Let n be a positive integer, I the n×n identity matrix over C, and J the 2n×2n matrix given by
J=[0−II0].
Let M be a 2n×2n matrix over C of the form
M=[ACBD]
where A, B, C, D are n×n matrices over C. Find necessary and sufficient conditions on A, B, C, D in order that MtJM=J.