# Growth Analysis of Composite Entire and Meromorphic Functions in the Light of Their Relative Orders

Authors: Sanjib Kumar Datta; Tanmay Biswas; Chinmay Biswas
Journal: International Scholarly Research Notices
 (2014)

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## Abstract

We study some comparative growth properties of composite entire and meromorphic functions on the basis of their relative orders (relative lower orders).


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## Body

## 1. Introduction

Letf be meromorphic and g be an entire function defined in the open complex plane C. The maximum modulus function corresponding to entire g is defined as Mg(r)=max⁡{|g(z)|:|z|=r}. For meromorphic f, Mf(r) cannot be defined as f is not analytic. In this situation one may define another function Tf(r) known as Nevanlinna’s characteristic function of f, playing the same role as maximum modulus function in the following manner:
(1)Tf(r)=Nf(r)+mf(r),
where the function Nf(r,a)(N¯f(r,a)) known as counting function of a-points (distinct a-points) of meromorphic f is defined as
(2)Nf(r,a)=∫0rnf(t,a)-nf(0,a)tdt+nf(0,a)log⁡⁡r,(N¯f(r,a)=∫0rn-f(t,a)-n-f(0,a)tdt+n¯f(0,a)log⁡⁡r).
Moreover, we denote by nf(r,a)(n¯f(r,a)) the number of a-points (distinct a-points) of f in |z|≤r and an ∞-point is a pole of f. In many occasions Nf(r,∞) and N¯f(r,∞) are denoted by Nf(r) and N¯f(r), respectively.The functionmf(r,∞) alternatively denoted by mf(r) known as the proximity function of f is defined as follows:
(3)mf(r)=12π∫02πlog⁡+|f(reiθ)|dθ,wherelog⁡+x=max⁡(log⁡x,0)∀x⩾0.
Also we may denote m(r,1/(f-a)) by mf(r,a).Whenf is an entire function, the Nevanlinna’s characteristic function Tf(r) of f is defined as
(4)Tf(r)=mf(r).Further, iff is a nonconstant entire function then Mf(r) and Tf(r) are both strictly increasing and continuous functions of r. Also their inverses Mf-1(r):(|f(0)|,∞)→(0,∞) and Tf-1:(Tf(0),∞)→(0,∞) exist, respectively, and are such that lim⁡s→∞⁡Mg-1(s)=∞ and lim⁡s→∞⁡Tf-1(s)=∞.In this connection we just recall the following definition which is relevant.Definition 1 (see [1]).
A nonconstant entire functionf is said to have the Property (A) if, for any σ>1 and for all sufficiently large r, [Mf(r)]2≤Mf(rσ) holds. For examples of functions with or without the Property (A), one may see [1].However, for any two entire functionsf and g, the ratio Mf(r)/Mg(r) as r→∞ is called the growth of f with respect to g in terms of their maximum moduli. Similarly, when f and g are both meromorphic functions, the ratio Tf(r)/Tg(r) as r→∞ is called the growth of f with respect to g in terms of their Nevanlinna’s characteristic functions. The notion of the growth indicators such asorder andlower order of entire or meromorphic functions which are generally used in computational purpose is defined in terms of their growth with respect to theexponential function as the following.Definition 2.
