A Free Triangle order is a partially ordered set in which every element can be represented by a triangle. All triangles lie between two parallel baselines, with each triangle intersecting each baseline in exactly one point. Two elements in the partially ordered set are incomparable if and only if their corresponding triangles intersect. A unit free triangle order is one with such a representation in which all triangles have the same area. In this paper, we present an example of a non-unit free triangle order.Key words geometric representation · free triangle order · unit vs. proper.The notion of a free triangle representation of a partially ordered set was first introduced by Laison [3] as a generalization of the ideas of interval and trapezoid representations. A free triangle representation assigns a triangle to each element of a partially ordered set, with all triangles having one vertex on each of two parallel baselines and a third 'free' vertex between the two baselines. In this paper, we examine and separate the classes of unit free triangle orders (those posets having a representation in which all triangles have the same area) and proper free triangle orders (those posets having a representation in which no free triangle contains another).

Abstract. Let a finite semiorder, or unit interval order, be given. All its numerical representations (when suitably defined) form a convex polyhedron. We show that the facets of the representation polyhedron correspond to the noses and hollows of the semiorder. Our main result is to prove that the coordinates of the vertices and the components of the extreme rays of the polyhedron are all integral multiples of a common value. The result follows from the total dual integrality of the system defining the representation polyhedron. Total dual integrality is in turn derived from a particular property of the oriented cycles in the directed graph of noses and hollows of a strictly upper diagonal step tableau. Our approach delivers also a new proof for the existence of the minimal representation, a concept originally discovered by Pirlot (1990). Finding combinatorial interpretations of the vertices and extreme rays of the representation polyhedron is left for future work.

Abstract:We explore the "oriented line graph" construction associated with a hypergraph, leading to a construction of pairs of strongly connected directed graphs whose adjacency operators have the same spectra. We give conditions on a hypergraph so that a hypergraph and its dual give rise to isospectral, but non-isomorphic, directed graphs. The proof of isospectrality comes from an argument centered around hypergraph zeta functions as defined by Storm. To prove non-isomorphism, we establish a Whitney-type result by showing that the oriented line graphs are isomorphic if and only if the hypergraphs are. ᭧

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