Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org Approximation of Entire Function of Slow Growth in Several Complex Variables Mushtaq Shakir A. Hussein (1, 2 ) (1) and Aiman Abdali Jaffar ( 2) Department of Mathematics, College of science, Mustansiriha University, Baghdad, Iraq. Abstract In the present paper, we study the polynomial approximation of entire function in Banach space ( B( p, q, k ) space, Hardy space and Bergman space). The coefficient characterizations of generalized type of entire function of slow growth in several complex variables have been obtained in terms of the approximation errors. Keyword: Entire function, generalized order, generalized type, approximation error. 1.Introduction Let f z a n 0 n z n be an entire function and M r , f max f z be its maximum modulus. z r The growth of f z is measured in terms of its order ρ and type τ defined as under ln ln M (r , f ) r ln ln M (r , f ) lim sup r r lim sup (1.1) r (1.2) for 0 < ρ < ∞. Various workers have given different characterizations for entire function of fast growth (ρ = ∞). M. N. Seremeta [6] defined the generalized order and generalized type with the help of general functions as follows. Let L0 denoted the class of functions h satisfying the following conditions (i) hx is defined on [a,∞) and is positive, strictly increasing, differentiable and tend to ∞ as x , (ii) h1 1 x x 1, h x for every function x such that x as x . lim x Let Λ denoted the class of function h satisfying condition (i) and hcx 1 x h x lim for every c > 0, that is, h(x) is slowly increasing. For the entire function f z and function x , x L0 , the generalized order of an entire function in the terms of maximum modulus is defined as , , f lim sup r ln M r , f . ln r (1.3) Further, for x L0 , 1 x L0 , x L0 , generalized type of an entire function f of finite generalized order ρ is defined as , , f lim sup r 152 ln M r , f . r (1.4) Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org where 0 < ρ < ∞ is a fixed number. Above relation were obtained under certain conditions which do not hold if . To overcome this difficulty, G. P. Kapoor and Nautiyal [4] defined generalized order ( ; f ) of slow growth with the help of general functions as follows Let Ω be the class of functions hx satisfying (i) and (iv) there exists a x and x0 , K1 and K 2 such that 0 K1 d (h( x)) K 2 for all x x0 . d ( (log x)) Let be the class of functions h(x) satisfying (i) and (v) d (h( x)) K , 0 < K < ∞. d (log x) lim x Kapoor and Nautiyal [4] showed that class Ω and are contained in Λ. Further, and they defined the generalized order ; f for entire function f z of slow growth as ; f lim sup r where x either belongs to Ω or to . (ln M (r , f )) , (ln r ) Let f ( z1 , z 2 ,, z n ) be an entire function, z ( z1 , z 2 ,, z n ) C n . Let G be a full region in Rn (positive hyper octant). Let GR C n denoted the region obtained from G by a similarity transformation about the origin, with ratio of similitude R. let d t (G) sup zG z z z1 t t1 t t , where t z 2 2 z n n , and let G denoted the boundary of the region G. Let f ( z ) f ( z1 , z 2 , , z