1 Introduction and main results

A meromorphic function means meromorphic in the whole complex plane. We assume that the reader is familiar with standard symbols and fundamental results of Nevanlinna Theory [1]. As usual, the abbreviation CM stands for "counting multiplicities", while IM means "ignoring multiplicities", and we denote the order of meromorphic function f by σ (f). For a non-constant meromorphic function f and a set S of complex numbers, we define the set E(S, f) = ∪aS{z|f(z) - a = 0}, where a zero of f - a with multiplicity m counts m times in E(S, f).

We define difference operator as Δ c f = f (z + c) - f (z), where c is a non-zero constant. In particular, we denote by S (f) the family of all meromorphic functions a (z) that satisfy T(r, a) = S(r, f) = o(T(r, f)), where r → ∞ outside a possible exceptional set of finite logarithmic measure. For convenience, we set Ŝ(f) := S(f) ∪ {∞}.

The difference Nevanlinna theory and its applications to the uniqueness theory have become a subject of great interest [24], recently. With these fundamental results, Heittokangas et al. considered a meromorphic function f (z) sharing values with its shift f(z + c), we recall a key result from [5].

Theorem A [[5], Theorem 2]. Let f be a non-constant meromorphic function of finite order, let c ∈ ℂ, and let a, b, cŜ(f) be three distinct periodic functions with period c. If f (z) and f (z + c) share a, b CM and c IM, then f (z) = f (z + c) for all z ∈ ℂ.

Recently, Yang and Liu and one of the present authors [6] considered the case F = fn, where f is a meromorphic function, assuming value sharing with F and F (z + c):

Theorem B [[6], Theorem 1.4]. Let f be a non-constant meromorphic function of finite order, n ≥ 7 be an integer, let c ∈ ℂ, and let F = fn. If F (z) and F (z + c) share aS(f)\{0} andCM, then f (z) = ωf (z + c), for a constant ω that satisfies ωn= 1.

Next, we consider the problem that related to the Theorem B, and have the following result, where a is a periodic function with period c. However, our proof is different to the one in [6].

Theorem 1.1. Let f be a non-constant meromorphic function of finite order, let c ∈ ℂ, and let aS(f) \ {0} be a periodic function with period c. If f (z)nand f(z + c)nshare a andCM, and n ≥ 4 is an integer, then f (z) = ωf (z + c), for a constant ω that satisfies ωn= 1.

Remarks.

  1. (1)

    Theorem 1.1 is not true, if a = 0. This can be seen by considering f ( z ) = e z 2 . Then f (z)nand f (z + c)nshare 0 and ∞ CM, however, f (z) ≠ ωf (z + c), where n is a positive integer.

  2. (2)

    Theorem 1.1 does not remain valid when n = 1. For example, f (z) = ez+ 1 and f (z + c) = ez+c+ 1, where c ≠ 2πi. Clearly, f(z) and f (z + c) share 1 and ∞ CM, however, f (z) ≠ ωf (z + c) for ωn= 1. Unfortunately, we have not succeeded in reducing the condition n ≥ 4 to n ≥ 2 in Theorem 1.1, and we also cannot give a counterexample when n = 2, 3 at present.

  3. (3)

    We give an example to show that the restriction of finite order in Theorem 1.1 cannot be deleted. This can be seen by taking f ( z ) = e e z ,n e c =-1. Then f (z)nand f (z + c)nshare 1 and ∞ CM, however, f(z) ≠ ωf (z + c), where n is a positive integer.

In 1976, Gross asked the following question [[7], Question 6]:

Question. Can one find (even one set) finite sets S j (j = 1, 2) such that any two entire functions f and g satisfying E(S j , f) = E(S j , g) (j = 1, 2) must be identical?

Since then, many results have been obtained for this and related topics (see [811]). We recall the following result given by Yi [9].

Theorem C [[9], Theorem 1]. Let S1 = {ω | ωn+ n- 1+ b = 0}, where n ≥ 7 is an integer, a and b are two non-zero constants such that the algebraic equation ωn+ n- 1+ b = 0 has no multiple roots. If f and g are two entire functions satisfying E(S1, f) = E(S1, g), then f = g.

Afterwards, Fang and Lahiri [12] got the result for meromorphic functions.

