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### Chapter 7 Relations : the second time around

Yen-Liang Chen

Dept of Information Management

National Central University

7.1. Relations revisited: properties of relations

- Definition 7.1. For sets A, B, any subset of AB is called a (binary) relation from A to B. Any subset of AA is called a binary relation on A .

Examples

- Ex 7.1
- Define the relation on the set Z by ab, if ab.
- For x, yZ and nZ+, the modulo n relation is defined by xy if x-y is a multiple of n.
- Ex 7.2: For x, y A, define xy if x is a prefix of y.

Ex 7.3.

- s1s2 if v(s1,x)=s2. Here, denotes the first level of reachability.
- s1s2 if v(s1,x1x2)=s2.Here, denotes the second level of reachability.
- 1-equivalence relation: s1E1s2 if w(s1,x)=w(s2,x) for xIA.
- k-equivalence relation: s1Eks2 if w(s1,x)=w(s2,x) for xIAk.
- equivalence relation: if two states are k-equivalent for all k.

reflexive

- Definition 7.2. A relation on a set A is called reflexive if for all xA, (x, x).
- Ex 7.4. For A={1, 2, 3, 4}, a relation AA will be reflexive if and only if {(1, 1), (2, 2), (3, 3), (4, 4)}.
- Ex 7.5. Given a finite set A with A=n, we have AA=n2, so there are relations on A. Among them, is reflexive.

symmetric

- Definition 7.3. A relation on a set A is called symmetric if (x, y) (y, x) for all x, yA.
- Ex 7.6. With A={1, 2, 3}, what properties do the following relations have?
- 1={(1, 2), (2, 1), (1, 3), (3, 1)}
- 2={(1, 1), (2, 2), (3, 3), (2, 3)}
- 3={(1, 1), (2, 2), (3, 3)}
- 4={(1, 1), (2, 2), (2, 3), (2, 3), (3, 2)}
- 5={(1, 1), (2, 3), (3, 3)}

symmetric

- To count the symmetric relations on A={a1, a2,…, an}.
- AA=A1A2, where A1={(a1, a1),…, (an, an)} and A2={(ai, aj)ij}.
- A1 contains n pairs, and A2 contains n2-n pairs.
- A2 contains (n2-n)/2 subsets Si,j of the form {(ai, aj), (aj, ai)ij}.
- So, we have totally symmetric relations on A.
- If the relations are both symmetric and reflexive, we have choices.

transitive

- Definition 7.4. A relation on a set A is called transitive if (x,y), (y,z) (x,z) for all x, y, zA.
- Ex 7.8. Define the relation on the set Z+ by ab if a divides b. This is a transitive and reflexive relation but not symmetric.
- Ex 7.9. Define the relation on the set Z by ab if ab0. What properties do they have?

anti-symmetric

- Definition 7.5. A relation on a set A is called anti-symmetric if (x, y) and (x, y) x=y for all x, yA.
- Ex 7.11. Define the relation (A, B) if AB. Then it is an anti-symmetric relation.
- Note that “not symmetric” is different from anti-symmetric.
- Ex 7.12. What properties do they have?
- ={(1, 2), (2, 1), (2, 3)}
- ={(1, 2), (2, 2)}

anti-symmetric

- To count the anti-symmetric relations on A={a1, a2,…, an}.
- AA=A1A2, where A1={(a1, a1),…, (an, an)} and A2={(ai, aj)ij}.
- A1 contains n pairs, and A2 contains n2-n pairs.
- A2 contains (n2-n)/2 subsets Si,j of the form {(ai, aj), (aj, ai)ij}.
- Each element in A1 can be selected or not.
- Each element in Si,j can be selected either one or none.
- So, we have totally anti-symmetric relations on A.
- Ex 7.13. Define the relation on the functions by f g if f is dominated by g (or fO(g)). What are their properties?

partial order relation

- Definition 7.6. A relation is called a partial order, if is reflexive, anti-symmetric and transitive.
- Ex 7.15. Define the relation on the set Z+ by ab if a divides b.

