On 8/20/2013 3:45 AM, Robin Chapman wrote: > On 19/08/2013 19:04, Nicolas Bonneel wrote: >> On 8/19/2013 12:34 AM, James Dow Allen wrote: >>> Let >>> ( A B 0 ) >>> ( 0 A B ) >>> ( B 0 A ) >>> be a 3Nx3N real matrix with A,B,0 each NxN and 0 an all-zeros matrix. >>> >>> What is the necessary and sufficient condition for that matrix to be >>> orthogonal, i.e. that its transpose also be its inverse? >>> >> >> by direct computation, AB=BA=0 and A^2+B^2=Id, right? > > No >
ok, but why not ? Noting the matrix above M, I just did M^T.M=Id and directly computed the matrix multiplication per block.