Theorem D [[12], Theorem 1]. Let S1 be defined as Theorem C and S2 = {∞}. Assume that f and g are two meromorphic functions satisfying E(S j , f) = E (S j , g) for j = 1,2. If f has no simple poles and n ≥ 7, then f = g.

Next, we give a difference analog of Theorem D that replacing g with f (z + c), and obtain the following result.

Theorem 1.2. Let S1 be defined as Theorem C and S2 = {∞}. Assume that f is a meromorphic function of finite order satisfying E(S j , f) = E(S j , f (z + c)) for j = 1,2. If n ≥ 6 and N ¯ ( r , f ) < n - 3 n - 1 T ( r , f ) +S ( r , f ) , then f (z) = f (z + c) for all z ∈ ℂ.

We investigate the value distribution of difference polynomials of meromorphic (entire) functions. Let f be a transcendental meromorphic function, and let n be a positive integer. Concerning to the value distribution of fnf", Hayman [[13], Corollary to Theorem 9] proved that fnf' takes every non-zero complex value infinitely often if n ≥ 3. Mues [[14], Satz 3] proved that f2f' - 1 has infinitely many zeros. Later on, Bergweiler and Eremenko [[15], Theorem 2] showed that ff' - 1 has infinitely many zeros also. As an analog result in difference, Laine and Yang [16] investigated the value distribution of difference products of entire functions, and obtained the following:

Theorem E [[16], Theorem 2]. Let f be a transcendental entire function of finite order, and let c be a non-zero complex constant. Then for n ≥ 2, f (z)nf (z + c) assumes every non-zero value a ∈ ℂ infinitely often.

In a recent article, one of the present authors considered the value distribution of f (z)n(f(z) - 1) f (z + c), the result may be stated as follows:

Theorem F [[17], Theorem 1]. Let f be a transcendental meromorphic function of finite order σ(f), let a ≠ 0 be a small function with respect to f, and let c be a non-zero complex constant. If the exponent of convergence of the poles of f satisfies λ ( 1 f ) <σ ( f ) and n ≥ 2, then f (z)n(f - 1) f (z + c) - a has infinitely many zeros.

In this article, we replace f (z + c) with Δ c f, and consider the value distribution of f (z)n(f(z) - 1)Δ c f. We get the following results:

Theorem 1.3. Let f be a transcendental meromorphic function of finite order σ(f) and Δ c f ≠ 0, let a ≠ 0 be a small function with respect to f, and let c be a non-zero complex constant. If the exponent of convergence of the poles off satisfies λ ( 1 f ) <σ ( f ) and n ≥ 3, then f (z)n(f - 1)Δ c f - a has infinitely many zeros.

Corollary 1.4. Let f be a transcendental entire function of finite order and Δ c f ≠ 0, let a ≠ 0 be a small function with respect to f, and let c be a non-zero complex constant. Then for n ≥ 3, f (z)n(f - 1)Δ c f - a has infinitely many zeros.

In particular, if a is a non-zero polynomial in Corollary 1.4, then Corollary 1.4 can be improved.

Theorem 1.5. Let f be a transcendental entire function of finite order and Δ c f ≠ 0, let a be a non-zero polynomial, and let c be a non-zero complex constant. Then for n ≥ 2, f (z)n(f - 1)Δ c f - a has infinitely many zeros.

2 Preliminary lemmas

Lemma 2.1. [[4], Theorem 2.1] Let f be a meromorphic function of finite order, and let c ∈ ℂ and δ ∈ (0, 1) . Then

m r , f ( z + c ) f ( z ) + m r , f ( z ) f ( z + c ) = o T ( r , f ) r δ = S ( r , f ) .

Chiang and Feng have obtained similar estimates for the logarithmic difference [[3], Corollary 2.5], and this study is independent from [4].

Lemma 2.2. [[4], Lemma 2.3] Let f be a meromorphic function of finite order and c ∈ ℂ. Then for any small function aS (f) with period c,

m r , Δ c f f - a = S ( r , f ) .

Lemma 2.3. [[3], Theorem 2.1] Let f be a meromorphic function of finite order σ (f), and let c be a non-zero constant. Then, for each ε > 0, we have

T ( r , f ( z + c ) ) = T ( r , f ( z ) ) + O ( r σ ( f ) - 1 + ε ) + O ( log r ) .

