Zeros and fixed points of the linear combination of shifts of a meromorphic function

R E S E A R C H

Open Access

Zeros and fixed points of the linear

combination of shifts of a meromorphic

function

Weiwei Cui

1

_{, Lianzhong Yang}

1*

_{and Xiaoguang Qi}

2

*_{Correspondence:}

lzyang@sdu.edu.cn

1_{School of Mathematics, Shandong}

University, Jinan, Shandong 250100, P.R. China

Full list of author information is available at the end of the article

Abstract

Letfbe transcendental and meromorphic in the complex plane. In this article, we investigate the existences of zeros and fixed points of the linear combination and quotients of shifts off(z) whenf(z) is of order one. We also prove a result concerning the linear combination which extends a result of Bergweiler and Langley. Some results concerning the order off(z) < 1 are also obtained.

MSC: 30D35; 39A05

Keywords: entire functions; meromorphic functions; complex difference; fixed points; zeros; shift

1 Introduction and main results

In this article, a function is called meromorphic if it is analytic in the whole complex plane except at possible isolated poles. We assume that readers are familiar with the basic results and notations of the Nevanlinna value distribution theory of meromorphic functions (see, e.g., [–]).

Letf be a transcendental meromorphic function in the plane. The forward differences

n(f) are defined in the standard way ([], p.) byf =f(z+ ) –f(z),n+(f) =nf(z+ ) –nf(z),n= , , , . . . .

Recently, many excellent results concerning the Nevanlinna theory for difference op-erators have been obtained (see,e.g., [–]). For example, Halburd and Korhonen [], respectively Chiang and Feng [] obtained the difference analogue of logarithmic deriva-tives lemma. Their results became a starting point of investigating Nevanlinna theory on difference operators and difference equations. Another important result belongs to Berg-weiler and Langley. In [], BergBerg-weiler and Langley firstly investigate the existence of zeros off and _ff. The results may be viewed as discrete analogue of the following existence theorem on the zeros off.

Theorem A([–]) Let f(z)be transcendental and meromorphic in the plane with

lim

r→+∞

T(r,f) r = ,

then fhas infinitely many zeros.

Hurwitz theorem implies that under the hypotheses of Theorem A,f(z+c) –f(z) has a zero nearzfor all sufficiently smallc∈C\ {}ifzis a zero off. Whenf is a transcen-dental entire function of order less than one, then the first differencef, and by repetition of this argument each differencenf, forn≥, is transcendental entire of order less than one and hence has infinitely many zeros. Bergweiler and Langley considered the divided difference case and obtained the following theorem.

Theorem B([]) There existsδ∈(,_)with the following property.Let f be a transcen-dental entire function with order

σ(f)≤σ≤ 

+δ< ,

then

G(z) =f(z) f(z) =

f(z+ ) –f(z) f(z)

has infinitely many zeros.

From the proof of Theorem B, it can be seen thatδis extremely small, so they con-jectured that the conclusion of Theorem B still holds forδ(f) < . Chen and Shon partly answered this question and obtained the following.

Theorem C([]) Let n∈N and f be a transcendental entire function of orderσ(f) < . Let c∈C\{}and a set H={zj}consist of all different zeros of f(z),satisfying any one of the

following two conditions:

(i) at most finitely many zeroszj,zksatisfyzj–zk=c;

(ii) lim_j→+∞|zj_z+ j |=l> .

Then

G(z) =f(z) f(z) =

f(z+c) –f(z) f(z)

has infinitely many zeros and infinitely many fixed points.

Bergweiler and Langley [] also obtained the following results.

Theorem D Let f be a function transcendental and meromorphic of lower orderλ(f) <λ< in the plane.Let c∈C\ {}be such that at most finitely many poles zj,zkof f satisfy

zj–zk=c.Then g(z) =f(z+c) –f(z)has infinitely many zeros.

In Theorem D, Bergweiler and Langley considered the existence of zeros of first differ-ence operator when the transcendental meromorphic function is of lower order less than one. Chen and Shon [] considered the case when the order off is equal to one. They proved the following results.

