1 Introduction and main results

A function f(z) is called meromorphic if it is analytic in the complex plane ℂ except at isolate poles. In what follows, we assume that the reader is familiar with the basic notion of Nevanlinna’s value distribution theory, see [1] and [2].

Let us consider the q-difference polynomial case. Let d j C for j=1,,n, and let I q be a finite set of multi-indexes γ=( γ 0 ,, γ n ). A q-difference polynomial of a meromorphic function w(z) is defined as follows:

P ( z , w ) = P ( z , w ( q z ) , w ( q 2 z ) , , w ( q n z ) ) = γ I q a γ ( z ) w ( z ) γ 0 w ( q z ) γ 1 w ( q n z ) γ n ,
(1.1)

where qC{0}, and the coefficients a γ (z) are small meromorphic functions with respect to w(z) such that T(r, a γ )=o(T(r,w)) on a logarithmic density 1, denoted by S q (r,w). The total degree of P(z,w) in w(z) and the q-shifts of w(z) is denoted by deg w q (P), and the order of zero of P(z, x 0 , x 1 ,, x n ), as a function of x 0 at x 0 =0, is denoted as ord 0 q (P), which can be found, e.g., in [3]. Moreover, the weight of difference polynomial (1.1) is defined by

K q (P)= max γ I q { j = 1 n γ j } ,

where γ and I q are the same as in (1.1) above. The q-difference polynomial P(z,w) is said to be homogeneous with respect to w(z) if the degree d γ = γ 0 ++ γ n of each term in the sum (1.1) is non-zero and the same for all γ I q .

We recall the following result of Zhang et al. [[4], Theorem 1].

Theorem A Let w(z) be a zero-order meromorphic solution of

H(z,w)P(z,w)=Q(z,w),

where P(z,w) is a homogeneous q-difference polynomial with polynomial coefficients, and H(z,w) and Q(z,w) are polynomials in w(z) with polynomial coefficients having no common factors. If

max { deg w q ( H ) , deg w q ( Q ) deg w q ( P ) } >min { deg w q ( P ) , ord 0 q ( Q ) } ord 0 q (P),

then N(r,w) S q (r,w), where ord 0 q (P) denotes the order of zero of P(z, x 0 , x 1 ,, x n ), as a function of x 0 at x 0 =0.

Now let us introduce some notation. Let q j C{0,} for j=1,,n, and let I and J be a finite set of multi-indexes I=( i 0 ,, i n ) and J=( j 0 ,, j n ). Two q-difference polynomials of a meromorphic function w(z) are defined as follows:

Ω 1 ( z , w 1 , w 2 ) = Ω 1 ( z , w 1 ( z ) , w 2 ( z ) , w 1 ( q 1 z ) , w 2 ( q 1 z ) , , w 1 ( q n z ) , w 2 ( q n z ) ) = i I a i ( z ) k = 1 2 w k ( z ) k i 0 w k ( q 1 z ) k i 1 w k ( q n z ) k i n

and

Ω 2 ( z , w 1 , w 2 ) = Ω 2 ( z , w 1 ( z ) , w 2 ( z ) , w 1 ( q 1 z ) , w 2 ( q 1 z ) , , w 1 ( q n z ) , w 2 ( q n z ) ) = j J b j ( z ) k = 1 2 w k ( z ) k i 0 w k ( q 1 z ) k i 1 w k ( q n z ) k i n ,

where the coefficients a i (z) and b j (z) are small with respect to w 1 (z) and w 2 (z) in the sense that T(r, a i )=o(T(r, w k )) and T(r, b j )=o(T(r, w k )), k=1,2, on a set of logarithmic density 1, as r tends to infinity outside of an exceptional set E of finite logarithmic measure

lim r E [ 1 , r ) d t t <.

The weights of Ω 1 (z, w 1 , w 2 ) and Ω 2 (z, w 1 , w 2 ) in w 1 (z), w 2 (z) are denoted by

λ 11 = max i { l = 0 n i 1 l } , λ 12 = max i { l = 0 n i 2 l }

and

λ 21 = max j { l = 0 n i 1 l } , λ 22 = max j { l = 0 n i 2 l } .

