Value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials

We discuss the value-sharing problem, versions of the Hayman conjecture, and the uniqueness problem for p-adic meromorphic functions and their difference operators and difference polynomials.

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Date:2012
Main Authors: An, Vu Hoai, Khoai, Ha Huy, Ан, Ву Гоай, Хоаї, Га Гуй
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Language:English
Published: Institute of Mathematics, NAS of Ukraine 2012
Online Access:https://umj.imath.kiev.ua/index.php/umj/article/view/2562
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Journal Title:Ukrains’kyi Matematychnyi Zhurnal
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Ukrains’kyi Matematychnyi Zhurnal
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author An, Vu Hoai
Khoai, Ha Huy
Ан, Ву Гоай
Хоаї, Га Гуй
author_facet An, Vu Hoai
Khoai, Ha Huy
Ан, Ву Гоай
Хоаї, Га Гуй
author_sort An, Vu Hoai
baseUrl_str https://umj.imath.kiev.ua/index.php/umj/oai
collection OJS
datestamp_date 2020-03-18T19:29:46Z
description We discuss the value-sharing problem, versions of the Hayman conjecture, and the uniqueness problem for p-adic meromorphic functions and their difference operators and difference polynomials.
first_indexed 2026-03-24T02:25:52Z
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fulltext UDC 517.9 Vu Hoai An (Hai Duong College, Hai Duong, Vietnam), Ha Huy Khoai (Ins. Math., Hanoi, Vietnam) VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS AND THEIR DIFFERENCE OPERATORS AND DIFFERENCE POLYNOMIALS * ЗАДАЧА ПРО СПIЛЬНI ЗНАЧЕННЯ ДЛЯ p-АДИЧНИХ МЕРОМОРФНИХ ФУНКЦIЙ ТА ЇХ РIЗНИЦЕВИХ ОПЕРАТОРIВ I РIЗНИЦЕВИХ ПОЛIНОМIВ We discuss the value-sharing problem, versions of the Hayman conjecture, and the uniqueness problem for p-adic meromorphic functions and their difference operators and difference polynomials. Дослiджено питання про спiльнi значення i єдинiсть та аналоги гiпотези Хеймена для p-адичних мероморфних функцiй та їх рiзницевих операторiв i рiзницевих полiномiв. 1. Introduction. The problem of determining a meromorphic (or entire) function on C by its sing- le pre-images, counting multiplicities, of finite sets is an important one and it has been studied by many mathematicians. For instance, in 1921 G. Polya showed that an entire function on C is deter- mined by the inverse images, counting multiplicities, of three distinct non-omitted values. In 1926, R. Nevanlinna showed that a meromorphic function on the complex plane is uniquely determined by the inverse images, ignoring multiplicities, of 5 distinct values. In [16] Hayman proved the following well-known result: Theorem 1.1. Let f be a meromorphic function on C. If f(z) 6= 0 and f (k)(z) 6= 1 for some fixed positive integer k and for all z ∈ C, then f is constant. Hayman also proposed the following conjecture (see [16]). Hayman Conjecture. If an entire function f satisfies fn(z)f ′(z) 6= 1 for a positive integer n and all z ∈ C, then f is a constant. It has been verified for transcendental entire functions by Hayman himself for n > 1 [16], and by Clunie for n ≥ 1 [5]. These results and some related problems caused increasingly attentions to the value-sharing problem of meromorphic functions and their derivatives (see [2, 4, 19, 21]). In 1997 Yang and Hua [23] studied the unicity problem for meromorphic functions and differential monomials of the form fnf ′, when they share only one value, and obtained the following theorem. Theorem 1.2. Let f and g be two non-constant meromorphic functions, let n ≥ 11 be an integer, and a ∈ C be a non-zero finite value. If fnf ′ and gng′ share the value a CM, then either f ≡ dg for some (n+1)-th root of unity d, or f = c1e cz and g = c2e −cz for three non-zero constants c1, c2 and c such that (c1c2)n+1c2 = −a2. *The work