The orderρf (the lower order λf) of an entire function f is defined as
(5)ρf=limsup⁡r→∞log⁡⁡log⁡⁡Mf(r)log⁡⁡log⁡⁡Mexp⁡z(r)=limsup⁡r→∞log⁡[2]⁡Mf(r)log⁡⁡r(λf=liminf⁡r→∞log⁡⁡log⁡⁡Mf(r)log⁡⁡log⁡⁡Mexp⁡z(r)=liminf⁡r→∞log⁡[2]⁡Mf(r)log⁡⁡r),
where log⁡[k]⁡x=log⁡⁡(log⁡[k-1]⁡x) for k=1,2,3,… and log⁡[0]⁡x=x. Further, if f is a meromorphic function one can easily verify that
(6)ρf=limsup⁡r→∞log⁡Tf(r)log⁡Texp⁡z(r)=limsup⁡r→∞log⁡Tf(r)log⁡(r/π)=limsup⁡r→∞log⁡Tf(r)log⁡r+O(1),(λf=liminf⁡r→∞log⁡Tf(r)log⁡Texp⁡z(r)=liminf⁡r→∞log⁡Tf(r)log⁡(r/π)=liminf⁡r→∞log⁡Tf(r)log⁡r+O(1)).Bernal [1, 2] introduced the definition of relative order of an entire function f with respect to another entire function g, denoted by ρg(f) to avoid comparing growth just with exp⁡z as follows:
(7)ρg(f)=inf⁡{μ>0:Mf(r)<Mg(rμ)∀r>r0(μ)>0}=limsup⁡r→∞log⁡⁡Mg-1Mf(r)log⁡⁡r.The definition coincides with the classical one [3] if g(z)=exp⁡z.Similarly, one can define the relative lower order of an entire functionf with respect to another entire function g denoted by λg(f) as follows:
(8)λg(f)=liminf⁡r→∞log⁡⁡Mg-1Mf(r)log⁡⁡r.Extending this notion, Lahiri and Banerjee [4] introduced the definition of relative order of a meromorphic function with respect to an entire function in the following way.Definition 3 (see [4]).
Letf be any meromorphic function and g be any entire function. The relative order of f with respect to g is defined as
(9)ρg(f)=inf⁡⁡{μ>0:Tf(r)<Tg(rμ)forallsufficientlylarger}=limsup⁡r→∞log⁡Tg-1Tf(r)log⁡r.
Likewise, one can define the relative lower order of a meromorphic function f with respect to an entire function g denoted by λg(f) as follows:
(10)λg(f)=liminf⁡r→∞log⁡Tg-1Tf(r)log⁡r.
It is known (cf. [4]) that if g(z)=exp⁡z then Definition 3 coincides with the classical definition of the order of a meromorphic function f.For entire and meromorphic functions, the notions of their growth indicators such asorder andlower order are classical in complex analysis and during the past decades, several researchers have already been exploring their studies in the area of comparative growth properties of composite entire and meromorphic functions in different directions using the classical growth indicators. But at that time, the concepts ofrelative orders andrelative lower orders of entire and meromorphic functions and their technical advantages of not comparing with the growths of exp⁡z are not at all known to the researchers of this area. Therefore the studies of the growths of composite entire and meromorphic functions in the light of theirrelative orders andrelative lower orders are the prime concern of this paper. In fact some light has already been thrown on such type of works by Datta et al. [5]. We do not explain the standard definitions and notations of the theory of entire and meromorphic functions as those are available in [6, 7].


