K. Åström, and B. Wittenmark. Addison-Wesley Series in Electrical and Computer Engineering: Control Engineering Addison-Wesley, Boston, US-MA, reprinted, corrected edition, (1989)
B. Wittenmark. 5th IFAC Symposium on Adaptive Systems in Control and Signal Processing, page 67--72. Budapest, HU, International Federation of Automatic Control, (1995)
J. Rayner. volume 195 of Progress in Astronautics and Aeronautics, chapter 11, page 217--230. AIAA American Institute Aeronautics and Astronautics, Reston, US-VA, (2001)