Labeling Schemes for Tetrahedron Equations and Dualities between Them

Zamolodchikov's tetrahedron equations, which were derived by considering the scattering of straight strings, can be written in three different labeling schemes: one can use as labels the states of the vacua between the strings, the states of the string segments, or the states of the particles at the intersections of the strings. We give a detailed derivation of the three corresponding tetrahedron equations and show also how the Frenkel-Moore equations fits in as a {\em nonlocal} string labeling. We discuss then how an analog of the Wu-Kadanoff duality can be defined between each pair of the above three labeling schemes. It turns out that there are two cases, for which one can simultaneously construct a duality between {\em all} three pairs of labelings.