It has recently become clear that many control problems are too difficult t
o admit analytic solutions. New results have also emerged to show that the
computational complexity of some "solved" control problems is prohibitive,
Many of these control problems can be reduced to decidability problems or t
o optimization questions. Even though such questions may be too difficult t
o answer analytically, or may not be answered exactly given a reasonable am
ount of computational resources, researchers have shown that we can "approx
imately'' answer these questions "most of the time", and have "high confide
nce" in the correctness of the answers. (C) 2001 Elsevier Science Inc. All
rights reserved.