Lemma 2.4. [[18], Theorem 2.4.2] Let f be a transcendental meromorphic solution of

f n A ( z , f ) = B ( z , f ) ,

where A(z, f), B(z, f) are differential polynomials in f and its derivatives with small meromorphic coefficients a λ , in the sense of m(r, a λ ) = S(r, f) for all λI. If the deg(B(z, f)) ≤ n, then m(r, A(z, f)) = S(r, f).

Lemma 2.5. Let f be a finite order entire function and Δ c f ≠ 0, and let c be a non-zero constant. Then

m ( r , f f Δ c f ) T ( r , f ) + S ( r , f ) .

Proof. Since f is an entire function with finite order, we deduce from Lemma 2.2 and the Lemma of logarithmic derivative that

3 T ( r , f ) = T ( r , f 3 ) = m ( r , f 3 ) + S ( r , f ) m ( r , f 3 f f Δ c f ) + m ( r , f f Δ c f ) + S ( r , f ) = m ( r , f 2 f Δ c f ) + m ( r , f f Δ c f ) + S ( r , f ) T ( r , f f ) + T ( r , Δ c f f ) + m ( r , f f Δ c f ) + S ( r , f ) 2 N ( r , 1 f ) + m ( r , f f Δ c f ) + S ( r , f ) 2 T ( r , f ) + m ( r , f f Δ c f ) + S ( r , f ) .

Hence, we get

m ( r , f f Δ c f ) T ( r , f ) + S ( r , f ) .
(1)

3 Proof of Theorem 1.1

Since f(z)nand f(z + c)nshare a and ∞ CM, we obtain that

f ( z + c ) n - a ( z + c ) f ( z ) n - a ( z ) = e Q ( z ) ,
(2)

where Q(z) is a polynomial. From Lemma 2.1, we know that T (r, eQ(z)) = m (r, eQ(z)) = S (r, f). Rewrite (2) as

f ( z + c ) n = e Q ( z ) ( f ( z ) n - a ( z ) + a ( z ) e - Q ( z ) ) .
(3)

Set

G ( z ) = f ( z ) n a ( z ) ( 1 - e - Q ( z ) ) .

If eQ(z)≢ 1, then we apply the Valiron-Mohon'ko theorem and the second main theorem to G (z), and get

n T ( r , f ) + S ( r , f ) = T ( r , G ) N ¯ r , 1 G + N ¯ ( r , G ) + N ¯ r , 1 G - 1 + S ( r , G ) N ¯ r , 1 f + N ¯ ( r , f ) + N ¯ r , 1 f ( z ) n - a ( z ) + a ( z ) e - Q ( z ) + S ( r , f ) N ¯ r , 1 f + N ¯ ( r , f ) + N ¯ r , 1 f ( z + c ) + S ( r , f ) 2 T ( r , f ) + T ( r , f ( z + c ) ) + S ( r , f ) .
(4)

Combining (4) with Lemma 2.3, we get

n T ( r , f ) 3 T ( r , f ) + O ( r σ ( f ) - 1 + ε ) + S ( r , f ) ,

which contradicts that n ≥ 4. Therefore, eQ(z)≡ 1, that is, f (z)n= f (z + c)n, so we have f (z) = ω f (z + c), for a constant ω with ωn= 1.

4 Proof of Theorem 1.2

From the assumption of Theorem 1.2, we get that

f ( z + c ) n + a f ( z + c ) n - 1 + b f ( z ) n + a f ( z ) n - 1 + b = e Q ( z ) ,
(5)

where Q(z) is a polynomial. Applying Lemma 2.1, we obtain that T (r, eQ(z)) = m (r, eQ(z)) = S (r, f). Rewrite (5) as

f ( z + c ) n + a f ( z + c ) n - 1 = e Q ( z ) f ( z ) n + a f ( z ) n - 1 + b - b e Q ( z ) .
(6)

If eQ(z)≢ 1, applying the second main theorem for three small functions, we get

n T ( r , f ) + S ( r , f ) = T ( r , f ( z ) n + a f ( z ) n - 1 ) N ¯ r , 1 f ( z ) n + a f ( z ) n - 1 + N ¯ ( r , f ( z ) n + a f ( z ) n - 1 ) + N ¯ r , 1 f ( z ) n + a f ( z ) n - 1 + b - b e Q ( z ) + S ( r , f ) N ¯ ( r , f ) + N ¯ r , 1 f ( z + c ) n - 1 ( f ( z + c ) + a ) + N ¯ r , 1 f ( z ) n - 1 ( f ( z ) + a ) + S ( r , f ) 3 T ( r , f ) + 2 T ( r , f ( z + c ) ) + S ( r , f ) .
(7)