n ) t t1 t 2 t n , be at1tn z1t1 zntn t1 ,t2 ,,tn 0 the power a t 0 series t zt , expansion of the function f (z ) . Let M G ( R, f ) max f ( z ) . To characterize the growth of f, order ( G ) and type ( G ) of f are defined z GR as [2] ln ln M G ( R, f ) , R ln R ln M G ( R, f ) G lim sup . R R G G lim sup For the entire function, f ( z ) a t 0 t z t , A. A. Gol'dberg [3, Th .1] obtained the order and type in terms of the coefficients of its Taylor expansion as G lim (e G G )1 G lim sup t t where d t (G) max r t ; r G r t r1t1 r2t2 rntn . 153 t 1 G t ln t ln at . 1 t [ at d t (G)] (1.5) , (0 < G < ∞) (1.6) Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org for an entire function of several complex variables f z a t t 0 z t , and functions x , x L , Seremeta [6, Th .1] proved that ( t ) [ln M G ( R, f )] . lim sup lim sup R k 1 (ln R) [ ln at d t (G ) ] 0 (1.7) t Further, for x L0 , 1 x L0 , x L0 , Seremeta [6, Th .1] proved that t lim sup R ( ) [ln M G ( R, f )] . lim sup t [( ( R)) ] [( {e1 [ at d t (G )]1 t }) ] (1.8) where 0 < ρ < ∞ is a fixed number. And let H q denote the Bergman space of functions f z satisfying the condition 1 q 1 f H ' f z d 1d 2 d n , q z G A where z ( z1 , z2 , , zn ) , d j dx j dy j , z j x j iy j , j 1,2, , n. and A is the area of G. For q = ∞, let f H f H sup{ f z , z U } . Then H q and H q are Banach space for q ≥ 1. In analogy with spaces of functions of one variable, we call H q and H q the Hardy and Bergman spaces respectively. The function f z analytic in U belong to the space B(p, q, k), where 0 p q , and 0 k , if 1k f p ,q ,k 1 (1 R) k (1 p1 q ) 1 M qk,G ( R, f ) dR 0 . And f p ,q , sup{(1 R)1 p1 q M q,G ( R, f ) ; 0 R 1} . It is known [1] that B(p, q, k) is a Banach space for p > 0 and q, k 1 , otherwise it is a Freachet space. Further , we have Let Pm { p : p a t m t q H q H q B( , q, q), 1 q . 2 z t } be the class polynomials of degree at most m and let X denote one of the Banach spaces defined. Then we defined error of an entire function f on the region G as E t ( f ) E t ( f , G, X ) inf{ f p X : p Pm } . Vakarchuk and Zhir [5] obtained the characterizations of generalized order and generalized type of f z in terms of the errors E t ( f ) defined above. These characterizations do not hold good when α = β = γ. i.e. for entire functions of slow growth. In this paper we have tried to fill this gap. We define the generalized type ( ; f ) of an entire function f z having finite generalized order as ( ; f ) lim sup r Where x either belongs to Ω or to . (ln M G ( R, f )) . [ (ln R)] 154 Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org 2. Main Results Theorem 2.1: Let x , then the entire function f z of generalized order ρ, 1 < ρ < ∞, is of generalized type τ if and only if (ln M G ( R, f )) lim sup lim sup , 1 t R t [ (ln R)] { [ 1 ln at d t (G ) ]} 1 t (2.1) provided dF ( x ; , ) d ln x O(1) as x for all τ, 0 < τ < ∞. Proof. Let (ln M G ( R, f )) . [ (ln R)] We suppose τ < ∞. Then for every ε > 0, R (ln M G ( R, f )) , R R . [ (ln R)] (or) ln M G ( R, f ) ( 1{ [ (ln R)] }) . Choose R Rt to be the unique root of the