In particular,if a set H={zj}consists of all different zeros of f(z),satisfying any one of the

following two conditions:

(i) at most finitely many zeroszj,zksatisfyzj–zk=c;

(ii) lim_j→+∞|zj_z+ j |=l> ,

then

G(z) =f(z) f(z) =

f(z+c) –f(z) f(z)

has infinitely many zeros and infinitely many fixed points.

In order to continue to investigate the existence of zeros and fixed points of difference polynomials and their associated difference quotients, we give the linear combination of difference shifts of a meromorphic function as follows:

g(z) =

n

j=

gj(z)f(z+cj), G(z) =

_n

j=

gj(z)f(z+cj)

f(z).

In fact,nf(z) is a special case ofg(z) since it can be rewritten as the following form:

nf(z) =

n

j=

C_nj(–)n–jf(z+j).

By letting the polynomial coefficients ofg(z) be constants andcjnonnegative integers,i.e.,

gj(z) =Cnj–(–)n–j+, cj=j– ,

forj= , . . . ,n+ , we could find thatg(z) andG(z) are general coefficient cases ofnf(z) and_fn₍f_z(₎z), respectively.

Hence, a natural question arises from Theorem E: What can be said about the existence of zeros and fixed points off and _ff, whenf(z) is a meromorphic function with order

σ(f) = ?

In [], Cui and Yang considered and gave answers to this question. Now, since we in-troduce the general form off and_ff, the above question acquires its new forms:

Do the conclusions of Theorems B-E still hold forg(z) andg(z)/f(z), whenf(z) is a mero-morphic function with orderσ(f) <  or evenσ(f)≤? In the present article, we answer this question and obtain the following results.

Theorem . Let f be a transcendental meromorphic function in the plane with the order

σ(f) =σ= .If f has finitely many poles and infinitely many zeros with the exponent of convergence of zerosλ(f) =λ= .If the polynomial coefficients{gj(z)}nj=of g(z)satisfy

deggl(z)

> max

≤j≤n,j=l

deggj(z) ,

then g(z) =j_j=_=ngj(z)f(z+cj)has infinitely many zeros and fixed points.

Theorem . Let f be a transcendental meromorphic function in the plane with the order

σ(f) =σ< .If f has finitely many poles and the polynomial coefficients{gj(z)}nj=of g(z) satisfy

deggl(z)

> max

≤j≤n,j=l

deggj(z) ,

then g(z) =j_j=_=ngj(z)f(z+cj)has infinitely many zeros and fixed points.

Now we consider the existence of zeros and fixed points ofG(z). From the proofs of Theorem . and Theorem ., we notice that the property ofg(z),i.e., whether it is tran-scendental or not, actually determines our conclusions. Therefore, in the following we still first consider the property ofG(z) and then investigate zeros and fixed points ofG(z) when f is of order equal to or less than one.

Theorem . Let G(z) =g_f(₍z_z)₎ and the polynomial coefficients{gj(z)}nj=of g(z)satisfy

deggl(z)

> max

≤l≤n,j=l

deggj(z) , cl= .

Then G(z)is transcendental provided f satisfies any one of the following two conditions:

(i) f is transcendental entire and the order of growth off satisfies

lim_r→+∞T(r,f)

r = ;

(ii) f is transcendental meromorphic and the order of growth off satisfies

lim_r→+∞T(r,f)

r

= .

Remark . In fact, Theorem . is a generalization of the following last part of Theo-rem F.

Theorem F([]) Let n∈N and let f be a transcendental entire function of order<_,and

set

G(z) =

n_f(z)

f(z) .

If G is transcendental,then G has infinitely many zeros.In particular,if f has order less thanmin{_n,_},then G is transcendental and has infinitely many zeros.

In the following,we will investigate the existence of zeros and fixed points of G(z)and obtain some results related to this.

Theorem . Let f be a function transcendental and meromorphic in the plane with the

of convergence of zerosλ(f) =λ= .If the polynomial coefficients{gj(z)}nj=of g(z)satisfy

deggl(z)

> max

≤j≤n,j=l

deggj(z) , cl= ,

then G(z) =g_f(₍z_z))has infinitely many zeros and fixed points.

The following theorem can be acquired by using Theorem . and using the same method as in the proof of Theorem ..