## 2. Lemmas

In this section we present some lemmas which will be needed in the sequel.Lemma 1 (see [8]).
Letf be meromorphic and g be entire; then, for all sufficiently large values of r,
(11)Tf∘g(r)⩽{1+o(1)}Tg(r)log⁡Mg(r)Tf(Mg(r)).Lemma 2 (see [9]).
Letf be meromorphic and g be entire and suppose that 0<μ<ρg≤∞. Then, for a sequence of values of r tending to infinity,
(12)Tf∘g(r)≥Tf(exp⁡(rμ)).Lemma 3 (see [10]).
Letf be meromorphic and g be entire such that 0<ρg<∞ and 0<λf. Then, for a sequence of values of r tending to infinity,
(13)Tf∘g(r)>Tg(exp⁡(rμ)),
where 0<μ<ρg.Lemma 4 (see [11]).
Letf be an entire function which satisfies the Property (A), β>0, δ>1, and α>2. Then
(14)βTf(r)<Tf(αrδ).


## 3. Theorems

In this section we present the main results of the paper.Theorem 1.
Letf be a meromorphic function and h be an entire function with 0<λh(f)≤ρh(f)<∞ and let g be an entire function with finite order. If h satisfies the Property (A), then, for every positive constant μ and each α∈(-∞,∞),
(15)lim⁡r→∞{log⁡Th-1Tf∘g(r)}1+αlog⁡Th-1Tf(exp⁡rμ)=0,whereμ>(1+α)ρg.Proof.
Let us suppose thatβ>2 and δ>1. If 1+α≤0, then the theorem is obvious. We consider 1+α>0.
SinceTh-1(r) is an increasing function of r, it follows from Lemma 1, Lemma 4, and the inequality Tg(r)≤log⁡Mg(r)  {cf. [6]} for all sufficiently large values of r that
(16)Th-1Tf∘g(r)⩽Th-1[{1+o(1)}Tf(Mg(r))],thatis,Th-1Tf∘g(r)⩽β[Th-1Tf(Mg(r))]δ,thatis,log⁡Th-1Tf∘g(r)⩽δlog⁡Th-1Tf(Mg(r))+O(1),thatis,log⁡Th-1Tf∘g(r)⩽δ(ρh(f)+ɛ)rρg+ɛ+O(1).
Again for all sufficiently large values of r we get that
(17)log⁡Th-1Tf(exp⁡rμ)≥(λh(f)-ɛ)rμ.
Hence for all sufficiently large values of r we obtain from (16) and (17) that
(18){log⁡Th-1Tf∘g(r)}1+αlog⁡Th-1Tf(exp⁡rμ)≤[δ(ρh(f)+ɛ)rρg+ɛ+O(1)]1+α(λh(f)-ɛ)rμ,
where we choose 0<ɛ<min⁡{λh(f),(μ/(1+α))-ρg}.
So from (18) we obtain that
(19)lim⁡r→∞{log⁡Th-1Tf∘g(r)}1+αlog⁡Th-1Tf(exp⁡rμ)=0.
This proves the theorem.Remark 2.
In Theorem1 if we take the condition 0<ρh(f)<∞ instead of 0<λh(f)≤ρh(f)<∞, the theorem remains true with “limit inferior” in place of “limit.”In view of Theorem1 the following theorem can be carried out.Theorem 3.
Letf be a meromorphic function and let g, h be any two entire functions where g is of finite order and λh(g)>0, ρh(f)<∞. If h satisfies the Property (A), then, for every positive constant μ and each α∈(-∞,∞),
(20)lim⁡r→∞{log⁡Th-1Tf∘g(r)}1+αlog⁡Th-1Tg(exp⁡rμ)=0,whereμ>(1+α)ρg.The proof is omitted.Remark 4.
In Theorem3 if we take the condition ρh(g)>0 instead of λh(g)>0, the theorem remains true with “limit” replaced by “limit inferior”.Theorem 5.
Letf be a meromorphic function and let g, h be any two entire functions such that 0<λh(f)≤ρh(f)<∞ and λg<μ<∞. Also suppose that h satisfies the Property (A). Then, for a sequence of values of r tending to infinity,
(21)Th-1Tf∘g(r)<Th-1Tf(exp⁡rμ).Proof.
Let us considerδ>1. Since Th-1(r) is an increasing function of r, it follows from Lemma 1 that, for a sequence of values of r tending to infinity,
(22)log⁡Th-1Tf∘g(r)⩽δ(ρh(f)+ɛ)rλg+ɛ+O(1).
Now from (17) and (22), it follows for a sequence of values of r tending to infinity that
(23)log⁡Th-1Tf(exp⁡rμ)log⁡Th-1Tf∘g(r)≥(λh(f)-ɛ)rμδ(ρh(f)+ɛ)rλg+ɛ+O(1).
As λg<μ we can choose ɛ (>0) in such a way that
(24)λg+ɛ<μ<ρg.
Thus from (23) and (24) we obtain that
(25)limsup⁡r→∞log⁡Th-1Tf(exp⁡rμ)log⁡Th-1Tf∘g(r)=∞.