equation 1 t F [ln R ; , ] . ln R lim sup R (2.2) Then t t 1 1 ln R 1[( ( ))1 ( 1) ] F [ ; , 1] . (2.3) By Cauchy's inequality, at d t (G ) R t M G ( R, f ) exp{ t ln R ( 1{ [ (ln R)] })} By using (2.5) and (2.6), we get at d t (G) exp{ t F t F} or 1 ln( at d t (G)) 1 t t 1 1{[( ( ))1 ( 1) ]} or ( ) t { [ 1 ln( at d t (G )) 1 t ]} 1 . Now proceeding to limits, we obtain ( ) t lim sup t { [ 1 ln( at d t (G )) 1 t Inequality (2.4) obviously holds when τ = ∞. Conversely, let ( ) t lim sup t { [ 1 ln at d t (G ) 1 t ]} 1 . Suppose σ < ∞. Then for every ε > 0 and for all t N , we have 155 ]} 1 . (2.4) Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org ( ) t { [ 1 ln at d t (G ) 1 t at d t (G) ]} 1 1 exp{( 1) at d t (G) R Re t t The inequality t F [ ; 1 , 1] t F [ 1 t ; 1 , 1] (2.5) 1 2 (2.6) 1. (2.7) . is fulfilled beginning with some t m(R) . Then at dt (G) R t m ( R ) 1 1 t t m ( R ) 1 2 t We now express m® in terms of R. From inequality (2.6), 2 R exp{( t 1 1 ) F [ ; , 1]} , m( R) E[ 1{ ( (ln R ln 2)) 1}] . x R x exp{( 1 ) x F[ x ; 1 , 1]}. Let we can take We consider the dF[ x ; 1 , 1] x 1 x 1 ln R ( ) F[ ; , 1] 0. x d ln x As x , by the assumption of the theorem, for finite (0 ) , dF[ x ; , 1] d ln x is bounded. So there is a A > 0 such that for x x1 we have dF [ x ; 1 , 1] A. d ln x function (2.8) (2.9) A ln 2 . It is then obvious that inequalities (2.6) and (2.7) hold for t m1 ( R) E[ 1{ ( (ln R ln 2))( 1) }] 1 . m0 We let designate the We can take number max( N ( ) , E[ x1 ] 1) . For R R1 (m0 ) we have (m0 ) (m0 ) 0 . From (2.9) and (2.8) it follows that (m1 ( R)) (m1 ( R)) 0 . We hence obtain that if for R R1 (m0 ) we let x * ( R) designate the point where ( x* ( R)) max m0 x m1 ( R ) ( x) , then m0 x* ( R) m1 ( R) and x* ( R) 1{ ( (ln R a( R))) 1} . Where A a( R) dF [ x ; 1 , 1] d ln x x x* ( R ) A. Further max m0 t m1 ( R ) ( at d t (G ) R t max ( x) R 1 { ( (ln R a ( R ))) 1 } 1 1 e { ( (ln Ra ( R ))) } (ln Ra ( R )) exp{a( R) 1{ ( (ln R a( R))) 1}} m0 x m1 ( R ) exp{ A 1{ ( (ln R A)) 1}}. It is obvious that (for R R1 (m0 ) ) M G ( R, f ) a t 0 t 156 d t (G ) R t Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 m0 a t 0 t www.iiste.org d t (G) R t m1 ( R ) a at d t (G) R t t t m1 ( R ) 1 t m0 1 O( R ) m1 ( R) m0 d t (G) R t ( at d t (G) R ) 1 t max m0 t m1 ( R ) M G ( R, f )(1 o(1)) exp{( A o(1)) 1[ ( (ln R A)) 1 ]} (ln M G ( R, f )) [ (ln R A)] 1 [ (ln R A)] . We then have [( A o(1)) 1 ln M G ( R, f )] . [ (ln R A)] Since (x) , now proceeding to limits we obtain lim sup R (ln M G ( R, f )) . [ (ln R)] (2.10) From inequality (2.4) and (2.10), we get the required the result. Now we prove Theorem 2.2: Let x , then a necessary and sufficient condition for an entire function f z B( p, q, k ) to be of generalized type τ having finite generalized order ρ, 1 < ρ < ∞ is ( ) t lim sup t [{ 1 ln(( E t ( B( p, q, k )) d t (G )) 1 t )}]( 1) . (2.11) Proof. First we consider the space B( p, q, k ) , q = 2, 0 < ρ < ∞ and k ≥ 1. Let f ( z) B( p, q, k ) be of generalized type τ with generalized order ρ. Then from the Theorem 2.1, we have ( ) t lim sup t { [ 1 