The purpose of this paper is to study the problem of the properties of Nevanlinna counting functions and proximity functions of meromorphic solutions of a type of systems of q-difference equations of the following form:

{ Ω 1 ( z , w 1 , w 2 ) = R 1 ( z , w 1 ) , Ω 2 ( z , w 1 , w 2 ) = R 2 ( z , w 2 ) ,
(1.2)

where

R 1 (z, w 1 )= P 1 ( z , w 1 ) Q 1 ( z , w 1 ) = i = 0 p 1 a i ( z ) w 1 i j = 0 q 1 b j ( z ) w 1 j

and

R 2 (z, w 2 )= P 2 ( z , w 2 ) Q 2 ( z , w 2 ) = i = 0 p 2 c i ( z ) w 2 i j = 0 q 2 d j ( z ) w 2 j ,

the coefficients { a i (z)}, { b i (z)}, { c i (z)}, { d i (z)} are meromorphic functions and small functions. The order of zero of Ω j (z, x 0 ,, x n ), as a function of x 0 at x 0 =0, is denoted by ord 0 ( Ω j ). The q-difference polynomial Ω k (z, w 1 , w 2 ), k=1,2, is said to be homogeneous with respect to w k (z) if the degree d k = i k 0 ++ i k n of each term in the sum is non-zero and the same for all iI. Finally, the order of growth of a meromorphic solution ( w 1 , w 2 ) is defined by

ρ( w 1 , w 2 )=max { ρ ( w 1 ) , ρ 2 ( w 2 ) } ,

where

ρ( w k )= lim sup r log T ( r , w k ) log r ,k=1,2.

In this paper, the main results are as follows.

Theorem 1 Let ( w 1 , w 2 ) be a zero-order meromorphic solution of system (1.2), where Ω k (z, w 1 , w 2 ) (k=1,2) are homogeneous q-difference polynomials in w 1 and w 2 , respectively, with meromorphic coefficients, and P k (z, w k ) and Q(z, w k ), k=1,2, are polynomials in w k (z) with meromorphic coefficients having no common factors. If

max{ q 1 , p 1 λ 11 }>min { λ 11 , ord w 1 ( P 1 ) } ord w 1 ( Ω 1 )+ λ 12
(1.3)

and

max{ q 2 , p 2 λ 22 }>min { λ 22 , ord w 2 ( P 2 ) } ord w 2 ( Ω 2 )+ λ 21 ,
(1.4)

then N(r, w 1 )= S q (r, w 1 ) and N(r, w 2 )= S q (r, w 2 ) cannot hold both at the same time, possibly outside of an exceptional set of finite logarithmic measure.

Theorem 2 Let ( w 1 , w 2 ) be a zero-order meromorphic solution of system (1.2), where Ω k (z, w 1 , w 2 ) (k=1,2) are homogeneous q-difference polynomials in w 1 and w 2 , respectively, with meromorphic coefficients, and P k (z, w k ) and Q(z, w k ), k=1,2, are polynomials in w k (z) with meromorphic coefficients having no common factors,

A=2 λ 11 ( max { p 1 , q 1 + λ 11 } min { λ 11 , ord w 1 ( Ω 1 ) } )

and

B=2 λ 22 ( max { p 2 , q 2 + λ 22 } min { λ 22 , ord w 2 ( Ω 2 ) } ) .

If A<0, B<0 and AB>9 λ 21 λ 12 , then m(r, w k )= S q (r, w k ) (k=1,2), where r runs to infinity outside of an exceptional set of finite logarithmic measure.

2 Some lemmas

Lemma 1 ([5], Theorem 1.2)

Let f(z) be a non-constant zero-order meromorphic function, and qC{0}. Then

m ( r , f ( q z ) f ( z ) ) = S q (r,f).

Lemma 2 ([6], Lemma 4)

If T: R + R + is a piecewise continuous increasing function such that

lim r log T ( r ) log r =0,

then the set

E:= { r : T ( C 1 r ) C 2 T ( r ) }

has logarithmic density 0 for all C 1 >1 and C 2 >1.

