CHAP. 2] RELATIONS 33
(b) LetR 5 be the relation of congruence modulo 5 on the setZof integers denoted byx≡y(mod 5)This means that the differencex−yis divisible by 5. ThenR 5 is an equivalence relation onZ. The quotient
setZ/R 5 contains the following five equivalence classes:A 0 ={...,− 10 ,− 5 , 0 , 5 , 10 ,...}
A 1 ={...,− 9 ,− 4 , 1 , 6 , 11 ,...}
A 2 ={...,− 8 ,− 3 , 2 , 7 , 12 ,...}
A 3 ={...,− 7 ,− 2 , 3 , 8 , 13 ,...}
A 4 ={...,− 6 ,− 1 , 4 , 9 , 14 ,...}Any integerx, uniquely expressed in the formx= 5 q+rwhere 0≤r<5, is a member of the equivalence
classAr, where r is the remainder.As expected,Zis the disjoint union of equivalence classesA 1 ,A 2 ,A 3 ,A 4.
Usually one chooses{ 0 , 1 , 2 , 3 , 4 }or{− 2 ,− 1 , 0 , 1 , 2 }as a set of representatives of the equivalence classes.2.9Partial Ordering Relations
A relationRon a setSis called apartial orderingor apartial orderofSifRis reflexive, antisymmetric, and
transitive. A setStogether with a partial orderingRis called apartially ordered setorposet. Partially ordered
sets will be studied in more detail in Chapter 14, so here we simply give some examples.EXAMPLE 2.14(a) The relation⊆of set inclusion is a partial ordering on any collection of sets since set inclusion has the three
desired properties. That is,(1)A⊆Afor any setA.
(2) IfA⊆BandB⊆A, thenA=B.
(3) IfA⊆BandB⊆C, thenA⊆C.(b) The relation≤on the setRof real numbers is reflexive, antisymmetric, and transitive. Thus≤is a partial
ordering onR.(c) The relation “adividesb,”writtena|b,is a partial ordering on the setNof positive integers. However, “a
dividesb” is not a partial ordering on the setZof integers sincea|bandb|aneed not implya=b. For
example, 3|−3 and− 3 |3 but 3=−3.2.10 n-ARY RELATIONS
All the relations discussed above were binary relations. By ann-ary relation, we mean a set of ordered
n-tuples. For any setS, a subset of the product setSnis called ann-ary relation onS. In particular, a subset of
S^3 is called aternary relationonS.
EXAMPLE 2.15
(a) LetLbe a line in the plane. Then “betweenness” is a ternary relationRon the points ofL; that is,(a,b,c)∈R
ifblies betweenaandconL.(b) The equationx^2 +y^2 +z^2 =1 determines a ternary relationTon the setRof real numbers. That is, a triple
(x,y,z)belongs toTif(x,y,z)satisfies the equation, which means(x,y,z)is the coordinates of a point in
R^3 on the sphereSwith radius 1 and center at the originO=( 0 , 0 , 0 ).