 1443-5756 c
2002 Victoria University. All rights reserved.
Supported by the National Nature Science Foundation of China (Grant No. 10071038) and “Qinglan Project”of the Educational Department of Jiangsu Province.
079-02
Theorem 1.1. Let f be a non-constant meromorphic function; let a, b and c be three finite complex numbers such thatb 6= 0,c6= 0andb 6=c; and letkbe a positive integer. Then
T(r, f)≤N
r, 1 f −a
+N
r, 1
f(k)−b
+N
r, 1 f(k)−c
−N
r, 1 f(k+1)
+S(r, f).
Recently, Yu [5] extended Theorem 1.1 as follows.
Theorem 1.2. Let f be a non-constant meromorphic function; and letb andcbe two distinct nonzero finite complex numbers; and letn, k be two positive integers. Ifφ(6≡ 0) is a meromor- phic function satisfyingT(r, φ) = S(r, f),n= 1orn≥k+ 3, then
(1.1) T(r, f)≤N
r, 1 f
+ 1
n
N
r, 1
φfn−1f(k)−b
+N
r, 1
φfn−1f(k)−c
− 1 n
N(r, f) +N
r, 1
(φfn−1f(k+1))0
+S(r, f).
Iff is entire, then (1.1) is valid for all positive integersn(6= 2).
In [5], the author expected that (1.1) is also valid forn = 2iff is entire.
In this note, we prove that (1.1) is valid for all positive integersneven iff is meromorphic.
Theorem 1.3. Let f be a non-constant meromorphic function; and letb andcbe two distinct nonzero finite complex numbers; and letn, k be two positive integers. Ifφ(6≡ 0) is a meromor- phic function satisfyingT(r, φ) = S(r, f), then
(1.2) T(r, f)≤N
r, 1 f
+ 1
n
N
r, 1
φfn−1f(k)−b
+N
r, 1
φfn−1f(k)−c
−N(r, f)− 1 n
(k−1)N(r, f) +N
r, 1
(φfn−1f(k+1))0
+S(r, f).
In [6], the author proved
Theorem 1.4. Letf be a transcendental meromorphic function; and letnbe a positive integer.
Then either fnf0 −a or fnf0 +a has infinitely many zeros, wherea(6≡ 0)is a meromorphic function satisfyingT(r, a) =S(r, f).
In this note, we will prove
Theorem 1.5. Letf be a transcendental meromorphic function; and letnbe a positive integer.
Then eitherfnf0 −aorfnf0 −b has infinitely many zeros, wherea(6≡0)andb(6≡ 0)are two meromorphic functions satisfyingT(r, a) =S(r, f)andT(r, b) =S(r, f).
2. PROOF OFTHEOREMS
For the proofs of Theorem 1.3 and 1.5, we require the following lemmas.
Lemma 2.1. [2]. Iff is a transcendental meromorphic function andK >1, then there exists a setM(K)of upper logarithmic density at most
δ(K) = min{(2eK−1−1)−1,(1 +e(K−1)) exp(e(1−K))}
such that for every positive integerk,
(2.1) lim sup
r→∞,r /∈M(K)
T(r, f)
T(r, f(k)) ≤3eK.
Lemma 2.2. If f is a transcendental meromorphic function and φ(6≡ 0) is a meromorphic function satisfyingT(r, φ) =S(r, f). Thenφfn−1f(k) 6≡constant for every positive integern.
Proof. Suppose that φfn−1f(k) ≡ constant. If n = 1, then φf(k) ≡ constant. Therefore, T(r, f(k)) =S(r, f), which implies that
lim sup
r→∞,r /∈M(K)
T(r, f)
T(r, f(k)) =∞.
This is contradiction to Lemma 2.1.
Ifn ≥2,thenT(r, fn−1f(k)) = S(r, f).On the other hand, nT(r, f)≤T(r, fn−1f(k)) +T
r, f
f(k)
+S(r, f)
≤T(r, fn−1f(k)) +T
r,f(k) f
+S(r, f)
≤T(r, fn−1f(k)) +N
r,f(k) f
+S(r, f)
≤T(r, fn−1f(k)) +N(r, 1
f) +N(r, fn−1f(k)) +S(r, f)
≤2T(r, fn−1f(k)) +T(r, f) +S(r, f).
HenceT(r, f) ≤ n−12 T(r, fn−1f(k)) +S(r, f),Therefore, T(r, f) = S(r, f),which is a con-
tradiction. Which completes the proof of this lemma.
