
doi: 10.1515/jaa.2006.259
In this paper, the authors prove (Theorem~1.1): Let \(f\), \(g\) be distinct nonconstant meromorphic functions and \(n\geq 10\) be an integer. Put \(h=f/g\). If \(E(\infty,f)=E(\infty,g)\) and \(E_{(3)}\bigl(1,f^n(f-1)f'\bigr)=E_{(3)}\bigl(1,g^n(g-1)g'\bigr)\), then \(f=R_n(1/h)\) and \(g=R_n(h)\) for the rational function \(R_n(w)=P_{n+1}(w)/P_{n+2}(w)\) with \(P_k(w)=(w^{k}-1)/k\). As its corollary, if one restricts both \(f\) and \(g\) to entire functions, the uniqueness \(f\equiv g\) is obtained from the \textit{set} equation \(E_{(3)}\bigl(1,f^n(f-1)f'\bigr)=E_{(3)}\bigl(1,g^n(g-1)g'\bigr)\) when \(n\geq 7\). Here \(E_{(k)}(a,\varphi)\) (\(k\in\mathbb{N}\cup\{\infty\}\), \(a\in\mathbb{C}\cup\{\infty\}\)) is the sequence of the \(a\)-points of a nonconstant meromorphic function~\(\varphi\) in~\(\mathbb{C}\) with multiplicity \(\leq k\), each points being repeated as many times as its multiplicity. Especially, \(E(a,\varphi)=E_{(\infty)}(a,\varphi)\). These results give extensions of some earlier results by \textit{M. L. Fang} and \textit{W. Hong} [Indian J. Pure Appl. Math. 32, No. 9, 1343--1348 (2001; Zbl 1005.30023)], \textit{H. L. Qiu} and \textit{Fang} [Bull. Korean Math. Soc. 41, No.1, 109-116 (2004; Zbl 1134.30325)], \textit{W. C. Lin} and \textit{H. X. Yi} in [Indian J. Pure Appl. Math. 35, No. 2, 121--132 (2004; Zbl 1056.30031)], and others. Related results are given by \textit{I. Lahiri} and \textit{N. Mandal} in [Int. J. Math. Math. Sci. 2005, No. 12, 1933--1942 (2005; Zbl 1084.30029)], too. The corollary itself seems to be proved also by \textit{I. Lahiri} and \textit{P. Sahoo} in [Georgian Math. J. 12, No. 1, 131--138 (2005; Zbl 1073.30022)]. One needs an additional assumption in order to exclude the case when \(f=R_n(1/h)\) and \(g=R_n(h)\) and then to conclude the uniqueness of transcendental meromorphic~\(f\) and~\(g\) under consideration. Some of the papers above limit the frequency of their poles with \(n\), such as \(\Theta(\infty,\varphi)>2/(n+1)\), since the \textit{deficiency} attains \(2/(n+1)\) for \(\varphi=R_n(h)\) with a suitable~\(h\). A proof of such results requires a \textit{variant} of the so-called Nevanlinna-Cartan Second Main Theorem for \textit{distinct} meromorphic functions~\(F\) and~\(G\) sharing \(1\)-points with \(FG\not\equiv 1\), as Lemma~2.2 in the paper under review. Then they apply their estimate to \(F=\Pi(f)\) and \(G=\Pi(g)\) with \(\Pi(\varphi):=\varphi^n(\varphi-1)\varphi'\). (For example, Lahiri and Mandal show that \(\Pi(f)\Pi(g)\not\equiv 1\) for any nonconstant meromorphic functions~\(f\) and~\(g\) when \(n\geq 5\).) This differential polynomial \(\Pi\bigl(\varphi(z)\bigr)\) is the derivative of \(P_{n+2}\bigl(\varphi(z)\bigr)-P_{n+1}\bigl(\varphi(z)\bigr)\) and the exceptional pair in Theorem~1.1, \(R_n(1/h)\) and \(R_n(h)\), is obtained when \(F\equiv G\) holds with \(h\not\equiv 1\). In fact, they prove that \(P_{n+2}(f)-P_{n+1}(f)\equiv P_{n+2}(g)-P_{n+1}(g)\) still holds, which implies directly the desired expression with \(h=f/g\). One could think it natural that the methods and discussions are overlapping those in many papers relating to the famous problems on \textit{uniqueness polynomials} posed by \textit{F. Gross} in [Complex Anal., Proc. Conf., Lexington 1976, Lect. Notes Math. 599, 51--67 (1977; Zbl 0357.30007)] and on differential polynomials by [\textit{W. K. Hayman}, Research problems in function theory. London: University of London. The Athlone Press (1967; Zbl 0158.06301)], respectively.
330, meromorphic function, uniqueness, sharing value, 510, Value distribution of meromorphic functions of one complex variable, Nevanlinna theory
330, meromorphic function, uniqueness, sharing value, 510, Value distribution of meromorphic functions of one complex variable, Nevanlinna theory
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