Now from (25), we obtain for a sequence of values of r tending to infinity and also for K>1(26)Th-1Tf(exp⁡rμ)>Th-1Tf∘g(r).
Thus the theorem follows.In the line of Theorem5, we may state the following theorem without its proof.Theorem 6.
Letg and h be any two entire functions with λh(g)>0 and let f be a meromorphic function with finite relative order with respect to h. Also suppose that λg<μ<∞ and h satisfies the Property (A). Then, for a sequence of values of r tending to infinity,
(27)Th-1Tf∘g(r)<Th-1Tg(exp⁡rμ).As an application of Theorem5 and Lemma 2, we may state the following theorem.Theorem 7.
Letf be a meromorphic function and let g, h be any two entire functions such that 0<λh(f)≤ρh(f)<∞ and λg<μ<ρg. If h satisfies the Property (A), then
(28)liminf⁡r→∞Th-1Tf∘g(r)Th-1Tf(exp⁡rμ)≤1≤limsup⁡r→∞Th-1Tf∘g(r)Th-1Tf(exp⁡rμ).The proof is omitted.Similary in view of Theorem6 and Lemma 3, the following theorem can be carried out.Theorem 8.
Letf be a meromorphic function and let g, h be any two entire functions with 0<λh(f)≤ρh(f)<∞, 0<λh(g)≤ρh(g)<∞, and 0<λg<μ<ρg<∞. Moreover h satisfies the Property (A). Then
(29)liminf⁡r→∞Th-1Tf∘g(r)Th-1Tg(exp⁡rμ)≤1≤limsup⁡r→∞Th-1Tf∘g(r)Th-1Tg(exp⁡rμ).The proof is omitted.Theorem 9.
Letf be a meromorphic function and let h, g be any two entire functions with λh(f)>0 and 0<ρh(g)<∞. Then
(30)limsup⁡r→∞log⁡Th-1Tf∘g(r)log⁡Th-1Tg(exp⁡rμ)=∞,
where 0<μ<ρg.Proof.
Let0<μ<μ/<ρg. As Th-1(r) is an increasing function of r, it follows from Lemma 2 for a sequence of values of r tending to infinity that
(31)log⁡⁡Th-1Tf∘g(r)≥log⁡⁡Th-1Tf(exp⁡(rμ/)),thatis,log⁡⁡Th-1Tf∘g(r)≥(λh(f)-ɛ)rμ/.
Again for all sufficiently large values of r we get that
(32)log⁡Th-1Tg(exp⁡rμ)≤(ρh(g)+ɛ)rμ.
So combining (31) and (32), we obtain for a sequence of values of r tending to infinity that
(33)log⁡Th-1Tf∘g(r)log⁡Th-1Tg(exp⁡rμ)≥(λh(f)-ɛ)rμ/(ρh(g)+ɛ)rμ.
Sinceμ<μ/, it follows from (33) that
(34)limsup⁡r→∞log⁡Th-1Tf∘g(r)log⁡Th-1Tg(exp⁡rμ)=∞.
Hence the theorem follows.Corollary 10.
Under the assumptions of Theorem9,
(35)limsup⁡r→∞Th-1Tf∘g(r)Th-1Tg(exp⁡rμ)=∞,0<μ<ρg.Proof.
In view of Theorem9, we get for a sequence of values of r tending to infinity that
(36)log⁡Th-1Tf∘g(r)≥Klog⁡⁡Th-1Tg(exp⁡rμ),forK>1,thatis,Th-1Tf∘g(r)≥log⁡{Th-1Tg(exp⁡rμ)}K,
from which the corollary follows.Theorem 11.
Letf be a meromorphic function and let h, g be any two entire functions such that (i) 0<ρh(g)<∞, (ii) λh(f)>0, and (iii) λh(f∘g)>0. Then
(37)limsup⁡r→∞[log⁡Th-1Tf∘g(r)]2{log⁡Th-1Tg(exp⁡rμ)}·{log⁡Th-1Tg(r)}=∞,
where 0<μ<ρg.Proof.
From the definition of relative order and relative lower order, we obtain for arbitrary positiveɛ and for all sufficiently large values of r that
(38)log⁡⁡Th-1Tf∘g(r)≥(λh(f∘g)-ɛ)log⁡⁡r,log⁡Th-1Tg(r)≤(ρh(g)+ɛ)log⁡r.
Therefore, from (38), it follows for all sufficiently large values of r that
(39)log⁡Th-1Tf∘g(r)log⁡Th-1Tg(r)≥(λh(f∘g)-ɛ)log⁡r(ρh(g)+ɛ)log⁡r,thatis,liminf⁡r→∞log⁡Th-1Tf∘g(r)log⁡Th-1Tg(r)≥λh(f∘g)ρh(g).
Thus the theorem follows from (34) and (39).Similarly, one may state the following theorems and corollary without their proofs as those can be carried out in the line of Theorems9 and 11 and Corollary 10, respectively.Theorem 12.
Letf be a meromorphic function and h be an entire function with 0<λh(f)≤ρh(f)<∞. Then, for any entire function g,
(40)limsup⁡r→∞log⁡Th-1Tf∘g(r)log⁡Th-1Tf(exp⁡rμ)=∞,
where 0<μ<ρg.Theorem 13.
