Summary: A Competitive Analysis of the List Update Problem
We consider the question of lookahead in the list update problem: What improvement
can be achieved in terms of competitiveness if an on-line algorithm sees not only the present
request to be served but also some future requests? We introduce two di erent models of
lookaheadandstudy the list update problemusing these models. We develop lowerbounds on
the competitiveness that can be achieved by deterministic on-line algorithmswith lookahead.
Furthermore we present on-line algorithmswith lookahead that are competitive against static
o -line algorithms.
In recent years there has been tremendous interest in the competitive analysis of on-line algo-
rithms. Many on-line problems have been studied in areas such as resource allocation, data
structures, graph problems, scheduling and navigation. In the context of data structures, the
list update problem is of fundamental importance. The problem is to maintain a set of items as
an unsorted linear list. A list of n items is given. A list update algorithm is presented with a
sequence of requests, where each request speci es an item of the list. In order to serve a request,
a list update algorithm must access the requested item, i.e., it has to start at the front of the
list and search linearly through the items until the desired item is found. Accessing the ith