Abstract. We give necessary and sufficient conditions in order that the sum of a double cosine series with nonnegative coefficients belong to the two-dimensional Zygmund classes $\Lambda_*(2)$ or $\lambda_*(2)$, respectively. Our theorem is the extension of the corresponding theorem by Boas from single to double cosine series.
AMS Subject Classification
(1991): 42B05, 42A16
summation by parts
Received February 6, 2003, and in revised form April 11, 2003. (Registered under 5801/2009.)