Combining (4.3) with Lemma 2.3, we get

n T ( r , f ) 5 T ( r , f ) + O ( r σ ( f ) - 1 + ε ) + S ( r , f ) ,

which contradicts n ≥ 6. Hence, eQ(z)≡ 1, we conclude by (5) that

f ( z + c ) n + a f ( z + c ) n - 1 = f ( z ) n + a f ( z ) n - 1 .
(8)

Set G ( z ) = f ( z ) f ( z + c ) . If G (z) is non-constant, then we have from (8)

f ( z ) = - a G ( G n - 1 - 1 ) G n - 1 = - a G n - 1 + + G G n - 1 + + 1 .
(9)

Making use of the standard Valiron-Mohon'ko lemma, we get from (9) that

T ( r , f ) = ( n - 1 ) T ( r , G ) + S ( r , f ) .
(10)

Noting that n ≥ 6, we deduce that 1 is not a Picard value of Gn. Suppose that a j ∈ {ℂ \ 1} (j = 1, 2,..., n - 1) are the distinct roots of equation hn- 1 = 0. Applying the second main theorem to G, we conclude by (9) that

( n - 3 ) T ( r , G ) j = 1 n - 1 N ¯ r , 1 G - a j + S ( r , G ) = N ¯ ( r , f ) .
(11)

From (10) and (11), we get N ¯ ( r , f ) n - 3 n - 1 T ( r , f ) +S ( r , f ) , which contradicts the assumption.

So G(z) is a constant, and we get f (z) = tf (z + c), where t is a non-zero constant. From (8), we know t = 1, therefore, f = g.

5 Proof of Theorem 1.3

The main idea of this proof is from [[17], Theorem 1], while the details are somewhat different. For the convenience of the reader, we give a complete proof.

Set F(z) = fn(z) (f(z) - 1)Δ c f. Since f is a transcendental meromorphic function with finite order σ(f), we conclude by Lemma 2.3 that

T ( r , F ) T ( r , f n ( z ) ( f ( z ) - 1 ) ) + T ( r , Δ c f ) + S ( r , f ) ( n + 2 ) T ( r , f ) + T ( r , f ( z + c ) ) + S ( r , f ) ( n + 3 ) T ( r , f ) + O ( r σ ( f ) - 1 + ε ) + S ( r , f ) .

Thus, we get S(r, F) = o(T(r, f)) = S(r, f). Moreover, we get

T ( r , Δ c f ) m ( r , Δ c f ) + O r λ ( 1 f ) + ε + S ( r , f ) m r , Δ c f f + m ( r , f ) + O r λ ( 1 f ) + ε + S ( r , f ) T ( r , f ) + O r λ ( 1 f ) + ε + S ( r , f ) .
(12)

On the other hand, we deduce by Lemma 2.2 that

( n + 2 ) T ( r , f ) = T ( r , f n + 1 ( f - 1 ) ) + S ( r , f ) = m ( r , f n + 1 ( f - 1 ) ) + O r λ ( 1 f ) + ε + S ( r , f ) m r , f n + 1 ( f - 1 ) F + m ( r , F ) + O r λ ( 1 f ) + ε + S ( r , f ) T r , Δ c f f + m ( r , F ) + O r λ ( 1 f ) + ε + S ( r , f ) m r , Δ c f f + N r , 1 f + m ( r , F ) + O r λ ( 1 f ) + ε + S ( r , f ) T ( r , f ) + T ( r , F ) + O r λ ( 1 f ) + ε + S ( r , f ) .

Hence

( n + 1 ) T ( r , f ) T ( r , F ) + O r λ ( 1 f ) + ε + S ( r , f ) .
(13)

The second main theorem yields

T ( r , F ) N ¯ ( r , F ) + N ¯ r , 1 F + N ¯ ( r , 1 F - a ) + S ( r , F ) N ¯ ( r , 1 F - a ) + N ¯ r , 1 f + N ¯ r , 1 f - 1 + N ¯ ( r , 1 Δ c f ) + O r λ ( 1 f ) + ε + S ( r , f ) N ¯ r , 1 F - a + 2 T ( r , f ) + T ( r , Δ c f ) + O r λ ( 1 f ) + ε + S ( r , f ) .

