# Errata: In Exercise 6.4, need to assume A is invertible and C A^-1 B is nonzero

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Location: page 198, line 14

If the vector is not in the range of then additional assumptions are required in order to show the desired result: Assume that the matrix is invertible and . In addition, it should be shown that the *output* error (with ) goes to zero (as opposed to the state error).

(Contributed by R. M. Murray, 14 Aug 08)