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ISSN 2291-8639

Volume 6, Number 2 (2014), 139-143 http://www.etamaths.com

ESTIMATION OF COMPARATIVE GROWTH PROPERTIES OF ENTIRE AND MEROMORPHIC FUNCTIONS IN TERMS OF

THEIR RELATIVE ORDER

ARKOJYOTI BISWAS

Abstract. In this paper we discuss some comparative growth properties of entire and meromorphic functions on the basis of their relative order which improve some earlier results.

1. Introduction, Definitions and Notations.

Letf be a non constant entire function in the open complex planeCandMf(r) = max{|f(z)|:|z|=r}.ThenMf(r) is strictly increasing,its inverse

Mf−1: (|f(0)|,∞)→(0,∞)

exists and is such that

lim r→∞M

−1

f (r) =∞.

Two entire functionsf andgare said to be asymptotically equivalent if there exists

l (0< l <∞) such that

Mf(r)

Mg(r)

→l as r→ ∞

and in that case we writef ∼g.Clearly iff ∼g theng ∼f.

The order and lower order of an entire function are defined in the following way:

Definition 1. The orderρf and lower orderλf of an entire functionf are defined as follows:

ρf = lim sup r→∞

log logMf(r)

logr and λf = lim infr→∞

log logMf(r)

logr .

The functionf is said to be of regular growth ifρf =λf.

The notion of order of an entire function was much improved by the introduction of the relative order of two entire functions.In this connection Bernal [1] gave the following definition.

2010Mathematics Subject Classification. 30D30, 30D35.

Key words and phrases. Entire and meromorphic functions; growth; relative order; asymptotic properties.

c

2014 Authors retain the copyrights of their papers, and all open access articles are distributed under the terms of the Creative Commons Attribution License.

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with respect tog is defined as follows:

ρg(f) = inf{µ >0 :Mf(r)< Mg(rµ)for all sufficiently large values ofr}

= lim sup

r→∞

logMg−1Mf(r)

logr .

If we take g(z) as expz then we see that ρg(f) =ρf and this shows that the relative order generalised the concept of the order of an entire function.

Similarly the relative lower orderλg(f) is defined as

λg(f) = lim inf r→∞

logMg−1Mf(r)

logr .

For the case of a meromorphic function this generalisation was due to Lahiri and Banerjee [5].They introduced the notion of relative order ρg(f) off with respect tog wheref is meromorphic as follows:

ρg(f) = inf{µ >0 :Tf(r)< Tg(rµ) for all sufficiently large values ofr}

= lim sup r→∞

logTg−1Tf(r)

logr .

Similarly the relative lower orderλg(f) off with respect tog is defined by

λg(f) = lim inf r→∞

logTg−1Tf(r)

logr .

In a recent paper Datta and Biswas [2] studied some growth properties of entire functions using relative order.In this paper we discuss some comparative growth properties of entire and meromorphic functions in terms of their relative order which improves some results of Datta and Biswas [2].

2. Lemmas.

In this section we present two lemmas which will be needed in the sequel.

Lemma 1. If g1 and g2 be two entire functions with property (A)such that g1∼ g2.Iff be meromorphic thenρg1(f) =ρg2(f).

Lemma 1 follows from Theorem 5{cf.[3]} on puttingL(r)≡1.

Lemma 2([4]). Iff, gbe two meromorphic function andgis of regular growth.Then

ρg(f) = ρf ρg.

3. Theorems.

In this section we present the main results of our paper.

Theorem 1. Let f be meromorphic andg, hbe two entire functions with non zero finite orders.Then

lim inf r→∞

logTg−1Tf(r)

logTh−1Tf(r)

≤ ρg(f)

ρh(f)

≤lim sup r→∞

logTg−1Tf(r)

logTh−1Tf(r)

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Proof. From the definition of relative order we get for arbitraryε(>0) and for all sufficiently large values ofrthat

(1) logTg−1Tf(r)<(ρg(f) +ε) logr.

Also for a sequence of values ofrtending to infinity we get that

(2) logTg−1Tf(r)>(ρg(f)−ε) logr.

Again for arbitraryε(>0) and for all sufficiently large values ofrwe obtain that

(3) logTh−1Tf(r)<(ρh(f) +ε) logr

and for a sequence of values ofrtending to infinity we get that

(4) logTh−1Tf(r)>(ρh(f)−ε) logr.

Now from (1) and (4) we get for a sequence of values ofrtending to infinity that

logTg−1Tf(r)

logTh−1Tf(r)

< (ρg(f) +ε)

(ρh(f)−ε)

.

Asε(>0) is arbitrary it follows that

(5) lim inf

r→∞

logTg−1Tf(r)

logTh−1Tf(r)

≤ ρg(f)

ρh(f)

.

Also from (2) and (3) we get for a sequence of values ofrtending to infinity that

logTg−1Tf(r)

logTh−1Tf(r)

> (ρg(f)−ε)

(ρh(f) +ε)

.

Asε(>0) is arbitrary it follows that

(6) lim sup

r→∞

logTg−1Tf(r)

logTh−1Tf(r)

≥ ρg(f)

ρh(f)

.

Thus from (5) and (6) Theorem 1 follows.This completes the proof.

