Given two determinantal rings over a field k. We consider the Rees algebra of
the diagonal ideal, the kernel of the multiplication map. The special fiber
ring of the diagonal ideal is the homogeneous coordinate ring of the join
variety. When the Rees algebra and the Symmetric algebra coincide, we show that
the Rees algebra is Cohen-Macaulay.Comment: This work is based on author's Ph. D. thesis from Purdue University
under the direction of Professor Bernd Ulric