Institute of Mathematics and Informatics Bulgarian Academy of Sciences
Serdica Mathematical Journal, Vol. 32, No 4, (2006), 375p-378p
In the proof of Lemma 3.1 in  we need to show that we may take the two points p and q with p ≠ q such that
where q1,…,qb-1 are points of C′, but in the paper  we did not show that p ≠ q. Moreover, we hadn't been able to prove this using the method of our paper . So we must add some more assumption to Lemma 3.1 and rewrite the statements of our paper after Lemma 3.1. The following is the correct version of Lemma 3.1 in  with its proof.