Corollary 1. If g and hare of regular growths then using Lemma 2 we get from Theorem 1 that

lim inf r→∞

logTg−1Tf(r)

logTh−1Tf(r) ≤ ρh

ρg

≤lim sup r→∞

logTg−1Tf(r)

logTh−1Tf(r)

.

Corollary 2. Ifg andhare of regular growths andg∼hthen using Lemma 1 we get from Theorem 1 that

lim inf r→∞

logTg−1Tf(r)

logTh−1Tf(r)

≤1≤lim sup r→∞

logTg−1Tf(r)

logTh−1Tf(r)

.

Remark 1. The converse of Corollary 2 is not always true which is evident from the following example.

Example 1. Let g (z) = expz and h(z) = exp(2z) so that Mg(r) = erand

Mh(r) =e2r.Now

Mg(r)

Mh(r)

→0 as r→ ∞

and sog1 g2 .Also

Tg(r) =

r

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Tg−1(r) =πr and Th−1(r) = π 2r.

But

lim r→∞

logTg−1Tf(r)

logTh−1Tf(r)

= lim

r→∞

logπTf(r) logπ2Tf(r)

= 1.

Theorem 2. Letf,hbe meromorphic andg be entire functions with non zero finite order.Then

lim inf r→∞

logTg−1Tf(r)

logTg−1Th(r)

≤ρg(f)

ρg(h)

≤lim sup r→∞

logTg−1Tf(r)

logTg−1Th(r)

.

Proof. From the definition of relative order we get for arbitraryε(>0) and for all sufficiently large values ofrthat

(7) logTg−1Th(r)<(ρg(h) +ε) logr

and for a sequence of values ofrtending to infinity we get that

(8) logTg−1Th(r)>(ρg(h)−ε) logr.

Now from (1) and (8) we get for a sequence of values ofrtending to infinity that

logT−1

g Tf(r) logTg−1Th(r)

< (ρg(f) +ε)

(ρg(h)−ε)

.

Asε(>0) is arbitrary it follows that

(9) lim inf

r→∞

logTg−1Tf(r)

logTg−1Th(r)

≤ ρg(f)

ρg(h)

.

Also from (2) and (7) we get for a sequence of values ofrtending to infinity that

logTg−1Tf(r)

logTg−1Th(r)

> (ρg(f)−ε)

(ρg(h) +ε)

.

Asε(>0) is arbitrary it follows that

(10) lim sup

r→∞

logTg−1Tf(r)

logTg−1Th(r)

≥ρg(f)

ρg(h)

.

From (9) and (10) we obtain Theorem 2.This completes the proof.

Corollary 3. If g and hare of regular growths then using Lemma 2 we get from Theorem 2 that

lim inf r→∞

logTg−1Tf(r)

logTg−1Th(r) ≤ ρf

ρh

≤lim sup r→∞

logTg−1Tf(r)

logTg−1Th(r)

.

Theorem 3. Let f, h be meromorphic andg, k be entire functions with non zero finite orders.Then

lim inf r→∞

logTg−1Tf(r)

logTk−1Th(r)

≤ρg(f)

ρk(h)

≤lim sup r→∞

logTg−1Tf(r)

logTk−1Th(r)

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Proof. From the definition of relative order we get for arbitraryε(>0) and for all sufficiently large values ofrthat

(11) logTk−1Th(r)<(ρk(h) +ε) logr.

Also for a sequence of values ofrtending to infinity we get that

(12) logTk−1Th(r)>(ρk(h)−ε) logr.

Now from (1) and (12) we get for a sequence of values ofrtending to infinity that

logTg−1Tf(r)

logTk−1Th(r)

< (ρg(f) +ε)

(ρk(h)−ε)

.

Asε(>0) is arbitrary it follows that

(13) lim inf

r→∞

logTg−1Tf(r)

logTk−1Th(r)

≤ ρg(f)

ρk(h)

.

Also from (2) and (11) we get for a sequence of values ofrtending to infinity that

logT−1

g Tf(r) logTk−1Th(r)

> (ρg(f)−ε)

(ρk(h) +ε)

.

Asε(>0) is arbitrary it follows that

(14) lim sup

r→∞

logTg−1Tf(r)

logTk−1Th(r)

≥ρg(f)

ρk(h)

.

From (13) and (14) we obtain Theorem 3.This completes the proof.

References

[1] L.Bernal: Orden relativo de crecimiento de functiones enteras, Collectanea Mathematica, 39(1988), 209-229.

[2] S.K.Datta and T.Biswas: Relative order of composite entire functions and some related growth properties, Bull. Cal. Math. Soc., 102 (2010), 259-266.

[3] S.K.Datta and A.Biswas: Estimation of relative order of entire and meromorphic functions in terms of slowly changing functions, Int.J.Contemp.Math.Sciences, 6 (2011), 1175-1186. [4] S.K.Datta and A.Biswas: A note on relative order of entire and meromorphic

function-s,International J. of Math. Sci. & Engg. Appls. 6 (2012), 413-421.

[5] B.K. Lahiri and D.Banerjee: Relative order of entire and meromorphic functions, Proc.Nat.Acad.Sci.India, 69 (1999), 339-354.

References

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