3 Proof of Theorem 1

Since Ω k (z, w 1 , w 2 ) are homogeneous in w 1 and w 2 , respectively, it follows by Lemma 1 that

m ( r , Ω 1 ( z , w 1 , w 2 ) w 1 λ 11 ) λ 12 m(r, w 2 )+ S q (r, w 1 )
(3.1)

and

m ( r , Ω 2 ( z , w 1 , w 2 ) w 2 λ 22 ) λ 21 m(r, w 1 )+ S q (r, w 2 )
(3.2)

for all r outside of an exceptional set of finite logarithmic measure. Moreover, from (1.2), we have

T ( r , Ω 1 ( z , w 1 , w 2 ) w 1 λ 11 ) = T ( r , P 1 ( z , w 1 ) Q 1 ( z , w 1 ) w 1 λ 11 ) = ( max { p 1 , q 1 + λ 11 } min { λ 11 , ord w 1 ( P 1 ) } ) T ( r , w 1 ) + S q ( r , w 1 )
(3.3)

and

T ( r , Ω 2 ( z , w 1 , w 2 ) w 2 λ 22 ) = T ( r , P 2 ( z , w 2 ) Q 2 ( z , w 2 ) w 2 λ 22 ) = ( max { p 2 , q 2 + λ 22 } min { λ 22 , ord w 2 ( P 2 ) } ) T ( r , w 2 ) + S q ( r , w 2 ) ,
(3.4)

where r approaches infinity outside of an exceptional set of finite logarithmic measure. By combining (3.1) and (3.3), (3.2) and (3.4), respectively, it follows that

N ( r , Ω 1 ( z , w 1 , w 2 ) w 1 λ 11 ) ( 1 + λ 12 + λ 11 ord w 1 ( Ω 1 ) ) T ( r , w 1 ) λ 12 m ( r , w 2 ) + S q ( r , w 1 )
(3.5)

and

N ( r , Ω 2 ( z , w 1 , w 2 ) w 2 λ 22 ) ( 1 + λ 21 + λ 22 ord w 2 ( Ω 2 ) ) T ( r , w 1 ) λ 21 m ( r , w 1 ) + S q ( r , w 2 ) .
(3.6)

From Lemma 2, we have

N ( r , Ω 1 ( z , w 1 , w 2 ) w 1 ord w 1 ( Ω 1 ( z , w 1 , w 2 ) ) ) ( λ 11 ord w 1 ( Ω 1 ) ) N ( q r , w 1 ) + λ 12 N ( q r , w 2 ) + S q ( r , w 1 ) = ( λ 11 ord w 1 ( Ω 1 ) ) N ( r , w 1 ) + λ 12 N ( r , w 2 ) + S q ( r , w 1 ) + S q ( r , w 2 )

and

N ( r , Ω 2 ( z , w 1 , w 2 ) w 1 ord w 2 ( Ω 2 ( z , w 1 , w 2 ) ) ) ( λ 22 ord w 2 ( Ω 2 ) ) N ( q r , w 2 ) + λ 21 N ( q r , w 1 ) + S q ( r , w 2 ) = ( λ 22 ord w 2 ( Ω 2 ) ) N ( r , w 2 ) + λ 11 N ( r , w 1 ) + S q ( r , w 1 ) + S q ( r , w 2 ) .

Therefore,

N ( r , Ω 1 ( z , w 1 , w 2 ) w 1 λ 11 ) N ( r , Ω 1 ( z , w 1 , w 2 ) w 1 ord w 1 ( Ω 1 ( z , w 1 , w 2 ) ) ) + N ( r , 1 w 1 λ 11 ord w 1 ( Ω 1 ) ) ( λ 11 ord w 1 ( Ω 1 ) ) N ( r , w 1 ) + λ 12 N ( r , w 2 ) + T ( r , 1 w 1 λ 11 ord w 1 ( Ω 1 ) ) + S q ( r , w 1 ) + S q ( r , w 2 ) ( λ 11 ord w 1 ( Ω 1 ) ) N ( r , w 1 ) + λ 12 N ( r , w 2 ) + ( λ 11 ord w 1 ( Ω 1 ) ) T ( r , w 1 ) + S q ( r , w 2 ) + S q ( r , w 2 )
(3.7)

and

N ( r , Ω 2 ( z , w 1 , w 2 ) w 2 λ 22 ) N ( r , Ω 2 ( z , w 1 , w 2 ) w 2 ord w 2 ( Ω 2 ( z , w 1 , w 2 ) ) ) + N ( r , 1 w 2 λ 22 ord w 2 ( Ω 2 ) ) ( λ 22 ord w 2 ( Ω 2 ) ) N ( r , w 2 ) + λ 21 N ( r , w 1 ) + T ( r , 1 w 2 λ 22 ord w 2 ( Ω 2 ) ) + S q ( r , w 1 ) + S q ( r , w 2 ) ( λ 22 ord w 2 ( Ω 2 ) ) N ( r , w 2 ) + λ 21 N ( r , w 1 ) + ( λ 22 ord w 2 ( Ω 2 ) ) T ( r , w 2 ) + S q ( r , w 2 ) + S q ( r , w 2 ) .
(3.8)

