Abstract. In this article, we characterize the UMD–property of a Banach space X by ideal norms associated with trigonometric orthonormal systems. The asymptotic behavior of that numerical parameters can be used to decide whether or not X is a UMD–space. Moreover, in the negative case, we obtain a measure that shows how far X is from being a UMD–space. The main result is, that all described para...