equivalence relation

- Definition 7.7. A relation is called an equivalence relation, if is reflexive, symmetric and transitive.
- Ex 7.16.(b)
- If A={1, 2, 3}, the following are all equivalence relations
- 1={(1, 1), (2, 2), (3, 3)}
- 2={(1, 1), (2, 2), (3, 3), (2, 3), (3,2)}
- 3={(1, 1), (1, 3), (2, 2), (3, 1), (3, 3)}
- 4={(1, 1), (1, 2), (1, 3), (2, 1), (2, 2), (2, 3), (3, 1), (3, 2), (3, 3)}

Examples

- Ex 7. 16(c). For a finite set A, A A is the largest equivalence relation on A. The equality relation is the smallest equivalence relation on A.
- Ex 7.16(d). Let f: AB be the onto function. Define the relation on A by ab if f(a)=f(b).
- Ex 7. 16(e). If is a relation on A, then is both an equivalence relation and a partial order relation iff is the equality relation on A.

7.2. Computer recognition: zero-one matrices and directed graphs

- Definition 7.8. Let relations 1AB and 2BC. The composite relation of 12 is a relation defined by 12={(x, z)y in B such that (x, y)1 and (y, z)2.
- Ex 7.17. Consider 1={(1, x), (2, x), (3, y), (3, z)} and 2={(w, 5), (x, 6)}. What is 12?

composite relation

- Ex 7. 18. Let A be the set of employees at a computer center, while B denotes a set of programming language, and C is a set of projects……
- Theorem 7.1.

1(23)= (12)3

the power of relation

- Definition 7.9. We define the powers of relation by (a) 1=; (b) n+1=n
- Ex 7.19. If ={(1, 2), (1, 3), (2, 4), (3, 2)}, then what is 2 and 3 and 4.

matrix representation

- A relation can be represented by an mn zero-one matrix.
- Ex 7.17. Consider 1={(1, x), (2, x), (3, y), (3, z)} and 2={(w, 5), (x, 6)}. What is 12?

matrix representation

- Ex 7.19. If ={(1, 2), (1, 3), (2, 4), (3, 2)}, then what is 2 and 3 and 4.

matrix representation

- Let A be a set with A=n and be a relation on A. If M() is the relation matrix for , then
- M()=0 if and only if =.
- M()=1 if and only if =AA.
- M(m)=[M()]m

less than

- Definition 7.11. Let E=(eij)mn F=(fij)mn be two zero-one matrices. We say that E precedes, or is less than , F, written as EF, if eij fij for all i, j.
- Ex 7.23. EF.

Theorem 7.2

- Let M denote the relation matrix for . Then

(A) R is reflexive if and only if InM.

(B) R is symmetric if and only if M=Mtr.

(C) R is transitive if and only if M2M.

(D) R is anti-symmetric if and only if MMtrIn.

Graph representation

- Definition 7.14. A directed graph can be denoted as G=(V, E), where V is the vertex set and E is the edge set.
- V={1,2,3,4,5}, E={(1,1),(1,2),(1,4),(3,2)}

Ex 7.27

- R={(1,1),(1,2),(2,3),(3,2),(3,3),(3,4),(4,2)}
- directed graph, undirected graph, connected, undirected cycle, directed cycle

Terms in graph

- strongly connected and loop-free
- disconnected graph, components

Graph representation for a relation

- Ex 7.30, Fig 7.8, is reflexive if and only if its directed graph contains a loop at each vertex
- Ex 7.31, Fig 7.9, is symmetric if and only if its directed graph may be drawn only by loops and undirected edges
- Ex 7.32, Fig 7.10, is anti-symmetric if and only if for any xy the graph contains at most one of the edges (x, y) or (y, x)
- Ex 7.33, Fig 7.11, a relation is an equivalence relation if and only if its graph consists of disjoint union of complete graphs augmented by loops at each vertex

7.3. Partial orders: Hasse Diagrams

- Definition: Let A be a set with a relation on A. The pair (A, ) is called a partially ordered set, or poset, if relation on A is partially ordered. If A is called a poset, we understand that there is a partially order on A that makes A into this set.

