A k-ranking of a graph G is a labeling of its vertices from
{1,…,k} such that any nontrivial path whose endpoints have the same
label contains a larger label. The least k for which G has a k-ranking is
the ranking number of G, also known as tree-depth. The list ranking number of
G is the least k such that if each vertex of G is assigned a set of k
potential labels, then G can be ranked by labeling each vertex with a label
from its assigned list. Rankings model a certain parallel processing problem in
manufacturing, while the list ranking version adds scheduling constraints. We
compute the list ranking number of paths, cycles, and trees with many more
leaves than internal vertices. Some of these results follow from stronger
theorems we prove about on-line versions of list ranking, where each vertex
starts with an empty list having some fixed capacity, and potential labels are
presented one by one, at which time they are added to the lists of certain
vertices; the decision of which of these vertices are actually to be ranked
with that label must be made immediately.Comment: 16 pages, 3 figure