Unifying unitary and hyperbolic transformations
Adam W. Bojańczyk, Sanzheng Qiao, Allan O. Steinhardt · Linear Algebra and its Applications · 2000
In this paper, we describe unified formulas for unitary and hyperbolic reflections and rotations, and show how these unified transformations can be used to compute a Hermitian triangular decomposition R̂HDR̂ of a strongly nonsingular indefinite matrix  given in the form Â=X1HX1+αX2HX2,α=±1. The unification is achieved by the introduction of signature matrices which determine whether the applicable transformations are unitary, hyperbolic, or their generalizations. We derive formulas for the condition numbers of the unified transformations, propose pivoting strategies for lowering the condition number of the transformations, and present a unified stability analysis for applying the transformations to a matrix.