ln( at d t (G )) 1 t ]} 1 . (2.12) . (2.13) For a given ε > 0, and all t m m( ) , we have at d t (G) Let g t ( f , z ) 1 exp{( 1) F [ ; 1 , 1]} t t t a j 0 j z j be the t th partial sum of the Taylor series of the function f (z ) . Following [5, p.1396], we get E t ( B( p,2, k ); f ) B1 k [( t 1)k 1; k (1 p 1 2)]{ (a j t 1 j d j (G)) 2 }1 2 (2.14) where B(a, b) (a, b 0) denotes the beta function. By using (2.13), we have E t ( B( p,2, k ); f ) B1 k [( t 1)k 1; k (1 p 1 2)] exp{( 1) t 1 F[ t 1 ; , 1]} 1 { j t 1 where j ( ) exp{ t 1 ( 1)[ 1{( exp{ ( 1)[ {( 1 j Set 157 ( t 1 ( j ) 1 ( 1) ) ) 1 ( 1) ) }]} }]} . 2 j ( )}1 2 , (2.15) Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org ( ) exp{ ( 1) ( 1 ) 1 ( 1) [ {( ) }]} . 1 Since (x) is increasing and j t 1 , we get j ( ) exp{ ( t 1 1 ( 1) ( 1)[ {( ) }]} (( t 1) j j ( t 1) 1 ( ) . (2.16) Since ( ) 1 , we get from (2.15) and (2.16), B1 k [( t 1)k 1; k (1 p 1 2)] E t ( B( p,2, k ); f ) (1 ( )) [exp{( 1) 2 12 t 1 [ {( 1 ( t 1 ) 1 ( 1) ) . (2.17) }]}] For t m , (2.17) yields ( t 1 ) { ( (1 1 )( 1) {ln(( E t d t ) 1 t ) ln( B1 k [( t 1) k 1;k (1 p 1 2 )] 1 t t })}( 1) ) (1 2 ( ))1 2 Now B[( t 1)k 1; k (1 p 1 2)] (( t 1)k 1)(k (1 p 1 2)) (( t 1 2 1 p)k 1) . Hence B[( t 1)k 1; k (1 p 1 2)] ~ e e [( t 1) k 1] ( t 1) k 3 2 [( t 1)k 1] [( t 1 21 p ) k 1] (1 p 1 2) [( t 1 2 1 p)k 1] ( t 1 2 1 p ) k 3 2 . Thus 1 ( t 1) {B[( t 1)k 1; k (1 p 1 2)]} 1. (2.18) )}]( 1) (2.19) Now proceeding to limits, we obtain ( ) t lim sup t [{ 1 ln(( E t d t (G )) 1 t For reverse inequality, by [5, p.1398], we have at 1 B1 k [( t 1)k 1; k (1 p 1 2)] E t ( B( p,2, k ); f ) . (2.20) Then for sufficiently large t , we have ( ) t [{ 1 ln(( E t d t (G )) 1 t )}]( 1) ( t ) [{ 1 {ln(( at 1 d t (G)) 1 t ) ln( B t k [( t 1)k 1; (1 p 1 2)])}}]( 1) ( t ) [ { 1 {ln(( at d t (G )) 1 t ) ln( B t k [( t 1)k 1; (1 p 1 2)])}}] ( 1) . By applying limits and from (2.12), we obtain ( ) t lim sup t [{ 1 ln(( E t d t (G )) 158 1 t )}]( 1) . (2.21) Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org From (2.19) and (2.21), we obtain the required relation ( ) t lim sup t [{ 1 ln(( E t d t (G)) 1 t )}]( 1) . (2.22) In the second step, we consider the spaces B( p, q, k ) for 0 p q, q 2, and q, k 1. Gvaradze [1] showed that, for p p1 , q q1 and k k1 , if at least one of the inequalities is strict, then the strict inclusion B( p, q, k ) B( p1 , q1 , k1 ) holds and the following relation is true: f p1 ,q1 ,k1 21 q1 q1 [k (1 p 1 q)]1 k 1 k1 f p ,q ,k . For any function f ( z) B( p, q, k ) , the last relation yields E t ( B( p1 , q1 , k1 ); f ) 21 q1 q1 [k (1 p 1 q)]1 k 1 k1 E t ( B( p, q, k ); f ). (2.23) For the general case B( p, q, k ), q 2, we prove the necessity of condition (2.13). Let f ( z) B( p, q, k ) be an entire transcendental function having finite generalized order ( ; f ) whose generalized type is defined by (2.12). Using the relation (2.13), for t m we estimate the value of the best polynomial approximation as follows E t ( B( p, q, k ); f ) f g t ( f ) 1 ( (1 R) ( k (1 p1 q )1 M qk,G dR)1 k . p ,q ,k 0 Now f q a t zt q ( at r t ) q (r a t 1 k t 1 k )q . Hence E t ( B( p, q, k ); f ) dt (G) B [( t 1)k 1; k (1 p 1 2)] a 1k k t 1 B1 k [( t 1)k 1; k (1 p 1 q)] (1 ( ))[exp{ t 1 ( 1)[ {( 1 ( t 1 ) 1 ( 1) ) k . (2.24) }]}] For t m , (2.24) yields ( t 1 ) { ( (1 1 )( 1) {ln (( E t d t (G )) 1 t t ) ln( B1 k [( t 1) k 1;k (1 p 1 2 )] 1 t (1 ( )) ) })}( 1) Since ( ) 1 , and , proceeding to limits and using (2.18), we obtain ( ) t lim sup t [ { 1 ln(( E t d t (G)) 1 t )}]( 1) . For the reverse inequality, let 0 p q 2 and k , q 1 . By (2.23), where p1 p, q1 2, and k1 k , and the condition (2.13) is already proved for the space B( p,2, k ) , we get ( ) t lim sup t [{ 1 ln(( E t ( B( p, q, k ); f ) d t (G )) 1 t )}]( 1) ( ) t lim sup t [{ 1 ln(( E t ( B( p,2, k ); f ) d t (G )) Now let 0 p 2 q . Since we have 159 1 t )}]( 1) . Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org M 2,G ( R, f ) M q,G ( R, f ), 0 R 1, therefore E t ( B( p, q, k ); f ) a t 1 B1 k [( t 1)k 1; k (1 p 1 q)]. (2.25) Then for sufficiently large t , we have ( ) t [{ 1 ln(( E t d t (G )) 1 t )}]( 1) ( ) t [ { 1 {ln(( a t 1 d t (G )) 1 t t k ) ln( B [( t 1)k 1; k (1 p 1 q)])}}] ( 1) ( ) t [{ 1 {ln(( a t d t (G)) 1 t ) ln( B t k [( t 1)k 1; k (1 p 1 q)])}}]( 1) . By applying limits and from (2.12), we obtain lim sup t ( t ) [{ 1 ln(( E t d t (G)) 1 t )}]( 1) ( t ) lim sup [{ 1 ln(( a t d t (G)) t 1 t )}]( 1) . Now we assume that 2 p q . Set q1 q, k1 k , and 0 p1 2 in the inequality (2.23), where p1 is an arbitrary fixed number. Substituting p1 for p in (2.25), we get E t ( B( p, q, k ); f ) a t 1 B1 k [( t 1)k 1; k (1 p1 1 q)] . (2.26) Using (2.26) and applying the same analogy as in the previous case 0 p 2 q , for sufficiently large t , we have ( ) t [{ 1 ln(( E t d t (G )) 1 t )}]( 1) ( ) t [{ 1 {ln(( a t 1 d t (G)) 1 t ) ln( B t k [( t 1)k 1; k (1 p1 1 q)])}}]( 1) ( ) t [{ 1 {ln(( a t d t (G )) 1 t ) ln( B t k [( t 1)k 1; k (1 p 1 q)])}}]( 1) By applying limits and using (2.12), we obtain ( ) t lim sup t [{ 1 ln(( E t d t (G )) 1 t )}]( 1) . From relation (2.19) and (2.21), and the above inequality, we obtain the required relation (2.22). Theorem 2.3: Assuming that the condition of Theorem 2.2 are satisfied and ( ) is a positive number, a necessary and sufficient condition for a function f ( z ) H q to be an entire function of generalized type ( ) having finite generalized order is that ( ) t lim sup t [{ 1 ln(( E t ( H q ; f ) d t (G )) 160 1 t )}]( 1) ( ). (2.27) Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 Proof. Let f ( z ) www.iiste.org a t 0 t z t be an entire transcendental function having finite generalized order ρ and generalized type τ. Since t lim t at 0 (2.28) f ( z) B( p, q, k ) , where 0 p q and q, k 1 . From relation (1.8), we get E t ( B(q 2 , q, q); f ) q E t ( H q ; f ), 1 q . (2.29) where q is a constant independent of t and f. In the case of Hardy space H , E t ( B( p, , ); f ) E t ( H ; f ), 1 p . (2.30) Since ( ) t ( ; f ) lim sup t [{ 1 ln(( E t ( H q ; f ) d t (G )) 1 t )}]( 1) ( ) t lim sup t [{ 1 ln(( E t (( B(q 2 , q, q); f ) d t (G )) 1 t )}]( 1) , 1 q . Using estimate (2.30) we prove inequality (2.31) in the case