Abstract
In a recent paper (Johnson 1985) the theory of disturbance-accommodating control was used to derive a new family of adaptive controllers. A critical step in the derivation presented by Johnson (1985) involves the assumption that certain higher-order perturbation terms are negligible with respect to terms that are linear in the perturbations. In the present paper the validity of that approximation is investigated and the special (singular) condition under which the assumption fails is identified and characterized in terms of a new observability-like determinant.

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