From (12) and above inequality, we get that

T ( r , F ) N ¯ r , 1 F - a + 3 T ( r , f ) + O r λ ( 1 f ) + ε + S ( r , f ) .
(14)

Combining (13) and (14), we have

( n - 2 ) T ( r , f ) N ¯ r , 1 F - a + O r λ ( 1 f ) + ε + S ( r , f ) ,

which is a contradiction to the fact that f is of order σ (f) if F - a has finitely many zeros. The conclusion follows.

6 Proof of Theorem 1.5

Suppose that fn(f - 1)Δ c f - a admits finitely many zeros only. Then, there are two non-zero polynomials P(z), Q(z) such that

f n ( f - 1 ) Δ c f - a = P ( z ) e Q ( z ) .
(15)

Differentiating (15) and eliminating eQ(z), we obtain

( f n - f n - 1 ) F ( z , f ) = a P ( z ) - a P * ( z ) - P ( z ) f ( z ) n - 1 f ( z ) Δ c f ,
(16)

where

F ( z , f ) = ( n + 1 ) P ( z ) f ( z ) Δ c f + P ( z ) f ( z ) ( Δ c f ) - P * ( z ) f ( z ) Δ c f

and P*(z) = P'(z) + P(z)Q'(z).

First, we conclude that a'P (z) - aP*(z) ≢ 0. Otherwise, if a'P(z) - aP*(z) = 0, by integrating, then we have

a P ( z ) = A e Q ( z ) ,

where A is a non-zero constant. Hence, we get eQ(z)is a constant and

f n ( z ) ( f ( z ) - 1 ) Δ c f = B P ( z ) + a ,
(17)

where B is a non-zero constant. Then, from Lemma 2.3 and (17), we obtain that

( n + 1 ) T ( r , f ) 2 T ( r , f ) + O r σ ( f ) - 1 + ε + S ( r , f ) ,

which is a contradiction when n ≥ 2.

If F(z, f) vanish identically, then

a P * ( z ) + P ( z ) f ( z ) n - 1 f ( z ) Δ c f - a P ( z ) 0 .
(18)

Rewrite (18), we get

f n 2 f f ( z ) Δ c f = a P ( z ) a P * ( z ) P ( z ) ,

hence

f n - 2 f 2 f ( z ) Δ c f f = a P ( z ) - a P * ( z ) P ( z ) .
(19)

Then, combining Lemmas 2.2, 2.4 and Equation (19), we conclude that

m ( r , f f ( z ) Δ c f ) = S ( r , f ) ,

which contradicts (1).

It remains to consider the case that F (z, f) ≢ 0. We rewrite (16) in the form that

( f ( z ) n + 2 - f ( z ) n + 1 ) F ( z , f ) f ( z ) 2 = a P ( z ) - a P * ( z ) - P ( z ) f ( z ) n - 1 f ( z ) Δ c f
(20)

and

f ( z ) n + 1 ( f ( z ) - 1 ) F ( z , f ) f ( z ) 2 = a P ( z ) - a P * ( z ) - P ( z ) f ( z ) n - 1 f ( z ) Δ c f f ( z ) 2 .

By Lemmas 2.2 and 2.4, we know that

m r , F ( z , f ) f ( z ) 2 = S ( r , f )

and

m r , ( f ( z ) - 1 ) F ( z , f ) f ( z ) 2 = S ( r , f ) .

As f (z) is entire, we get that the poles of F ( z , f ) f ( z ) 2 may be located only at the zeros of f (z). If F ( z , f ) f ( z ) 2 has infinitely many poles, then from that a zero of f (z) with multiplicity t should be a pole of t + 1 of F ( z , f ) f ( z ) 2 . Since n ≥ 2, we know that the left side of (20) must have infinitely many zeros, which is a contradiction that a'P(z) - aP*(z) is a non-zero polynomial. We get

N r , F ( z , f ) f ( z ) 2 = O ( log r ) and N r , ( f ( z ) - 1 ) F ( z , f ) f ( z ) 2 = O ( log r ) .

Hence

T r , F ( z , f ) f ( z ) 2 = S ( r , f )

and

T r , ( f ( z ) - 1 ) F ( z , f ) f ( z ) 2 = S ( r , f )

as well. Combining these two estimates, we obtain

T ( r , f ) = S ( r , f )

contradiction. This completes the proof of Theorem 1.5.