Theorem . Let f be a function transcendental and meromorphic in the plane with the orderσ(f) =σ <_.If f(z)has only finitely many poles and if the polynomial coefficients {gj(z)}nj=of g(z)satisfy

deggl(z)

> max

≤j≤n,j=l

deggj(z) , cl= ,

then G(z) =g_f(₍z_z))has infinitely many zeros and fixed points.

In particular,if f is transcendental entire withσ(f) < ,and the polynomial coefficients {gj(z)}nj=of g(z)satisfy

deggl(z)

> max

≤j≤n,j=l

deggj(z) , cl= ,

then G(z) =g_f(₍z_z))has infinitely many zeros and fixed points. 2 Some lemmas

Lemma .([]) Let g be a function transcendental and meromorphic in the plane of order less than one.Let h> .Then there exists anε-set E such that

g(z+c) g(z+c) →,

g(z+c) g(z) →,

as z→ ∞in C\E,uniformly in c for|c| ≤h.Further,E may be chosen so that for large z not in E,the function g has no zeros or poles in|ζ–z| ≤h.

Remark .([]) Following Hayman ([], pp.-), define anε-setEto be a countable union of discs

E= ∞

j= B(bj,rj)

such that

lim

j→∞|bj|=∞,

∞

j= rj

|bj|

<∞.

Lemma .([]) Let fj(z) (j= , . . . ,n)be meromorphic functions,gj(z) (j= , . . . ,n)be entire

functions and satisfy:

(i) n_j=fj(z)egj(z)≡;

(ii) when≤j<k≤n,gj(z) –gk(z)is not a constant;

(iii) when≤j≤n,≤h<k≤n,

T(r,fj) =o

Tr,egh–gk _(r→ ∞_,r∈_/E),

whereE⊂(,∞)is of finite linear measure or finite logarithmic measure.

Then fj(z)≡ (j= , . . . ,n).

Lemma .([]) Let f be a transcendental meromorphic function in the plane of order less than one.Let h> .Then there exists anε-set E such that

f(z+c) –f(z) =cf(z) +o()

as z→ ∞in C\E,uniformly in c for|c| ≤h.

Lemma .([]) If f is transcendental and meromorphic of order less than one in the plane.Let n∈N.Then there exists anε-set Ensuch that

nf(z)∼f(n)(z) as z→ ∞in C\En.

Lemma .([]) Let f be a transcendental meromorphic function with finite orderσ(f) =

σ,H={(k,j), (k,j), . . . , (kq,jq)}be a finite set of distinct pairs of integers that satisfy ki>

ji≥for i= , . . . ,q,and letε> be a given constant.Then there exists a set E⊂(,∞)with

finite logarithmic measure such that for all z satisfying|z|∈/E∪[, ]and for all(k,j)∈H, we have

f_f((kj))₍(_zz₎)

≤ |z|(k–j)(σ–+ε).

Lemma .([]) Letη,ηbe two arbitrary complex numbers and let f(z)be a meromor-phic function of finite orderσ.Letε> be given,then there exists a subset E⊂R with finite logarithmic measure such that for all r∈/E∪[, ],we have

exp–rσ–+ε≤f(z+η)

f(z+η)

≤exprσ–+ε.

The following Lemma . can be easily obtained from Chiang and Feng (Theorem . []), here we omit its proof.

Lemma . Let g(z), . . . ,gn(z)be polynomials such that there exists an integer k, ≤k≤n,

so that

deggk(z)

> max

≤j≤n,j=k

holds.Suppose that f(z)is a meromorphic solution to

gn(z)y(z+cn) +· · ·+g(z)y(z+c) = ,

where cj,j= , . . . ,n,are complex numbers of which at least two are distinct.Thenσ(f)≥.

Lemma . Let f(z)be a transcendental meromorphic function satisfying σ(f) =σ < , and{gj(z)}be polynomials.If there exists an integer l, ≤l≤n,such that

deggl(z)

> max

≤j≤n,j=l

deggj(z)

holds,then g(z)is transcendental and meromorphic.

Proof Suppose thatg(z) is not transcendental. Then we divide our proof into two cases. Case. Ifg(z)≡, that is,

gn(z)f(z+cn) +· · ·+g(z)f(z+c)≡.

