The query complexity of finding local minima in the lattice

Abstract

In this paper we study the query complexity of finding local minimum points of a boolean function. This task occurs frequently in exact learning algorithms for many natural classes, such as monotone DNF, O (log n)-term DNF, unate DNF, and decision trees. On the negative side, we prove that any (possibly randomized) algorithm that produces a local minimum of a function f chosen from a sufficiently rich concept class, using a membership oracle for f, must ask (n2) membership queries in the worst case. In particular, this lower bound applies to the class of decision trees. A simple algorithm is known that achieves this lower bound. On the positive side, we show that for the class O (log n)-term DNF finding local minimum points requires only (n log n) membership queries (and more generally (tn) membership queries for t-term DNF with tn). This efficient procedure improves the time and query complexity of known learning algorithms for the class O (log n)-term DNF. 2001 Elsevier Science.