Examples of Poset

- Ex 7.34. Let A be the set of courses offered at a college. Define the relation on A by xy if x ,y are the same course or if x is a prerequisite for y.
- Ex 7.35. Define on A={1, 2, 3, 4} by xy if x divide y. Then (A, ) is a poset.
- Ex 7.36. Let A be the set of tasks that must be performed to build a house. Define the relation on A by xy if x ,y are the same task or if x must be performed before y.

Hasse Diagram

- If (A, ) is a poset, we construct a Hasse diagram for on A by drawing a line segment from x up to y, if
- xy
- there is no z such that xz and zy.
- Ex 7.38, Fig 7.18. The relation on (a) is the subset relation, while the relations on the others are the divide relations.

totally ordered

- Definition 7.16. If (A, ) is a poset, we say that A is totally ordered if for all x, y A either xy or yx. In this case, is called a total order.
- Ex 7.40.
- On the set N, the relation defined by xy if xy is a total order.
- The subset relation is a partial order but not total order.
- Fig 7.19 is a total order.

Topological sorting

- Given a Hasse diagram for a partial order relation , how to find a total order for which .

maximal and minimal

- Definition 7.17. If (A, ) is a poset, then x is a maximal element of A if for all aA, axx a. Similarly, y is a minimal element of A if for all bA, byb y .
- Ex 7.42.
- For the poset (P(U), ), U is the maximal and is the minimal.
- Let B be the proper subsets of {1, 2, 3}. Then we have multiple maximal elements for the poset (B, ).

Examples

- Ex 7.43. For the poset (Z, ), we have neither a maximal nor a minimal element. For the poset (N, ), we have no maximal element but a minimal element 0.
- Ex 7.44. How about the poset in Fig. 7.18? Do they have maximal or minimal elements?
- Theorem 7.3. If (A, ) is a poset and A is finite, then A has both a maximal and a minimal element.

Least and greatest

- Definition 7.18. If (A, ) is a poset, then x is a least element of A if for all aA, xa. Similarly, y is a greatest element of A if for all aA, ay.
- Ex 7.45.
- For the poset (P(U), ), U is the greatest and is the least.
- Let B be the nonempty subsets of {1, 2, 3}. Then we have U as the greatest maximal element and three minimal elements for the poset (B, ).
- Theorem 7.4. If poset (A, ) has a greatest or a least element, then that element is unique.

Lower bound and upper bound

- Definition 7.19. If (A, ) is a poset with BA, then xA is called a lower bound of B if xb for all bB. Likewise, yA is called an upper bound of B if by for all bB.
- An element xA is called a greatest lower bound of B if for all other lower bounds x of B we have xx. Similarly, an element xA is called a least upper bound of B if for all other upper bounds x of B we have xx.
- Theorem 7.5. If (A, ) is a poset and BA, then B has at most one lub (glb).

Examples

- Ex 7.47. Let U={1, 2, 3, 4} with A=P(U) and let be the subset relation on B. If B={{1}, {2}, {1, 2}}, then what are the upper bounds of B, lower bounds of B, the greatest lower bound and the least upper bound?
- Ex 7.48. Let be the “” relation on A. What are the results for the following cases?
- A=R and B=[0, 1]
- A=R and B={qQq2<2}
- A=Q and B={qQq2<2}

Lattice

- Definition 7.20. The poset (A, ) is called a lattice if for all x, yA the elements lub{x, y} and glb{x, y} both exist in A.
- Ex 7.49. For A=N and x, yN, define xy by xy. Then lub{x, y}=max{x, y} and glb{x, y}=min{x, y}. (N,) is a lattice.
- Ex 7.50. For the poset (P(U), ), if S, TU, we have lub{S, T}=ST and glb{S, T}=ST and it is a lattice.