Since

deggl(z)

> max

≤j≤n,j=l

deggj(z) (.)

holds, and by using Lemma ., we can get that

σ(f)≥.

Clearly, this contradicts withσ(f) < . Henceg(z)≡.

Case. Ifg(z) is rational, without loss of generality, we may suppose thatg(z) is a nonzero polynomial.

By Lemma . andσ(f) < , we know that there exists anε-setEsuch that

f(z+cj) =f(z)

 +o() (.)

asz→ ∞inC\E. Therefore,

g(z) =f(z)g(z)

 +o()+· · ·+gn(z)

 +o(). (.)

Since (.) holds, we know that

g(z) +· · ·+gn(z)≡. (.)

So, by (.) and (.), we obtain that

f(z) = g(z)

g(z)( +o()) +· · ·+gn(z)( +o())

(.)

SetH={r:|z|> ,z∈C\E}. Thus by Remark ., we see thatH⊂(, +∞) is of finite logarithmic measure. Sinceg(z),gj(z),j= , . . . ,n, are polynomials, by (.) we get that

T(r,f)≤T(r,g) +Tr,g(z) +o()+· · ·+gn(z)

 +o()+O()

≤T(r,g) +

n

j=

T(r,gj) +O(), r∈/H.

Thus, for all sufficient larger,

T(r,f) =O(logr),

which contradicts with the fact thatf is transcendental.

Hence,g(z) is transcendental.

Lemma .([, ]) If f is transcendental entire satisfyinglim_r→+∞T(_rr,f)= ,or transcen-dental meromorphic withlim_r→+∞T(r,f)

r

= ,thenf_f has infinitely many zeros.

3 Proofs of Theorem 1.1 and Theorem 1.2

Proof of Theorem. According to the condition, suppose that{bv},v= , . . . ,n, are poles

off(z) with multiplicityτv, and we let

p(z) =

n

v=

(z–bv)τv.

Thenp(z)f(z) is entire and transcendental.

SetF(z) =p(z)f(z). Then, by the Hadamard factorization theorem andλ(f) <σ(f), we haveF(z) =h(z)eaz+b_{, wherea=  andbare constants,h(z) is an entire function satisfying}

σ(h) =λ(h) =λ(f) <σ(f) = .

Sinceg(z) =n_j=gj(z)f(z+cj) andF(z) =p(z)f(z) =h(z)eaz+b, we know that

g(z) =

n

j= gj(z)

h(z+cj)

p(z+cj)

eaz+b+acj

=

_n

j= gj(z)

h(z+cj)

p(z+cj)

eacj

eaz+b.

Note thatσ(h) < , by Lemma ., there exists anε-setEsuch that

h(z+ )∼h(z)

asz→ ∞inC\E. Thus

g(z)∼

_n

j= gj(z)

 p(z+cj)

eacj

Since

deggl(z)

> max

≤j≤n,j=l

deggj(z)

holds, we know that

n

j= gj(z)

 p(z+cj)

eacj≡_.

Ifg(z) has only finitely many zeros, then there exists a nonzero rational functionR(z) such that

n

j= gj(z)

h(z+cj)

p(z+cj)

eacj_{=R(z). (.)}

By Lemma . andσ(h) < , we know that (.) is impossible. Henceg(z) has infinitely many zeros.

Now we prove thatg(z) =n_j=gj(z)f(z+cj) has infinitely many fixed points. Set

g∗(z) =g(z) –z.

Thus we only need to prove thatg∗(z) has infinitely many zeros. Ifg∗(z) has only finitely many zeros, then we have

g∗(z) =R∗(z)edz+α_,

whereR∗(z) is a rational function,d=  andαare constants. Without loss of generality, we may suppose thatα=b. In fact, ifα=b, thenR∗(z)edz+α_=eα–b_R∗_(z)edz+b_{, ande}α–b_R∗_(z)

is also a rational function. So,

R∗(z)edz+b=h(z)eaz+b–z,

whereh(z) =n_j=gj(z) h(z+cj)

p(z+cj)e

acj_{. Since}

deggl(z)

> max

≤j≤n,j=l

deggj(z)

andσ(h) < , by Lemma ., we know thath(z) is transcendental, andσ(h) < . We affirm thata=d.

