James Dow Allen <firstname.lastname@example.org> might have writ, in news:XnsA22175B33BFDDjamesdowallen@18.104.22.168:
> 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? > > This problem statement can be considered ambiguous. > *But if you derive a good parametric form for (A B) you will know it.* > > (I already "know the answer." I post from curiosity: Is this a VERY easy > problem, or just an easy problem?)
James Dow Allen <email@example.com> might have writ, in news:XnsA2236C8E3E26Ajamesdowallen@22.214.171.124:
> I found it fruitful to introduce M = A + B; S = A - B
I will bump my own thread one time. I'm certain that this is an easy problem for many sci.math'ers, especially with the hint. The solution involves a particular type of matrix which might lead to an interesting discussion.