Abstract: We establish a relationship between the online
mistake-bound model of learning and resource-bounded dimension. This
connection is combined with the Winnow algorithm to obtain new results
about the density of hard sets under adaptive reductions. This
improves previous work of Fu (1995) and
Lutz
and Zhao (2000), and solves one of Lutz and Mayordomo's "Twelve
Problems in Resource-Bounded Measure" (1999).