Interval Orders with Two Interval Lengths

A poset P=(X,≺) has an interval representation if each x∈X can be assigned a real interval Ix so that x≺y in P if and only if Ix lies completely to the left of Iy. Such orders are called interval orders. In this paper we give a surprisingly simple forbidden poset characterization of those posets that have an interval representation in which each interval length is either 0 or 1. In addition, for posets (X,≺) with a weight of 1 or 2 assigned to each point, we characterize those that have an interval representation in which for each x∈X the length of the interval assigned to x equals the weight assigned to x. For both problems we can determine in polynomial time whether the desired interval representation is possible and in the affirmative case, produce such a representation.