Unitary perturbation theory: a generalization of two-by-two rotations

A generalization of the two-by-two rotation technique is proposed, permitting a whole row (column) of the matrix to be treated simultaneously. The method is based on the explicit analytical evaluation of the matrix exponent representing a symmetric combination of the individual rotations. Besides constructing the unitary transformation matrices, a new orthogonalization algorithm is also proposed. The resulting “unitary perturbation theory” and orthogonalization method may be useful in different areas. Received: 15 November 1999 / Accepted: 30 January 2000 / Published online: 19 April 2000