Combining (3.5) and (3.7), (3.6) and (3.8), respectively, we have

( 1 + λ 12 + λ 11 ord w 1 ( Ω 1 ) ) T ( r , w 1 ) < ( λ 11 ord w 1 ( Ω 1 ) ) N ( r , w 1 ) + λ 12 T ( r , w 2 ) + ( λ 11 ord w 1 ( Ω 1 ) ) T ( r , w 1 ) + S q ( r , w 1 ) + S q ( r , w 2 )
(3.9)

and

( 1 + λ 21 + λ 22 ord w 2 ( Ω 2 ) ) T ( r , w 2 ) < ( λ 22 ord w 2 ( Ω 2 ) ) N ( r , w 2 ) + λ 21 T ( r , w 1 ) + ( λ 22 ord w 2 ( Ω 2 ) ) T ( r , w 2 ) + S q ( r , w 1 ) + S q ( r , w 2 ) .
(3.10)

Suppose that N(r, w 1 )= S q (r, w 1 ) and N(r, w 2 )= S q (r, w 2 ), according to (3.9) and (3.10), we have

(1+ λ 12 )T(r, w 1 )< λ 12 T(r, w 2 )+ S q (r, w 1 )+ S q (r, w 2 )

and

(1+ λ 21 )T(r, w 2 )< λ 21 T(r, w 1 )+ S q (r, w 1 )+ S q (r, w 2 ).

That is,

( 1 + λ 12 + o ( 1 ) ) T(r, w 1 )< ( λ 12 + o ( 1 ) ) T(r, w 2 )
(3.11)

and

( 1 + λ 21 + o ( 1 ) ) T(r, w 2 )< ( λ 12 + o ( 1 ) ) T(r, w 1 ).
(3.12)

By (3.11) and (3.12), we conclude that

1+ λ 12 +1+ λ 21 +o(1)< λ 12 + λ 21 ,

which is impossible, we prove the assertion.

4 Proof of Theorem 2

It follows by Lemma 1 that

m ( r , Ω 1 ( z , w 1 , w 2 ) w 1 λ 11 ) λ 12 m(r, w 2 )+ S q (r, w 1 )
(4.1)

and

m ( r , Ω 2 ( z , w 1 , w 2 ) w 2 λ 22 ) λ 21 m(r, w 1 )+ S q (r, w 2 )
(4.2)

for all r outside of an exceptional set of finite logarithmic measure.

Suppose now that ( w 1 (z), w 2 (z)) is a finite-order meromorphic solution of (1.2). Denoting C= max j = 1 , , n {| c j |} in Ω 1 (z, w 1 , w 2 ) and Ω 2 (z, w 1 , w 2 ), by Lemma 2, we obtain

N ( r , Ω 1 ( z , w 1 , w 2 ) w 1 λ 11 ) λ 11 ( N ( | q | r , w 1 ) + N ( r , 1 w 1 ) ) + λ 12 ( N ( | q | r , w 2 ) + N ( r , 1 w 2 ) ) + λ 12 N ( r , w 2 ) + S q ( r , w 1 ) + S q ( r , w 2 ) = λ 11 ( N ( r , w 1 ) + N ( r , 1 w 1 ) ) + λ 12 ( N ( r , w 2 ) + N ( r , 1 w 2 ) ) + λ 12 N ( r , w 2 ) + S q ( r , w 1 ) + S q ( r , w 2 )
(4.3)

for all r outside of a set E of finite logarithmic measure. By (4.1) and (4.3), we have

