The quasitransitive relation x≤⁠5/4⁠y. Its symmetric and transitive part is shown in blue and green, respectively.

The mathematical notion of quasitransitivity is a weakened version of transitivity that is used in social choice theory and microeconomics. Informally, a relation is quasitransitive if it is symmetric for some values and transitive elsewhere. The concept was introduced by Sen (1969) to study the consequences of Arrow's theorem.

Formal definition

A binary relation T over a set X is quasitransitive if for all a, b, and c in X the following holds:

( a T ⁡ b ) ∧ ¬ ( b T ⁡ a ) ∧ ( b T ⁡ c ) ∧ ¬ ( c T ⁡ b ) ⇒ ( a T ⁡ c ) ∧ ¬ ( c T ⁡ a ) . {\displaystyle (a\operatorname {T} b)\wedge \neg (b\operatorname {T} a)\wedge (b\operatorname {T} c)\wedge \neg (c\operatorname {T} b)\Rightarrow (a\operatorname {T} c)\wedge \neg (c\operatorname {T} a).}

If the relation is also antisymmetric, T is transitive.

Alternately, for a relation T, define the asymmetric or "strict" part P:

( a P ⁡ b ) ⇔ ( a T ⁡ b ) ∧ ¬ ( b T ⁡ a ) . {\displaystyle (a\operatorname {P} b)\Leftrightarrow (a\operatorname {T} b)\wedge \neg (b\operatorname {T} a).}

Then T is quasitransitive if and only if P is transitive.

Examples

Preferences are assumed to be quasitransitive (rather than transitive) in some economic contexts. The classic example is a person indifferent between 7 and 8 grams of sugar and indifferent between 8 and 9 grams of sugar, but who prefers 9 grams of sugar to 7. Similarly, the Sorites paradox can be resolved by weakening assumed transitivity of certain relations to quasitransitivity.

Properties

  • A relation R is quasitransitive if, and only if, it is the disjoint union of a symmetric relation J and a transitive relation P. J and P are not uniquely determined by a given R; however, the P from the only-if part is minimal.
  • As a consequence, each symmetric relation is quasitransitive, and so is each transitive relation. Moreover, an antisymmetric and quasitransitive relation is always transitive.
  • The relation from the above sugar example, {(7,7), (7,8), (7,9), (8,7), (8,8), (8,9), (9,8), (9,9)}, is quasitransitive, but not transitive.
  • A quasitransitive relation needn't be acyclic: for every non-empty set A, the universal relation A×A is both cyclic and quasitransitive.
  • A relation is quasitransitive if, and only if, its complement is.
  • Similarly, a relation is quasitransitive if, and only if, its converse is.

See also

  • Sen, A. (1969). "Quasi-transitivity, rational choice and collective decisions". Rev. Econ. Stud. 36 (3): 381–393. doi:. JSTOR . Zbl .
  • Frederic Schick (Jun 1969). "Arrow's Proof and the Logic of Preference". Philosophy of Science. 36 (2): 127–144. doi:. JSTOR . S2CID .
  • Amartya K. Sen (1970). Collective Choice and Social Welfare. Holden-Day, Inc.
  • Amartya K. Sen (Jul 1971). (PDF). The Review of Economic Studies. 38 (3): 307–317. doi:. JSTOR . Archived from (PDF) on 2016-09-10.
  • A. Mas-Colell and H. Sonnenschein (1972). (PDF). The Review of Economic Studies. 39 (2): 185–192. doi:. JSTOR . S2CID . Archived from (PDF) on 2018-04-12.
  • D.H. Blair and R.A. Pollak (1982). "Acyclic Collective Choice Rules". Econometrica. 50 (4): 931–943. doi:. JSTOR .
  • Bossert, Walter; Suzumura, Kotaro (Apr 2005). (PDF) (Technical Report). Université de Montréal, Hitotsubashi University Tokyo. Archived from (PDF) on 2018-04-12.
  • Bossert, Walter; Suzumura, Kotaro (Mar 2009). (PDF). Social Choice and Welfare (Technical Report). 39 (2–3). Université de Montréal, Waseda University Tokyo: 323–334. doi:. S2CID . Archived from (PDF) on 2018-04-12.
  • Bossert, Walter; Suzumura, Kōtarō (2010). Consistency, choice and rationality. Harvard University Press. ISBN 978-0674052994.
  • Alan D. Miller and Shiran Rachmilevitch (Feb 2014). (PDF) (Working paper). University of Haifa.