Abstract
Global convergence of a number of discrete-time adaptive control algorithms with persistently exciting inputs has been established in the literature. A key element of the convergence proof is the block-invariance feature of the algorithm. While the parameters are estimated at every sample, the controller parameters remain invariant throughout a block of samples and are updated only at the end of each such block. The necessity of this block-invariance feature for guaranteeing convergence is established here by a counterexample.

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