# A Removability Result for Holomorphic Functions of Several Complex Variables.

Byline: Juhani Riihentaus

Abstract: Suppose that fi is a domain of On , n S 1 , E c f i closed in fi , the Hausdorff measure H2 n -1 (E )= 0 , and f is holomorphic in fi \ E . It is a classical result of Besicovitch that if n =1 and f is bounded, then f has a unique holomorphic extension to fi . Using an important result of Federer, Shiffman extended Besicovitch's result to the general case of arbitrary number of several complex variables, that is, for n S 1 . Now we give a related result, replacing the boundedness condition of f by certain integrability conditions of f and of (EQ.).

Keywords: Holomorphic function, subharmonic function, Hausdorff measure, exceptional sets.

1. INTRODUCTION

1.1. Previous Results

The following result of Besicovitch is well-known:

Theorem 1. (, Theorem 1, p. 2) Let D be a domain in O. Let E c D be closed in D and let H1 (E) = 0. If f: D \ E c O is holomorphic and bounded, then f has a unique holomorphic extension to D.

Above and below Ha is the a - dimensional Haus- dorff (outer) measure in k, kS2.

Much later Shiffman gave the following general result:

Theorem 2. (, Lemma 3, p. 115) Let fi be a domain in On , n S 1 . Let E c f i be closed in fi and let H2 n-1 (E )= 0 . If f : fi\E c O is holomorphic and bounded, then f has a unique holomorphic extension to fi.

Shiffman's proof was based on Besicovitch's result, Theorem 1 above, on coordinate rotation, on the use of Cauchy integral formula and on the following result of Federer:

Lemma 1. (, Theorem 2.10.25, p. 188, and , Corollary 4, Lemma 2, p. 114) Suppose that E c k , k S 2 , is such that Hk -1 (E )= 0 . Then for all j , 1 S j S k , and for Hk -1 -almost all Xj c k -1 the set E( X j ) is empty. For slightly more general versions of Shiffman's result with different proofs, see , Theorem 3.1, p. 49, Corollary 3.2, p. 52, and , Theorem 3.1, p. 333, Corollary 3.3, p. 336.

1.2. Notation

Our notation is more or less standard, see [6-8]. However and for the convenience of the reader, we recall here the following. If x = (x ,..., x ) c n , n > 2 and j j c D , 1 S j S n , then we write x = (x j , X j ) , where X j = ( x1 ,..., x j -1 , x j +1 ,..., xn ). Moreover, if E c R, (Eq) and (Eq) , we write

(EQUATIONS)

If (EQ) and p > 0 , then (EQ) , p > 0 , is the space of functions u in (Eq.) for which (Eq.) is locally integrable on fi . We identify (EQ) , with (EQ). We use the common convention (Eq.).

For the definition and properties of subharmonic functions, see e.g. [9-12], for the definition of holomorphic functions see e.g. [13-15].

2. AN EXTENSION RESULT FOR HOLOMORPHIC FUNCTIONS

2.1. Our result is related to Theorem 2 above, and reads as follows:

Theorem 3. Suppose that (Eq.) is a domain in On , n S 1 . Let (Eq.) be closed in fi and let H2 n-1 (E )= 0. Let (Eq.) be holomorphic and such that the following conditions are satisfied:

(EQUATIONS)

Then f has a holomorphic extension to (EQ)

2.2. The proof will be based, in addition to Federer's cited Lemma 1 above, also on the following recent result:

Lemma 2. (, Theorem, p. 568) Suppose that (EQ) is a domain in (EQ) Let E c (EQ) be closed in fi and let Hn-1 (E )less than (Eq.) . Let u : (Eq.) be such that the following conditions are satisfied:

(EQUATIONS)

Proof of Theorem 3. Write f = u + iv . It is sufficient to show that u and v have subharmonic extensions to (EQ). As a matter of fact, then f will be locally bounded in (EQ) , and thus the claim will follow from Theorem 2 or also from the already cited slightly more general results from [4, 5]. To see that u and v have indeed subharmonic extensions to (EQ) , we use our Lemma 2 as follows.

It is sufficient to show that the assumption (iv) of Lemma 2 is satisfied. For that purpose take j , arbitrarily. By Federer's result, Lemma 1 above, we know that for H2n-1 almost all (EQ) the set (EQ) is empty. Thus for H2n-1 almost all (EQ) the functions (EQ) and (EQ) are (EQ) functions. Therefore, the assumption (iv) is satisfied both for u and for v, concluding the proof.

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