q . For the reverse inequality (2.31) ( ; f ) , (2.32) we use the relation (2.13), which is valid for t m , and estimate from above, the generalized type τ of an entire transcendental function f (z ) having finite generalized order ρ, as follows. We have E t ( H q ; f ) f gt a j t 1 Hq j 1 [exp{( 1) t 1 [ {( 1 ( t 1 ) 1 ( 1) ) j j t 1 }]}] ( ) Using (2.16), E t ( H q ; f ) f gt Hq 1 d t (G) (1 ( ))[exp{( 1) t 1 [ {( 1 1 (1 ( ))[exp{( 1) E t ( H q ; f ) d t (G ) ( t 1 t 1 ) 1 ( 1) ) [ {( 1 . }]}] ( t 1 ) 1 ( 1) ) }]} This yields ( [{ 1 [ln(( E t ( H q ; f ) d t (G)) t 1 1 t 1 ) ) ln((1 ( )) 1 t 1 )]}]( 1) . (2.33) Since ( ) 1 and by applying the properties of the function α, passing to the limit as t in (2.33), we obtain inequality (2.32). thus we have finally 161 ( ) . (2.34) Mathematical Theory and Modeling ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.4, No.5, 2014 www.iiste.org This prove Theorem 2.2. Remark : An analog of Theorem 2.3 for the Bergman space follows from (1.8) for 1 q and from Theorem 2.2 for q = ∞. References [1] M.I.Gvaradze, On the class of spaces of analytic functions, Mat. Zametki, (21)(2)(1994), 141150. [2] B.A.Fuks, Introduction to the theory of analytic functions of several complex variables, Trasl. of Math. Monographs. Vol.8, Amer. Math. Soc., Providence, R.I., 1963. [3] A.A.Gol'dberg, Elemntary remarks on the formulas for defining order and type of functions of several variables, Akad. Nauk Armjan. SSR. Dokl., 29(1959), 145-152. [4] G.P.Kapoor and A.Nautiyal, Polynomial approximation of an entire function of slow growth, J.Approx.Theory, (32)(1981), 64-75. [5] S.B.Vakarchuk and S.I.Zhir, On some problems of polynomial approximation of entire transcendental functions, Ukrainian Mathematical Journal, (54)(9)(2002), 1393-1401. [6] M.N.Seremeta, On the connection between the growth of the maximum modulus of an entire function and the moduli of the coefficients of its power series expansion, Amer. Math. Soc. Transl., (2)(88)(1970), 291-301. 162 The IISTE is a pioneer in the Open-Access hosting service and academic event management. The aim of the firm is Accelerating Global Knowledge Sharing. More information about the firm can be found on the homepage: http://www.iiste.org CALL FOR JOURNAL PAPERS There are more than 30 peer-reviewed academic journals hosted under the hosting platform. Prospective authors of journals can find the submission instruction on the following page: http://www.iiste.org/journals/ All the journals articles are available online to the readers all over the world without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. Paper version of the journals is also available upon request of readers and authors. MORE RESOURCES Book publication information: http://www.iiste.org/book/ Recent conferences: http://www.iiste.org/conference/ IISTE Knowledge Sharing Partners EBSCO, Index Copernicus, Ulrich's Periodicals Directory, JournalTOCS, PKP Open Archives Harvester, Bielefeld Academic Search Engine, Elektronische Zeitschriftenbibliothek EZB, Open J-Gate, OCLC WorldCat, Universe Digtial Library , NewJour, Google Scholar