A generalized chordal metric making strong stabilizability a robust   property
  An abstract chordal metric is defined on linear control systems described by their transfer functions. Analogous to a previous result due to Jonathan Partington ("Robust control and approximation in the chordal metric", in Robust Control, LNCIS 183, Springer, 1992) for H^infty, it is shown that strong stabilizability is a robust property in this metric. 
Amol Sasane
math.OC math.FA