In this paper we extend previous results for the single-input/single-o
utput (SISO) mixed sensitivity problem for distributed parameter syste
ms to a more general setting in which the transfer function of the pla
nt to be controlled has both irrational stable and irrational unstable
parts. These developments were originally motivated by the computatio
n of the infimal H(infinity)-norm of the weighted mixed sensitivity fo
r the irrational transfer function model of a damped flexible beam.