Lemma 2.3. [1]. Iff is a meromorphic function, anda1, a2, a3 are distinct meromorphic func- tions satisfyingT(r, aj) = S(r, f)forj = 1,2,3.Then
T(r, f)≤
3
X
j=1
N
r, 1 f−aj
+S(r, f).
Proof of Theorem 1.3. By Lemma 2.2, we have φfn−1f(k) 6≡constant if n andk are positive integers. By (4.17) of [1], we have
m
r, 1 fn
+m
r, 1
φfn−1f(k)−b
+m
r, 1
φfn−1f(k)−c (2.2)
≤m
r, 1
φfn−1f(k)
+m
r,f(k) f
+m
r, 1
φfn−1f(k)−b
+m
r, 1
φfn−1f(k)−c
+S(r, f)
≤m
r, 1
φfn−1f(k)
+m
r, 1
φfn−1f(k)−b
+m
r, 1
φfn−1f(k)−c
+S(r, f)
≤m
r, 1
(φfn−1f(k))0
+S(r, f)
≤T(r,(φfn−1f(k))0)−N
r, 1
(φfn−1f(k))0
+S(r, f)
≤T(r, φfn−1f(k)) +N(r, f)−N
r, 1
(φfn−1f(k))0
+S(r, f) By (2.2), we have
T(r, fn) +T(r, φfn−1f(k))
≤N
r, 1 fn
+N
r, 1
φfn−1f(k)−b
+N
r, 1
φfn−1f(k)−c
+N(r, f)−N
r, 1
(φfn−1f(k))0
+S(r, f).
Therefore,
nT(r, f)≤nN
r, 1 f
+N
r, 1
φfn−1f(k)−b
+N
r, 1
φfn−1f(k)−c
+N(r, f)−N
r, 1
(φfn−1f(k))0
−N(r, fn−1f(k)) +S(r, f)
≤nN
r, 1 f
+N
r, 1
φfn−1f(k)−b
+N
r, 1
φfn−1f(k)−c
−nN(r, f)−(k−1)N(r, f)−N
r, 1
(φfn−1f(k))0
+S(r, f),
thus we get (1.2). This completes the proof of Theorem 1.3.
Proof of Theorem 1.5. By Nevanlinna’s first fundamental theorem, we have 2T(r, f) = T
r, f f0· f f0
≤T(r, f f0) +T
r, f f0
+S(r, f)
≤T(r, f f0) +T
r,f0 f
+S(r, f)
≤T(r, f f0) +N
r,f0 f
+S(r, f)
=T(r, f f0) +N
r, 1 f
+N(r, f) +S(r, f)
≤T(r, f f0) +T(r, f) + 1
3N(r, f f0) +S(r, f).
Thus we get
T(r, f)≤ 4
3T(r, f f0) +S(r, f).
Hence we getT(r, a) = S(r, f f0)andT(r, b) =S(r, f f0).
By Lemma 2.3, we have
T(r, f f0)≤N(r, f) +N
r, 1 f f0−a
+N
r, 1
f f0−b
+S(r, f f0)
≤ 1
3N(r, f f0) +N
r, 1 f f0−a
+N
r, 1
f f0 −b
+S(r, f f0).
Hence we get
T(r, f)≤ 3 2
N
r, 1
f f0 −a
+N
r, 1 f f0−b
+S(r, f f0).
Thus we know that eitherf f0 −aorf f0−bhas infinitely many zeros.
REFERENCES
[1] W.K. HAYMAN, Meromorphic Functions, Clarendon Press, Oxford, 1964.
[2] W.K. HAYMAN AND J. MILES, On the growth of a meromorphic function and its derivatives, Complex Variables, 12(1989), 245–260.
[3] K.L. HIONG, Sur la limitation deT(r, f)sans intervention des pôles, Bull. Sci. Math., 80 (1956), 175–190.
[4] L. YANG, Value Distribution Theory, Springer-Verlag, Berlin, 1993.
[5] K.W. YU, On the distribition ofφ(z)fn−1(z)f(k)(z), J. of Ineq. Pure and Appl. Math., 3(1) (2002), Article 8. [ONLINE:http://jipam.vu.edu.au/v3n1/037_01.html]
[6] K.W. YU, A note on the product of a meromorphic function and its derivative, Kodai Math. J., 24(3) (2001), 339–343.