Eigenvectors of a 2n x 2n Matrix
It has been known that the eigenvalues of a
certain 2n x 2n matrix can be obtained by use of 
two smaller matrices of order n which can be easily
constructed.  An algorithm is given to obtain the 
eigenvectors of the 2n x 2n matrix by use of
the eigenvectors of the smaller matrices.
CACM December, 1967
Charmonman, S.
