We would like to thank the referees for their reports and their helpful suggestions. Corrections were made according to these suggestions. Two short remarks regarding REPORT 1: - We agree that the result concerning the polynomials $s_1,s_2,s_3$ (page 6, second paragraph) is a bit surprising at a first glance. Anyhow, the statement is true -- it was proven by Blekherman in Corollary 2.3. of "Nonnegative Polynomials and Sums of Squares" before. The source and the Corollary are also cited on Page 6 of our article. - We added some sentences in order to clarify the reasons for a symbolic investigation of the problem at Page 2. We hope that the motivation for the symbolic approach becomes - in connection with the rest of the introduction and the achieved results - more obvious now.