was supported by a NAFOSTED grant. c© VU HOAI AN, HA HUY KHOAI, 2012 ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 147 148 VU HOAI AN, HA HUY KHOAI Recently, there has been an increasing interest in studying value-sharing and uniqueness for mero- morphic functions and their difference operators and difference polynomials. Halburd and Korhonen [14] established a version of Nevanlinna theory based on difference operators. For an analog of Hay- man Conjecture for difference, Laine and Yang [20] investigated the value distribution of difference products of entire functions, and obtained the following theorem. Theorem 1.3. Let f(z) be a transcendental entire function of finite order, and c be a non-zero complex constant. Then n ≥ 2, f(z)nf(z + c) assumes every non-zero value a ∈ C infinitely often. In recent years the similar problems are investigated for functions in a non-Archimedean fields (see, for example, [3, 5]). In [22] J. Ojeda proved that for a transcendental meromorphic function f in an algebraically closed fields of characteristic zero, complete for a non-Archimedean absolute value K, the function f ′fn − 1 has infinitely many zeros, if n ≥ 2. Ha Huy Khoai and Vu Hoai An [12] established a similar results for a differential monomial of the form fn(f (k)) m , where f is a meromorphic function in Cp. Now let K be an algebraically closed field of characteristic zero, complete for a non-Archimedean absolute value. We denote by A(K) the ring of entire functions in K, byM(K) the field of meromor- phic functions, i.e., the field of fractions of A(K), and K̂ = K∪{∞}. The value-sharing problem for meromorphic functions in K was investigated first in [1] and [8]. In recent years, many interesting results on the value-sharing problem for meromorphic functions in K were obtained (see [17, 13]). Let us first recall some basic definitions. For f ∈M(K) and S ⊂ K̂, we define Ef (S) = ⋃ a∈S {(z,m)|f(z) = a with multiplicity m}. Let F be a nonempty subset of M(K). Two functions f, g of F are said to share S, counting multiplicity (share S CM), if Ef (S) = Eg(S). Now for f ∈M(K). We define difference operators of f as ∆cf = f(z + c)− f(z), ∆1 cf = ∆cf and ∆n+1 c f = ∆nf(z + c)−∆nf(z), n = 1, 2, . . . , where c ∈ Cp is a non-zero constant; and difference polynomial of f as A(z, f) = ∑ Λ∈I aΛ(z)f(z)Λ0f(z)Λ1 . . . f(z)Λn , where I be a finite set of multiindex Λ = (Λ0,Λ1, . . . ,Λn) and the coefficients aΛ(z) are small with respect to f(z) in the sence that TaΛ(r) = o(Tf (r)). From now on, we assume P (z) is a non-zero polynomial on Cp of degree n. Write P (z) = = a0(z − a1)m1(z − a2)m2 . . . (z − as)ms , a0 6= 0. In this paper we discuss the value-sharing and versions of the Hayman Conjecture and uniqueness for p-adic meromorphic functions and their difference operators and difference polynomials, and prove a p-adic analog of Laine – Yang’s result. Namely, we prove the following theorems. ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS . . . 149 Theorem 1.4 (A version of the Hayman Conjecture for p-adic meromorphic functions and their difference operators). Let f be a meromorphic function on Cp and n, ki, s, q, i = 1, . . . , q, be are integers, s ≥ 1, q ≥ 1, ki ≥ 1, n ≥ q∑ i=1 (2ki + 1)2i + q + s+ 1− 3 q∑ i=1 ki, and ∆qf is not identically zero. Then P (f)(∆1 cf)k1 . . . (∆q cf)kq − a has zeros, where a ∈ Cp is a non-zero. Theorem 1.5 (A version of the Hayman Conjecture for p-adic meromorphic functions and their difference polynomials). Let f be a meromorphic function on Cp and n, qi, s, k, i = 1, . . . , k, be are integers, and s ≥ 1, k ≥ 1, qi ≥ 1, n ≥ k∑ i=1 qi + 2k + s+ 1. Then P (f)(f(z + c))q1 . . . (f(z + kc))qk − a has zeros, where a ∈ Cp is a non-zero. Theorem 1.6 (A version of the Yang and Hua’s Theorem 1.2 for p-adic meromorphic functions and their difference polynomials). Let f and g be two non-constant p-adic meromorphic functions. (1) If Efnf(z+c)...f(z+kc)(1) = Egng(z+c)...g(z+kc)(1), with k ≥ 1 and n ≥ 5k+8 be are integers, then f = hg with hn+k = 1 or fg = l with ln+k = 1. (2) If Efn(f(z+c))q1 ...