N ( r , Ω 1 ( z , w 1 , w 2 ) w 1 λ 11 ) λ 11 ( N ( r , w 1 ) + N ( r , 1 w 1 ) ) + λ 12 ( N ( r , w 2 ) + N ( r , 1 w 2 ) ) + S q ( r , w 1 ) + S q ( r , w 2 ) λ 12 ( 2 T ( r , w 1 ) m ( r , w 1 ) ) + λ 12 ( 3 T ( r , w 2 ) 2 m ( r , w 2 ) ) + S q ( r , w 1 ) + S q ( r , w 2 )
(4.4)

for all rE. On the other hand, by (4.1) and (4.3),

N ( r , Ω 1 ( z , w 1 , w 2 ) w 1 λ 11 ) + λ 12 m ( r , w 2 ) T ( r , P 1 ( r , w 1 ) w 1 λ 11 Q 1 r , w 1 ) = ( max { p 1 , q 1 + λ 11 } min { λ 11 , ord w 1 ( Ω 1 ) } ) T ( r , w 1 ) + S q ( r , w 1 ) ,
(4.5)

where r lies outside of a set F of finite logarithmic measure. Combining inequalities (4.4) and (4.5) with the assumption in Theorem 2, we have

( max { p 1 , q 1 + λ 11 } min { λ 11 , ord w 1 ( Ω 1 ) } ) T ( r , w 1 ) λ 12 m ( r , w 2 ) + S q ( r , w 1 ) + S q ( r , w 2 ) λ 11 ( 2 T ( r , w 1 ) m ( r , w 1 ) ) + λ 12 ( 3 T ( r , w 2 ) 2 m ( r , w 2 ) ) + S q ( r , w 1 ) + S q ( r , w 2 ) .
(4.6)

Similarly, we obtain

( max { p 2 , q 2 + λ 22 } min { λ 22 , ord w 2 ( Ω 2 ) } ) T ( r , w 2 ) λ 21 m ( r , w 1 ) + S q ( r , w 1 ) + S q ( r , w 2 ) λ 22 ( 2 T ( r , w 2 ) m ( r , w 2 ) ) + λ 21 ( 3 T ( r , w 1 ) 2 m ( r , w 1 ) ) + S q ( r , w 1 ) + S q ( r , w 2 ) .
(4.7)

By (4.6) and (4.7), we obtain

λ 11 m ( r , w 1 ) ( 2 λ 11 ( max { p 1 , q 1 + λ 11 } min { λ 11 , ord w 1 ( Ω 1 ) } ) + o ( 1 ) ) T ( r , w 1 ) + ( 3 λ 12 + o ( 1 ) ) T ( r , w 2 )
(4.8)

and

( ( max { p 2 , q 2 + λ 22 } min { λ 22 , ord w 2 ( Ω 2 ) } ) 2 λ 22 + o ( 1 ) ) T ( r , w 2 ) ( 3 λ 21 + o ( 1 ) ) T ( r , w 1 ) 2 λ 21 m ( r , w 2 ) .
(4.9)

Combining (4.8) and (4.9), we have

λ 11 m ( r , w 1 ) ( 2 λ 11 ( max { p 1 , q 1 + λ 11 } min { λ 11 , ord w 1 ( Ω 1 ) } ) + o ( 1 ) ) T ( r , w 1 ) + 3 λ 12 ( 3 λ 21 + o ( 1 ) ) T ( r , w 1 ) 6 λ 12 λ 21 m ( r , w 1 ) ( max { p 2 , q 2 + λ 22 } min { λ 22 , ord w 2 ( Ω 2 ) } ) 2 λ 22 ,

that is,

( λ 11 6 λ 12 λ 21 B ) m(r, w 1 ) ( A 9 λ 12 λ 21 + o ( 1 ) B ) T(r, w 1 ),
(4.10)

where A=2 λ 11 (max{ p 1 , q 1 + λ 11 }min{ λ 11 , ord w 1 ( Ω 1 )}) and B=2 λ 22 (max{ p 2 , q 2 + λ 22 }min{ λ 22 , ord w 2 ( Ω 2 )}). Combining the assumption and (4.10), we have

m(r, w 1 )= S q (r, w 1 )

for all r outside of EF, a set of finite logarithmic measure.

Similarly, we obtain

m(r, w 2 )= S q (r, w 2 )

for all r outside of EF, we have proved the assertion.