I'm trying to understand Ridgeway Scott's "A survey of displacement methods for the plate bending problem" [1]. In chapter four he is talking about polygonal approximation and states that the error in H2 norm is of order 3/2. He also gives the assumptions that are necessary but there is no detailed explanation on how he gets there. I would be grateful for anyone pointing me to a resource that maybe explains this with more detail. I tried proving it myself, but I don't really know how to start and what proper tools to use. I also had a look in the literature, but I could only find some estimates for the Poisson problem and was unable to transfer to fourth order problems.
[1] in: Formulations and Computational Algorithms in Finite Element Analysis, K.-J. Bathe, J. T. Oden, and W. Wunderlich, eds., pages 855-876. Cambridge: M.I.T. Press, 1977