(f(z+kc))qk (1) = Egn(g(z+c))q1 ...(g(z+kc))qk (1), with qi > 1, i = 1, . . . , k, k ≥ 1, n ≥ k∑ i=1 qi + 8k + 8 be are integers, then f = hg with hn+q1+...+qk = 1 or fg = l with ln+q1+...+qk = 1. (3) If Efn(f(z+e1c)...f(z+emc)(f(z+t1c))q1 ...(f(z+tkc)) qk (1) = = Egn(g(z+e1c)...g(z+emc)(g(z+t1c))q1 ...(g(z+tkc)) qk (1), with ej ≥ 1, j = 1, . . . ,m, ti ≥ 1, qi > 1, i = 1, . . . , k, k ≥ 1, n ≥ 5m+ k∑ i=1 qi + 8k + 8 be are integers, then f = hg with hn+m+q1+...+qk = 1 or fg = l with ln+m+q1+...+qk = 1. The main tool of the proof is the p-adic Nevanlinna theory ([8 – 3, 17]). Therefore, in the next section we first establish some properties of the height function (a p-adic analog of the Nevanlinna characteristic functionrm) for p-adic meromorphic functions and their difference operator and differ- ence polynomials for later use. ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 150 VU HOAI AN, HA HUY KHOAI 2. Height of p-adic meromorphic functions. Let f be a non-constant holomorphic function on Cp. For every a ∈ Cp, expanding f around a as f = ∑ Pi(z − a) with homogeneous polynomials Pi of degree i, we define vf (a) = min{i : Pi 6≡ 0}. For a point d ∈ Cp we define the function vdf : Cp → N by vdf (a) = vf−d(a). Fix a real number ρ with 0 < ρ ≤ r. Define Nf (a, r) = 1 ln p r∫ ρ nf (a, x) x dx, where nf (a, x), as usually, is the number of the solutions of the equation f(z) = a (counting multiplicity) in the disk Dx = {z ∈ Cp : |z| ≤ x}. If a = 0, then set Nf (r) = Nf (0, r). For l a positive integer, set Nl,f (a, r) = 1 ln p r∫ ρ nl,f (a, x) x dx, where nl,f (a, r) = ∑ |z|≤r min { vf−a(z), l } . Let k be a positive integer. Define the function v≤kf from Cp into N by v≤kf (z) = 0 if vf (z) > k, vf (z) if vf (z) ≤ k, and n≤kf (r) = ∑ |z|≤r v≤kf (z), n≤kf (a, r) = n≤kf−a(r). Define N≤kf (a, r) = 1 ln p r∫ ρ n≤kf (a, x) x dx. If a = 0, then set N≤kf (r) = N≤kf (0, r). Set ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS . . . 151 N≤kl,f (a, r) = 1 ln p r∫ ρ n≤kl,f (a, x) x dx, where n≤kl,f (a, r) = ∑ |z|≤r min { v≤kf−a(z), l } . In a like manner we define N<k f (a, r), N<k l,f (a, r), N>k f (a, r), N≥kf (a, r), N≥kl,f (a, r), N>k l,f (a, r). Recall that for a holomorphic function f(z) in Cp, represented by the power series f(z) = ∞∑ 0 anz n, for each r > 0, we define |f |r = max{|an|rn, 0 ≤ n <∞}. Now let f = f1 f2 be a non-constant meromorphic function on Cp, where f1, f2 be holomorphic functions on Cp having no common zeros, we set |f |r = |f1|r |f2|r . For a point d ∈ Cp ∪ {∞} we define the function vdf : Cp → N by vdf (a) = vf1−df2(a) with d 6=∞, and v∞f (a) = vf2(a). For a point a ∈ C define mf (∞, r) = max { 0, log |f |r } , qmf (a, r) = m1/f−a(∞, r), Nf (a, r) = Nf1−af2(r), Nf (∞, r) = Nf2(r), Tf (r) = max 1≤i≤2 log |fi|r. In a like manner we define Nl,f (a, r), N≤kf (a, r), N≤kl,f (a, r), N<k f (a, r), N<k l,f (a, r), N>k f (a, r), N≥kf (a, r), N≥kl,f (a, r), N>k l,f (a, r), with a ∈ Cp ∪ {∞}. Then we have (see [9]) Nf (a, r) +mf (a, r) = Tf (r) +O(1) ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 152 VU HOAI AN, HA HUY KHOAI with a ∈ Cp ∪ {∞}, Tf (r) = T1/f (r) +O(1), |f (k)|r ≤ |f |r rk , mf (k)/f (∞, r) = O(1). The following two lemmas were proved in [9]. Lemma 2.1. Let f be a non-constant holomorphic function on Cp. Then Tf (r)− Tf (ρ) = Nf (r), where 0 < ρ ≤ r. Lemma 2.2. Let f be a non-constant meromorphic function on Cp and let a1, a2, . . . , aq be distinct points of Cp. Then (q − 1)Tf (r) ≤ N1,f (∞, r) + q∑ i=1 N1,f (ai, r)−N0,f ′(r)− log r +O(1), where N0,f ′(r) is the counting function of the zeros of f ′ which occur at