Letf be a meromorphic function and let h, g be any two entire functions such that (i) 0<λh(f)≤ρh(f)<∞ and (ii) λh(f∘g)>0. Then
(41)limsup⁡r→∞[log⁡Th-1Tf∘g(r)]2{log⁡Th-1Tf(exp⁡rμ)}·{log⁡Th-1Tf(r)}=∞,
where 0<μ<ρg.Corollary 14.
Under the assumptions of Theorem12,
(42)limsup⁡r→∞Th-1Tf∘g(r)Th-1Tf(exp⁡rμ)=∞,0<μ<ρg.Theorem 15.
Letf be a meromorphic function and let h be an entire function with 0<λh(f)≤ρh(f)<∞. Then, for any entire function g,
(43)limsup⁡r→∞log⁡[2]Th-1Tf∘g(exp⁡rB)log⁡[2]Th-1Tf(exp⁡rμ)=∞,
where 0<μ<ρg and B>0.Proof.
Let0<μ/<ρg. As Th-1(r) is an increasing function of r, it follows from (31) for a sequence of values of r tending to infinity that
(44)log⁡[2]Th-1Tf∘g(r)≥O(1)+μ/log⁡r.
So for a sequence of values of r tending to infinity we get from above that
(45)log⁡[2]Th-1Tf∘g(exp⁡rB)≥O(1)+μ/rB.
Again we have for all sufficiently large values of r that
(46)log⁡[2]Th-1Tf(exp⁡⁡rμ)≤O(1)+μlog⁡⁡r.
Now combining (45) and (46), we obtain for a sequence of values of r tending to infinity that
(47)log⁡[2]Th-1Tf∘g(exp⁡rB)log⁡[2]Th-1Tf(exp⁡rμ)≥O(1)+μ/rBO(1)+μlog⁡r,
from which the theorem follows.In view of Theorem15 the following theorem can be carried out.Theorem 16.
Letf be a meromorphic function and let h, g be any two entire functions with λh(f)>0 and 0<ρh(g)<∞. Then
(48)limsup⁡r→∞log⁡[2]Th-1Tf∘g(exp⁡rB)log⁡[2]Th-1Tg(exp⁡rμ)=∞,
where 0<μ<ρg and B>0.The proof is omitted.Theorem 17.
Letl be an entire function satisfying the Property (A) and let h be a meromorphic function such that λl(h)>0. Also let g and k be any two entire functions with finite nonzero order such that ρg<ρk. Then, for every meromorphic function f with 0<ρl(f)<∞,
(49)limsup⁡r→∞log⁡Tl-1Th∘k(r)log⁡Tl-1Tf∘g(r)+log⁡Tl-1Tf(r)=∞.Proof.
Sinceρg<ρk, we can choose ɛ (>0) in such a way that
(50)ρg+ɛ<μ<ρk-ɛ.
As Tl-1(r) is an increasing function of r, it follows from Lemma 2 for a sequence of values of r tending to infinity that
(51)log⁡Tl-1Th∘k(r)≥log⁡Tl-1Th(exp⁡rμ),where0<μ<ρk≤∞;thatis,log⁡Tl-1Th∘k(r)≥(λl(h)-ɛ)rμ.
Now from the definition of relative order of f with respect to l we have for arbitrary positive ɛ and for all sufficiently large values of r that
(52)log⁡Tl-1Tf(r)≤(ρl(f)+ɛ)log⁡r.
Now for any δ>1, we get from (16), (51), (52), and in view of (50) for a sequence of values of r tending to infinity that
(53)log⁡Tl-1Th∘k(r)log⁡Tl-1Tf∘g(r)+log⁡Tl-1Tf(r)≥(λl(h)-ɛ)rμδ(ρh(f)+ɛ)rρg+ɛ+(ρl(f)+ɛ)log⁡r+O(1),thatis,log⁡Tl-1Th∘k(r)log⁡Tl-1Tf∘g(r)+log⁡Tl-1Tf(r)=∞,
which proves the theorem.In the line of Theorem17 the following theorem can be carried out.Theorem 18.
Letl be an entire function satisfying the Property (A) and let h be a meromorphic function such that λl(h)>0. Also let g and k be any two entire functions with finite nonzero order and also ρg<ρk. Then, for every meromorphic function f with 0<ρl(f)<∞,
(54)limsup⁡r→∞log⁡Tl-1Th∘k(r)log⁡Tl-1Tf∘g(r)+log⁡Tl-1Tg(r)=∞.


## 4. Conclusion

Actually this paper deals with the extension of the works on the growth properties of composite entire and meromorphic functions on the basis of theirrelative orders andrelative lower orders. These theories can also be modified by the treatment of the notions ofgeneralized relative orders (generalized relative lower orders) and (p,q)threlative orders ((p,q)threlative lower orders). Moreover, some extensions of the same type may be done in the light of slowly changing functions.



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