Split semiorders

A poset P = (X, 4 ) is a split semiorder if there are maps a , f : X --* • with a(x)<~f(x)<~a(x) + 1 for every x E X such that x -< y if and only if f ( x ) < a ( y ) and a(x) + 1 < f ( y ) . A split interval order is defined similarly with a(x)+ 1 replaced by b(x), a(x)<~ f(x)<~ b(x), such that x -< y if and only if f ( x ) < a ( y ) and b(x) < f ( y ) . We investigate these generalizations of semiorders and interval orders through aspects of their numerical representations, three notions of poset dimensionality, minimal forbidden posets, and inclusion relationships to other classes of posets, including several types of tolerance orders. (~) 1999 Elsevier Science B.V. All rights reserved