points other than roots of the equations f(z) = ai, i = 1, . . . , q, and 0 < ρ ≤ r. 3. Two versions of the Hayman Conjecture for p-adic meromorphic functions and their difference operators and difference polynomials. We are going to prove Theorems 1.4 – 1.6. We need the following lemmas. Lemma 3.1. Let f be a non-constant p-adic meromorphic function and ∆f is not identically zero and k, q be a positive integer. Then: (1) mf(z+c)/f(z)(∞, r) = O(1); (2) mf(z+kc)/f(z)(∞, r) = O(1); (3) m∆cf/f (∞, r) = O(1); (4) m(∆cf)q/f (∞, r) = O(1); (5) Tf(z+c)(r) = Tf(z)(r) +O(1); (6) Tf(z+qc)(r) = Tf(z)(r) +O(1); (7) T∆cf/f (r) ≤ 2Tf (r) +O(1). Proof. Set Ac = f(z + c) f(z) . Then: (1) If |c| < r. Notice that the set of r ∈ R+ such that there exist z ∈ Cp with |z| = r is dense in R+. Therefore, without loss of generality one may assume that there exist z ∈ Cp such that |z| = r. Then |c + z| = |z| = r. So |f(z)|r = |f(z + c)|r and |Ac| = 1. If r ≤ |c|, then |c + z| ≤ ≤ max { |c|, |z| } ≤ |c|. Thus |Ac| = O(1). Therefore mAc(∞, r) = max { 0, log |Ac|r } = O(1). (2) Similarly as the arguments of (1), we obtain mf(z+kc)/f(z)(∞, r) = O(1). (3) By mf(z+c)/f(z)(∞, r) = O(1), m∆cf/f (∞, r) ≤ max { mf(z+c)/f(z)(∞, r), 0 } , we have m∆cf/f (∞, r) = O(1) . ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS . . . 153 (4) Bym∆cf/f (∞, r) = O(1), m(∆cf)q/f (∞, r) = qm∆cf/f (∞, r), we havem(∆cf)q/f (∞, r) = = O(1). (5) Let f = f1 f2 be a non-constant meromorphic function on Cp, where f1, f2 be holomorphic functions on Cp having no common zeros. Similarly as the arguments of (1), we have: If |c| < r, then |f1(z)|r = |f1(z + c)|r and |f2(z)|r = |f2(z + c)|r. If r ≤ |c|, then |f1(z)|r ≤ ≤ |f1(z)|c, |f1(z + c)|r ≤ |f1(z)|c|, and |f2(z)|r ≤ |f2(z)|c, |f2(z + c)|r ≤ |f2(z)|c. Moreover, Tf (r) = max1≤i≤2 log |fi|r. So Tf(z+c)(r) = Tf(z)(r) +O(1). (6) Similarly as the arguments of (5), we obtain Tf(z+qc)(r) = Tf(z)(r) +O(1). (7) We have T∆cf f (r) = m f(z+c)−f(z) f(z) (∞, r) +N f(z+c)−f(z) f(z) (∞, r) ≤ ≤ m f(z+c) f(z) (∞, r) +N f(z+c) f(z) (∞, r) +O(1) ≤ ≤ mf(z)(∞, r) +Nf(z)(∞, r) +mf(z+c)(∞, r) +Nf(z+c)(∞, r) +O(1) = = Tf(z+c)(r) + Tf(z)(r) +O(1) ≤ 2Tf (r) +O(1). Lemma 3.1 is proved. Lemma 3.2. Let f be a non-constant p-adic meromorphic function and ∆qf is not identically zero and k, q, m be a positive integer and P (z) is the above. Then: (1) T∆q cf (r) ≤ 2qTf (r) +O(1); (2) T∆q cf/f (r) ≤ 2(2q − 1)Tf (r) +O(1); (3) ( n+ 3 ∑q i=1 ki − ∑q i=1 ki2 i+1 ) Tf (r) ≤ TP (f)(∆1 cf)k1 ...(∆q cf)kq (r) +O(1); (4) ( n− ∑k i=1 qi ) Tf (r) ≤ TP (f)(f(z+c))q1 ...(f(z+kc))qk (r) +O(1). Proof. We will prove (1) by induction on j, 1 ≤ j ≤ q − 1. With j = 1, we have T∆cf (∞, r) ≤ ≤ Tf(z+c)(r) +Tf(z)(r) +O(1). By Tf(z+c)(r) = Tf(z)(r) +O(1), T∆cf (r) ≤ 2Tf (r) +O(1). Now assume that T ∆j cf (r) ≤ 2jTf (r) + O(1). Moreover we have ∆j+1 c f = ∆c(∆ j cf). From this and by induction, T ∆j+1 c f (r) = T ∆c(∆j cf) (r) ≤ T ∆j cf(z+c) (r) + T ∆j cf(z) (r) + O(1) ≤ 2.2jTf (r) + O(1) = = 2j+1Tf (r) +O(1). We will prove (2) by induction on j, 1 ≤ j ≤ q−1. With j = 1, by 3.1 (7) we have T∆cf/f (r) ≤ ≤ 2Tf (r) +O(1. Now assume that T ∆j cf/f (r) ≤ 2(2j − 1)Tf (r) +O(1). Moreover we have T ∆ j+1 c f f (r) = T ∆ j+1 c f ∆ j cf ∆ j cf f (r) ≤ T ∆ j+1 c f ∆ j cf + T ∆ j cf f +O(1) ≤ ≤ T ∆c(∆ j cf) ∆ j cf + T ∆ j cf f +O(1) ≤ 2T ∆j c + 2(2j − 1)Tf (r) +O(1). By (1), T∆q cf (r) ≤ 2qTf (r) +O(1). Thus 2T ∆j c + 2(2j − 1)Tf (r) ≤ 2(2j + 2j − 1)Tf (r) +O(1) ≤ ≤ 2(2j+1 − 1)Tf (r) +O(1). Therefore ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 154 VU HOAI AN, HA HUY KHOAI T ∆ j+1 c f f (r) ≤ 2(2j+1 − 1)Tf (r) +O(1). (3) Set G = P (f)(∆1 cf)k1 . . . (∆q cf)kq . We have fk1 . . . fkqG = fk1+...+kqP (f)(∆1 cf)k1 . . . (∆q cf)kq and 1 fk1+...+kqP (f) = 1 G ( ∆1 cf f )k1 . . . ( ∆q cf f )kq . From this and (2), we obtain( n+ q∑ i=1 ki ) Tf (r) = T 1 fk1+...