Ifa=d, then

R∗(z)edz+b–h(z)eaz+b+ze= .

So,

R∗(z) –h(z)eaz+b=z,

again by Lemma ., we get

R∗(z) –h(z)≡, z≡.

This also is a contradiction.

Hence,g(z) has infinitely many fixed points.

Proof of Theorem. According to Lemma ., we know thatg(z) is transcendental under conditions of Theorem . and the orderσ(g) < , which meansg(z) has either infinitely many zeros or infinitely many poles. Sincef(z) has only finitely many poles,i.e.,g(z) has finitely many poles. Henceg(z) must have infinitely many zeros.

The same discussion also holds when we consider the fixed points ofg(z). Hence,g(z)

has infinitely many fixed points.

We complete our proofs of Theorem . and Theorem ..

4 Proofs of Theorem 1.3 and Theorem 1.4

Proof of Theorem. First we prove thatG(z) is transcendental. Suppose thatGis rational. Then we obtain that

n

j= gj(z)

f(z+cj) –f(z)

f(z)

=R(z) –

n

j= gj(z),

whereR(z) is rational.

According to Lemma ., we know that

f(z+cj) –f(z) =cjf(z)

 +o()

asz→ ∞inC\E. So we get

n

j= cjgj(z)

f(z) f(z)

 +o()=R(z) –

n

j=

gj(z). (.)

Sincecl=  and

deggl(z)

> max

≤j≤n,j=l

deggj(z) ,

we know that

n

j=

So,

Sincegj(z) are polynomials. Hence the right-hand side of (.) is rational. We can easily get

a contradiction from the two sides of (.) since condition (i) or (ii) separately guarantees that the left-hand side f_f((_zz₎) has infinitely many zeros by Lemma .. Therefore,G(z) is transcendental. We complete the proof of Theorem ..

Proof of Theorem. Now we prove thatG(z) has infinitely many zeros. We continue to use symbols in the proofs of Theorem . and Theorem ..

Since know that there exists anε-setEsuch that

We could find a meromorphic functionq(z) such thatclh

h = q

q. In fact, by integrating both

sides ofclh

According to the Rouché theorem, we obtain that

n poles or infinitely many zeros and poles. Thus, we obtain that

nr,G(z)→ ∞

gdzis also of finite order. Therefore,

we can obtain that

G∗(z) –g(z)∼g(z)m _(z)

The following proof is similar to the above. We can get a similar equation to that of (.) by applying the Rouché theorem. At last, we get our conclusion thatG(z) has infinitely many fixed points.

Theorem . follows.

5 Proof of Theorem 1.5

Whenf(z) is meromorphic withσ(f) <_, since

deggl(z)

> max

≤j≤n,j=l

deggj(z) , cl= 

by Theorem . we could immediately find thatG(z) is transcendental and the order of G(z) is less than _, which meansG(z) has either infinitely many zeros or infinitely many poles. In what follows, we could obtain

n

r,  m(z)

–nr,m(z)=n

r,  g(z)

–nr,g(z)=deg(gl)≥

by using the same method as in the proof of Theorem ., wherem(z) representsG(z) or G(z) –z. Therefore,G(z) must have infinitely many zeros and fixed points.

Whenf(z) is transcendental and entire withσ(f) < , we can also obtain the same con-clusion by the same discussion as above. Theorem . follows.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

Cui and Yang completed the main part of the article, Qi corrected the main theorems. All authors read and approved the final manuscript.

Author details

1_{School of Mathematics, Shandong University, Jinan, Shandong 250100, P.R. China.}2_{School of Mathematics, University of}

Jinan, Jinan, Shandong 250022, P.R. China.

Acknowledgements

The authors thank the referee for his/her valuable suggestions to improve the present article. The authors also thank Jun Wang for valuable suggestions and discussions. This work was supported by the NNSF of China (No. 11171013 & No. 11371225). The third author is also supported by the NNSF of China (No. 11301220), the Tianyuan Fund for Mathematics (No. 11226094), the NSF of Shandong Province, China (No. ZR2012AQ020) and the Fund of Doctoral Program Research of University of Jinan (XBS1211).

Received: 14 January 2015 Accepted: 2 September 2015 References

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