M.Aleknovich et al. have recently proposed a model of algorithms, called BT model, which generalizes both the priority model of Borodin, Nielson and Rackoff, as well as a simple dynamic programming model by Woeginger. BT model can be further divided into three kinds of fixed, adaptive and fully adaptive ones. They have proved exponential time lower bounds of exact and approximation algorithms under adaptive BT model for Knapsack problem. Their exact lower bound is $Omega(2^{0.5n}/sqrt{n})$, in this paper, we slightly improve the exact lower bound to about $Omega(2^{0.69n}/sqrt{n})$, by the same technique, with related parameters optimized.
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