7.4. Equivalence relation and partitions

- For any set A, the relation of equality is an equivalence relation on A.
- Let the relation on Z defined by xy if x-y is a multiple of 2, then is an equivalence relation on Z, where one contains all even integers and the other odd integers.

partition

- Definition 7.21. Given a set A and index set I, let AiA for iI. Then {Ai}iI is a partition of A if (a) A=iIAi and (b) AiAj= for ij.
- Ex 7.52, A={1,…,10}….
- Ex 7.53. A partition of R

equivalence class

- Definition 7.22. the equivalence class of x, denoted [x], is defined by [x]={yAyx}
- Ex 7.54. Define the relation on Z by xy if 4(x-y).
- Ex 7.55. Define the relation on Z by ab if a2=b2.

equivalence class

- Theorem 7.6. If is an equivalence relation on a set A and x, yA, then (a) x[x]; (b) xy if and only if [x]=[y]; and (c) [x]=[y] or [x][y]=.
- Ex 7.56.
- Let A={1, 2, 3, 4, 5}, ={(1, 1), (2, 2), (2, 3), (3, 2), (3, 3), (4, 4), (4, 5), (5, 4), (5, 5)}, Then, we have A=[1][2][4].
- Consider an onto function f:AB. f({1, 3, 7})=x; f({4, 6})=y; f({2, 5})=z. The relation defined on A by ab if f(a)=f(b). A=[1][4][2].
- Ex 7.58. If an equivalence relation on A={1, 2, 3, 4, 5, 6, 7} induces the partition A={1, 2} {3}{4, 5, 7}{6}, what is ?

Theorems

- Theorem 7.7. If A is a set, then any equivalence relation on A induces a partition of A, and any partition of A gives rise to an equivalence relation on A.
- Theorem 7.8. For any set A, there is one-to-one correspondence between the set of equivalence relations on A and the set of partitions of A.

7.5. Finite state machine: the minimization process

- Two finite state machines of the same function may have different number of internal states.
- Some of these states are redundant.
- A process of transforming a given machine into one that has no redundant internal states is called the minimization process.

1-equivalence

- For the states S, we define the relation E1 on S by s1E1s2 if w(s1, x)=w(s2, x) for all xI. The relation E1 is an equivalence relation on S, and it partitions S into subsets such that two states are in the same subset if they produce the same output for each xI.

k-equivalence

- For the states S, we define the k-equivalence relation Ek on S by s1Eks2 if w(s1, x)=W(s2, x) for all xIk. The relation Ek is an equivalence relation on S, and it partitions S into subsets such that two states are in the same subset if they produce the same output for each xIk.
- Finally, we call two states equivalent if they are k-equivalent for all k1.

Goal and tips

- Hence, our objective is to determine the partition of S induced by E and to select one state for each equivalent class.
- Observations:
- If two states are not 2-equivalent, they can not be 3-equivalent.
- For s1, s2S, where s1Eks2, we find that s1Ek+1s2 if and only if v(s1, x)Ekv(s2,x) for all xI.

The procedure for the minimization

- Set k=1. We determine the states that are 1-equivalent.
- Having determined Pk, we determine the states that are (k+1)-equivalent. Note that if s1Eks2, then s1Ek+1s2 if and only if v(s1, x)Ekv(s2,x) for all xI.
- If Pk+1=Pk, the process is completed.

Ex 7.60.

- the original table in Table 7.1 and P1:{s1}, {s2, s5, s6}, {s3, s4}
- Table 7.2. P2:{s1}, {s2, s5}, {s6}, {s3, s4}, P2=P3

refinement

- Definition 7.23. If P1 and P2 are partitions of set A, then P2 is called a refinement of P1, denoted as P2P1, if every cell of P2 is contained in a cell of P1. When P2P1 and P2P1, we write P2
- Theorem 7.9. In the minimization process, if Pk+1=Pk, then Pr+1=Pr for all rk+1.

distinguishing string

- If s1Eks2 but s1Ek+1s2, then we have a string x=x1x2…xkxk+1Ik+1 such that w(s1, x)w(s2, x) but w(s1, x1x2…xk)=w(s2, x1x2…xk). We call this string as distinguishing string.
- s1Ek+1s2x1I [v(s1, x1) Ek v(s2, x1)]

Ex 7.61

- s2E1s6 but s2E2s6.
- X=00 is the minimal distinguishing string for s2 and s6

Ex 7.62

- s1 and s4 are 2 equivalent but are not 3-equivalent.
- X=111 is the minimal distinguishing string for s1 and s4

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