Jw. Hoffmann et Pa. Fuhrmann, ON BALANCED REALIZATIONS OF BOUNDED REAL AND POSITIVE REAL FUNCTIONS, Linear algebra and its applications, 245, 1996, pp. 107-146
Based on normalized coprime factorizations with respect to indefinite
metrics and the construction of suitable characteristic functions, the
Ober balanced canonical forms for the classes of bounded real and pos
itive real functions are derived. This uses a matrix representation of
the shift realization with respect to a basis related to sets of orth
ogonal polynomials.