+kqP (f) (r) +O(1) = T 1 G ( ∆1 cf f )k1 ... ( ∆ q cf f )kq (r) +O(1) ≤ ≤ T1/G(r) + q∑ i=1 T(∆i cf/f)ki (r) +O(1) ≤ T1/G(r) + q∑ i=1 kiT∆i cf/f (r) +O(1) ≤ ≤ TG(r) + q∑ i=1 ki2(2i − 1)Tf (r) +O(1). So ( n+ 3 q∑ i=1 ki − q∑ i=1 ki2 i+1 ) Tf (r) ≤ TP (f)(∆1 cf)k1 ...(∆q cf)kq (r) +O(1). (4) Set F = P (f)(f(z+ c))q1 . . . (f(z+ kc))qk . We have f q1 . . . f qkF = f q1+...+qkP (f)(f(z+ +c))q1 . . . (f(z+kc))qk and f q1+...+qkP (f) = F. ( f(z) f(z+c) )q1 . . . ( f(z) f(z+kc) )qk . From this and 3.1 (5), 3.1 (6), we obtain ( n+ k∑ i=1 qi ) Tf (r) = Tfq1+...+qkP (f)(r) +O(1) = = T F. ( f(z) f(z+c) )q1 ... ( f(z) f(z+kc) ) qk (r) +O(1) ≤ TF (r) + k∑ i=1 T( f(z) f(z+ic) )qi (r) +O(1) ≤ ≤ TF (r) + k∑ i=1 qiT f(z) f(z+ic) (r) +O(1) ≤ TF (r) + k∑ i=1 qi(Tf (r) + Tf(z+ic)(r)) +O(1). Therefore ( n+ k∑ i=1 qi ) Tf (r) ≤ TF (r) + 2 k∑ i=1 qiTf (r) +O(1). So ( n− k∑ i=1 qi ) Tf (r) ≤ TP (f)(f(z+c))q1 ...(f(z+kc))qk (r) +O(1). Lemma 3.2 is proved. ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS . . . 155 Lemma 3.3. Let f and g be non-constant p-adic meromorphic functions. If Ef (1) = Eg(1), then one of the following three cases holds: (1) Tf (r) ≤ N1,f (∞, r) + N≥2 1,f (∞, r) + N1,f (0, r) + N≥2 1,f (0, r) + N1,g(∞, r) + N≥2 1,g (∞, r) + +N1,g(0, r) +N≥2 1,g (0, r)− log r +O(1), the same inequality holding for Hg(r); (2) f ≡ g; (3) fg ≡ 1. Proof. Set F = 1 f − 1 , G = 1 g − 1 , L = f ′′ f ′ − 2 f ′ f − 1 − g” g′ + 2 g′ g − 1 . (3.1) Then L = F ′′ F ′ − G′′ G′ . (3.2) Next we consider the following two cases: Case 1: L 6≡ 0. Since Ef (1) = Eg(1), if f(a) = 1, g(a) = 1 and v1 f (a) = v1 g(a), then L(a) = 0. We now consider the poles of L. It is clear that all poles of L are of order 1. We can easily see from (3.1) that any simple pole of f and g is not a pole of L and the poles of L only occur at zeros of f ′ and g′ and the multiple poles of f and g. From (3.1) we have mL(∞, r) = O(1), and N≤1 f (1, r) = N≤1 g (1, r) ≤ NL(0, r) ≤ TL(r) +O(1) ≤ NL(∞, r) +O(1). (3.3) On the other hand, by Lemma 2.2, Tf (r) ≤ N1,f (∞, r) +N1,f (0, r) +N1,f (1, r)−N0,f ′(r)− log r +O(1), where N0,f ′(r) denotes the counting function of those zeros of f ′ but not that of f(f − 1). Also, N1,0,f ′(r) is defined similarly, where in counting, each zero of f ′ is counted with multiplicity 1. From (3.1), (3.2) and (3.3) we deduce that N≤1 f (1, r) ≤ N≥2 1,f (∞, r)+ +N≥2 1,g (∞, r) +N1,0,f ′(r) +N1,0,g′(r) +N≥2 1,f (0, r) +N≥2 1,g (0, r) +O(1). (3.4) Since Ef (1) = Eg(1), ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 156 VU HOAI AN, HA HUY KHOAI N1,f (1, r) = N≤1 f (1, r) +N≥2 1,g (1, r). Then Tf (r) ≤ N1,f (∞, r) +N1,f (0, r) +N≤1 f (1, r) +N≥2 1,g (1, r)−N0,f ′(r)− log r +O(1). (3.5) Now we consider N≥2 1,g (1, r). By Lemma 2.1, Ng′(0, r)−Ng(0, r) +N1,g(0, r) = N g′ g (0, r) ≤ T g′ g (r) +O(1) = = N g′ g (∞, r) +m g′ g (∞, r) +O(1) = = N1,g(∞, r) +N1,g(0, r) +O(1). Therefore Ng′(0, r) ≤ N1,g(∞, r) +Ng(0, r) +O(1). Moreover N0,g′(r) +N≥2 1,g (1, r) +N≥2 g (0, r)−N≥2 1,g (0, r) ≤ Ng′(0, r). The above two inequalities yield N0,g′(r) +N≥2 1,g (1, r) ≤ N1,g(∞, r) +N1,g(0, r) +O(1). Combining this inequality and (3.4) and (3.5), we obtain (1). Case 2: L ≡ 0. Then f ′′ f ′ − 2 f ′ f − 1 ≡ g′′ g′ − 2 g′ g − 1 . (3.6) By (3.6) we have F ′′ F ′ ≡ G′′ G′ . Thus f ≡ ag + b cg + d , where a, b, c, d ∈ Cp satisfying ad− bc 6= 0. Then Tf (r) = Tg(r) +O(1). Next we consider the following subcases: ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS . . . 157 Subcase 1: ac 6= 0. Then f − a c ≡ b− ad c cg + d . By Lemma 2.3 Tf (r) ≤ N1,f (∞, r) +N1,f−a c (0, r) +N1,f (0, r) +O(1) = = N1,f (∞, r) +N1,g(∞, r) +N1,f (0, r) +O(1). We get (1). Subcase 2: a 6= 0, c = 0. Then f ≡ ag + b d . If b 6= 0, by Lemma 2.2, Tf (r) ≤ N1,f (∞, r) +N1,f− b d (0, r) +N1,f (0, r) +O(1) = = N1,f (∞, r) +N1,g(0, r) +N1,f (0, r) +O(1). We get (1). If b = 0, then f ≡ ag d . If a d = 1, then f ≡ g. We obtain (2). If a d 6= 1, then by Ef (1) = Eg(1) and Lemma 2.3 f 6= 1, f 6= a d , Tf (r) ≤ N1,f (∞, r) +N1,f ( a d , r) +N1,f (1, r) +O(1) = N1,f (∞, r) +O(1). We get (1). Subcase 3: a = 0, c 6= 0. Then f ≡ b cg + d . If d 6= 0 , by Lemma 2.2, Tf (r) ≤ N1,f (∞, r) +N1,f− b d (0, r) +N1,f (0, r) +O(1) = = N1,f (∞, r) +N1,g(0, r) +N1,f (0, r) +O(1). We obtain (1). If d = 0, then f ≡ b cg . If b c = 1, then fg ≡ 1. We obtain (3). If b c 6= 1, then by Ef (1) = Eg(1) and Lemma 2.2, f 6= 1, f 6= b c , Tf (r) ≤ N1,f (∞, r) +N1,f ( b c , r ) +N1,f (1, r) +O(1) = N1,f (∞, r) +O(1). We get (1). Lemma 3.3 is proved. ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 158 VU HOAI AN, HA HUY KHOAI Now we use the above lemmas to prove the main result of the paper. Proof of Theorem 1.4. From P (z) = a0(z − a1)m1(z − a2)m2 . . . (z − as)ms , a0 6= 0, P (f) = = a0(f −a1)m1(f −a2)m2 . . . (f −as)ms . Set G = P (f)(∆1 cf)k1 . . . (∆q cf)kq . We see that any pole of G can occur only at poles of f, f(z + c), f(z + 2c), . . . , f(z + qc), and any zero of G can occur only at zeros of f − a1, f − a2, . . . , f − as, ∆1 cf, . . . ,∆ q cf . From this and by Lemmas 2.1, 2.2, 3.1 (5), 3.1 (6), 3.2 (1), 3.2 (3) we have( n+ 3 q∑ i=1 ki − q∑ i=1 ki2 i+1 ) Tf (r) ≤ TG(r) +O(1) ≤ ≤ N1,G(∞, r) +N1,G(0, r) +N1,G(a, r)− log r +O(1) ≤ ≤ N1,f (∞, r) + q∑ i=1 N1,f(z+ic)(∞, r) + s∑ i=1 N1,f (ai, r)+ + q∑ i=1 N1,∆i cf +N1,G(a, r)− log r +O(1) ≤ ≤ Tf (r) + qTf (r) + sTf (r) + q∑ i=1 2iTf (r) +N1,G(a, r)− log r +O(1) = = ( q∑ i=1 2i + q + s+ 1 ) Tf (r) +N1,G(a, r)− log r +O(1). Therefore( n+ 3 q∑ i=1 ki − q∑ i=1 (2ki + 1)2i − q − s− 1 ) Tf (r) + log r ≤ N1,G(a, r) +O(1). Since and n ≥ q∑ i=1 (2ki + 1)2i + q + s+ 1− 3 q∑ i=1 ki, we obtain P (f)(∆1 cf)k1 . . . (∆q cf)kq − a has zeros. Theorem 1.4 is proved. Proof of Theorem 1.5. From P (z) = a0(z − a1)m1(z − a2)m2 . . . (z − as)ms , a0 6= 0, P (f) = = a0(f − a1)m1(f − a2)m2 . . . (f − as)ms . Set F = P (f)(f(z+ c))q1 . . . (f(z+ kc))qk . We see that any pole of F can occur only at poles of f, f(z + c), f(z + 2c), . . . , f(z + kc), and any zero of G can occur only at zeros of f − a1, f − a2, . . . , f − as, f(z + c), f(z + 2c), . . . , f(z + kc). From this and by Lemmas 2.1, 2.2, 3.1 (5), 3.1 (6), 3.2 (4) we have ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS . . . 159( n− k∑ i=1 qi ) Tf (r) ≤ TF (r) +O(1) ≤ ≤ N1,F (∞, r) +N1,F (0, r) +N1,F (a, r)− log r +O(1) ≤ ≤ N1,f (∞, r) + k∑ i=1 N1,f(z+ic)(∞, r) + s∑ i=1 N1,f (ai, r) + k∑ i=1 N1,f(z+ic)(0, r)+ +N1,F (a, r)− log r +O(1) ≤ Tf (r) + kTf (r) + sTf (r) + kTf (r) +N1,F (a, r)− − log r +O(1) = (2k + s+ 1)Tf (r) +N1,F (a, r)− log r +O(1). Therefore ( n− k∑ i=1 qi − 2k − s− 1 ) Tf (r) + log r ≤ N1,F (a, r) +O(1). Since and n ≥ k∑ i=1 qi + 2k + s+ 1 we obtain P (f)(f(z + c))q1 . . . (f(z + kc))qk − a has zeros. Theorem 1.5 is proved. Proof of Theorem 1.6. (1) Set A = fnf(z + c) . . . f(z + kc), B = gng(z + c) . . . g(z + kc). It suffices to consider the following cases: Case 1: TA(r) +O(1) ≤ N1,A(∞, r) +N≥2 1,A(∞, r) +N1,A(0, r) +N≥2 1,A(0, r)+ +N1,B(∞, r) +N≥2 1,B(∞, r) +N1,B(0, r) +N≥2 1,B(0, r)− log r +O(1). By Lemmas 3.2 (4), 3.3, (n− k)Tf (r) ≤ TA(r) +O(1) ≤ N1,A(∞, r) +N≥2 1,A(∞, r) +N1,A(0, r) +N≥2 1,A(0, r)+ +N1,B(∞, r) +N≥2 1,B(∞, r) +N1,B(0, r) +N≥2 1,B(0, r)− log r +O(1), (n− k)Tg(r) ≤ TB(r) +O(1) ≤ N1,A(∞, r) +N≥2 1,A(∞, r) +N1,A(0, r)+ +N≥2 1,A(0, r) +N1,B(∞, r) +N≥2 1,B(∞, r) +N1,B(0, r) +N≥2 1,B(0, r)− log r +O(1). (3.7) We see that any pole of A can occur only at poles of ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 160 VU HOAI AN, HA HUY KHOAI f, f(z + c), f(z + 2c), . . . , f(z + kc). From this and by Lemmas 2.1, 3.1 (5), 3.1 (6) we have N1,A(∞, r) +N≥2 1,A(∞, r) ≤ ≤ 2N1,f (∞, r) + k∑ i=1 (N1,f(z+ic)(∞, r) +N≥2 1,f(z+ic)(∞, r)) +O(1) ≤ ≤ 2Nf (∞, r) + k∑ i=1 Nf(z+ic)(∞, r) +O(1) ≤ ≤ 2Tf (r) + k∑ i=1 Tf(z+ic)(r) +O(1) ≤ (k + 2)Tf (r) +O(1). So N1,A(∞, r) +N≥2 1,A(∞, r) ≤ (k + 2)Tf (r) +O(1). (3.8) Similarly, and note that any zero of A can occur only at zeros of f, f(z + c), f(z + 2c), . . . , f(z + kc), we obtain N1,A(0, r) +N≥2 1,A(0, r) ≤ (k + 2)Tf (r) +O(1). (3.9) Similarly we obtain N1,B(∞, r) +N≥2 1,B(∞, r) ≤ (k + 2)Tg(r) +O(1), N1,B(0, r) +N≥2 1,B(0, r) ≤ (k + 2)Tg(r) +O(1). (3.10) From (3.7) – (3.10) we have (n− k)Tf (r) ≤ (2k + 4)(Tf (r) + Tg(r))− log r +O(1). Similarly (n− k)Tg(r) ≤ (2k + 4)(Tf (r) + Tg(r))− log r +O(1). So (n− k)(Tf (r) + Tg(r)) ≤ (4k + 8)(Tf (r) + Tg(r))− 2 log r +O(1), (n− 5k − 8)(Tf (r) + Tg(r)) + 2 log r ≤ O(1). By n ≥ 5k + 8 we obtain a contradiction. ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS . . . 161 Case 2: A = fnf(z + c) . . . f(z + kc) ≡ B = gng(z + c) . . . g(z + kc). Set h = f g . Assume that h is not a constant. Then we get hn = 1 h(z + c) . . . h(z + kc) . Thus, by Lemma 3.1 (5), we get nTh(r) = Thn = T 1 h(z+c)...h(z+kc) ≤ k∑ i=1 Th(z+ic)(r) +O(1) ≤ kTh(r), which is a contradiction with n ≥ 5k+ 8. Hence h must be a constant, which implies that hn+k = 1, thus f = hg with hn+k = 1. Case 3: fnf(z+ c) . . . f(z+kc).gng(z+ c) . . . g(z+kc) ≡ 1. From this we have (fg)n(f(z+ + c)g(z + c)) . . . (f(z + kc)g(z + kc)) = 1. Set l = fg. Assume that l is not a constant. Then we get ln = 1 l(z + c) . . . l(z + kc) . Similar as above, l must be a constant. Thus fg = l with ln+k = 1. (2) Set C = fn(f(z + c))q1 . . . (f(z + kc))qk , D = gn(g(z + c))q1 . . . (g(z + kc))qk . It suffices to consider the following cases: Case 1: TC(r) +O(1) ≤ N1,C(∞, r) +N≥2 1,C(∞, r) +N1,C(0, r) +N≥2 1,C(0, r)+ +N1,D(∞, r) +N≥2 1,D(∞, r) +N1,D(0, r) +N≥2 1,D(0, r)− log r +O(1). By Lemmas 3.2 (4), ( n− k∑ i=1 qi ) Tf (r) ≤ TC(r) +O(1), ( n− k∑ i=1 qi ) Tg(r) ≤ TD(r) +O(1). (3.11) By qi ≥ 2, i = 1, . . . , k, N1,(f(z+ic))qi (∞, r) +N≥2 1,(f(z+ic))qi (∞, r)) ≤ 2Nf(z+ic)(∞, r). From this and similar as (3.8) we obtain N1,C(∞, r) +N≥2 1,C(∞, r) ≤ (2k + 2)Tf (r) +O(1), N1,C(0, r) +N≥2 1,C(0, r) ≤ (2k + 2)Tf (r) +O(1), N1,D(∞, r) +N≥2 1,D(∞, r) ≤ (2k + 2)Tg(r) +O(1), N1,D(0, r) +N≥2 1,D(0, r) ≤ (2k + 2)Tg(r) +O(1). (3.12) ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 162 VU HOAI AN, HA HUY KHOAI Since (3.11), (3.12), and similar as in (1) we obtain TC(r) ≤ (4k + 4)(Tf (r) + Tg(r))− log r +O(1), TD(r) ≤ (4k + 4)(Tf (r) + Tg(r))− log r +O(1),( n− k∑ i=1 qi − 8k − 8 ) (Tf (r) + Tg(r)) + 2 log r ≤ O(1). By n ≥ k∑ i=1 qi + 8k + 8 we obtain a contradiction. Case 2: C = fn(f(z + c))q1 . . . (f(z + kc))qk ≡ D = gn(g(z + c))q1 . . . (g(z + kc))qk . Similar as Case 2 of (1) we get f = hg with hn+q1+...+qk = 1. Case 3: fn(f(z + c))q1 . . . (f(z + kc))qk .gn(g(z + c))q1 . . . (g(z + kc))qk ≡ 1. Similar as Case 3 of (1) we get fg = l with ln+q1+...+qk = 1. (3) Set E = fnf(z + e1c) . . . f(z + emc)(f(z + t1c)) q1 . . . (f(z + tkc)) qk , H = gng(z + + e1c) . . . g(z + emc)(g(z + t1c)) q1 . . . (g(z + tkc)) qk . It suffices to consider the following cases: Case 1: TE(r) +O(1) ≤ N1,E(∞, r) +N≥2 1,E(∞, r) +N1,E(0, r) +N≥2 1,E(0, r)+ +N1,H(∞, r) +N≥2 1,H(∞, r) +N1,H(0, r) +N≥2 1,H(0, r)− log r +O(1). By Lemma 3.2 (4) ( n−m− k∑ i=1 qi ) Tf (r) ≤ TE(r) +O(1), ( n−m− k∑ i=1 qi ) Tg(r) ≤ TD(r) +O(1). (3.13) Similar as Case 1 of (1) and (2) we have ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 VALUE-SHARING PROBLEM FOR p-ADIC MEROMORPHIC FUNCTIONS . . . 