  function Mar28()
% To see what is going on, assume p=1 and q=0 and consider
% Anew = V12'*A*V12. 
% Note that V12 = [G zeros(2,n-2) ; zeros(n-2,2) eye(n-2,n-2)] where 
% G is a 2x2 rotation. Look at V'*AV where A = [A11 A12; A12' A22] where
% A11 = A(1:2,1:2), A12 = A(1:2,3:n), and A22 = A(3:n,3:n). It follows that

%             G'*A11*G   G'*A12
%   Anew  =   
%             A12'*G       A22
%
% Thus, once you have G'*A12 you have A12*G = (G'*A12)'