Separation relation

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In mathematics, a separation relation is a formal way to arrange a set of objects in an unoriented circle. It is defined as a quaternary relation S(abcd) satisfying certain axioms, which is interpreted as asserting that a and c separate b from d.[1]

Whereas a linear order endows a set with a positive end and a negative end, a separation relation forgets not only which end is which, but also where the ends are. In this way it is a final, further weakening of the concepts of a betweenness relation and a cyclic order. There is generally nothing else that can be forgotten: up to the relevant sense of interdefinability, these three relations are the only nontrivial reducts of the ordered set of rational numbers.[2]


  1. ^ Huntington, Edward V. (July 1935), "Inter-Relations Among the Four Principal Types of Order", Transactions of the American Mathematical Society 38 (1): 1–9, doi:10.1090/S0002-9947-1935-1501800-1, retrieved 8 May 2011 
  2. ^ Macpherson, H. Dugald (2011), "A survey of homogeneous structures", Discrete Mathematics, doi:10.1016/j.disc.2011.01.024, retrieved 28 April 2011