163 N1,E(∞, r) +N≥2 1,E(∞, r) ≤ ≤ 2Nf (∞, r) + m∑ i=1 Nf(z+eic)(∞, r) + 2 k∑ i=1 Nf(z+tic)(∞, r)O(1) ≤ ≤ (m+ 2k + 2)Tf (r) +O(1), N1,E(0, r) +N≥2 1,E(0, r) ≤ (m+ 2k + 2)Tf (r) +O(1), N1,H(∞, r) +N≥2 1,H(∞, r) ≤ (m+ 2k + 2)Tg(r) +O(1), N1,H(0, r) +N≥2 1,H(0, r) ≤ (m+ 2k + 2)Tg(r) +O(1). (3.14) Since (3.13), (3.14), and similar as in (1), (2), we obtain( n−m− k∑ i=1 qi ) Tf (r) ≤ 2(m+ 2k + 2)(Tf (r) + Tg(r))− log r +O(1), ( n−m− k∑ i=1 qi ) Tg(r) ≤ 2(m+ 2k + 2)(Tf (r) + Tg(r))− log r +O(1), ( n−m− k∑ i=1 qi ) (Tf (r) + Tg(r)) ≤ 4(m+ 2k + 2)(Tf (r) + Tg(r))− 2 log r +O(1), ( n− 5m− k∑ i=1 qi − 8k − 8 ) (Tf (r) + Tg(r)) + 2 log r ≤ +O(1). Which is a contradiction with n ≥ 5m+ k∑ i=1 qi + 8k + 8. Case 2: Prove is similarly as in Case 2 of (1) and (2) we get f = hg with hn+m+q1+...+qk = 1. Case 3: Prove is similarly as in Case 2 of (1) and (2) we get fg = l with ln+m+q1+...+qk = 1. Theorem 1.6 is proved. 1. Adam W. W., Straus E. G. Non-Archimedean analytic functions taking the same values at the same points // Ill. J. Math. – 1971. – 15. – P. 418 – 424. 2. Alotaibi A. On the Zeros of af(f (k)) n for n ≥ 2 // Comput. Methods and Funct. Theory. – 2004. – 4, № 1. – P. 227 – 235. 3. Boutabaa A., Escassut A. Uniqueness problems and applications of the ultrametric Nevanlinna theory // Contemp. Math. – 2003. – 319. – P. 53 – 74. 4. Chen H. H., Fang M. L. On the value distribution of fnf ′ // Sci. China Ser. A. – 1995. – 38. – P. 789 – 798. 5. Clunie J. On a result of Hayman // J. London Math. Soc. – 1967. – 42. – P. 389 – 392. ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2 164 VU HOAI AN, HA HUY KHOAI 6. Escassut A., Ojeda J., Yang C. C. Functional equations in a p-adic context // J. Math. Anal. and Appl. – 2009. – 351, № 1. – P. 350 – 359. 7. Gross F., Yang C. C. On pre-images and range sets of meromorphic functions // Proc. Jap. 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Hayman’s conjecture in a p-adic field // Taiwan. J. Math. – 2008. – 12, № 9. – P. 2295 – 2313. 23. Yang C.-C., Hua X. H. Uniqueness and value-sharing of meromorphic functions // Ann. Acad. Sci. Fenn. Math. – 1997. – 22. – P. 395 – 406. Received 21.08.11 ISSN 1027-3190. Укр. мат. журн., 2012, т. 64, № 2
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spelling umjimathkievua-article-25622020-03-18T19:29:46Z Value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials Задача про спiльнi значення для p-адичних мероморфних функцiй та їх рiзницевих операторiв i рiзницевих полiномiв An, Vu Hoai Khoai, Ha Huy Ан, Ву Гоай Хоаї, Га Гуй We discuss the value-sharing problem, versions of the Hayman conjecture, and the uniqueness problem for p-adic meromorphic functions and their difference operators and difference polynomials. Дослiджено питання про спiльнi значення i єдинiсть та аналоги гiпотези Хеймена для p-адичних мероморфних функцiй та їх рiзницевих операторiв i рiзницевих полiномiв. Institute of Mathematics, NAS of Ukraine 2012-02-25 Article Article application/pdf https://umj.imath.kiev.ua/index.php/umj/article/view/2562 Ukrains’kyi Matematychnyi Zhurnal; Vol. 64 No. 2 (2012); 147-164 Український математичний журнал; Том 64 № 2 (2012); 147-164 1027-3190 en https://umj.imath.kiev.ua/index.php/umj/article/view/2562/1883 https://umj.imath.kiev.ua/index.php/umj/article/view/2562/1884 Copyright (c) 2012 An Vu Hoai; Khoai Ha Huy
spellingShingle An, Vu Hoai
Khoai, Ha Huy
Ан, Ву Гоай
Хоаї, Га Гуй
Value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials
title Value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials
title_alt Задача про спiльнi значення для p-адичних мероморфних функцiй та їх рiзницевих операторiв i рiзницевих полiномiв
title_full Value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials
title_fullStr Value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials
title_full_unstemmed Value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials
title_short Value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials
title_sort value-sharing problem for p-adic meromorphic functions and their difference operators and difference polynomials
url https://umj.imath.kiev.ua/index.php/umj/article/view/2562
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