1. Equivalence Relations
A relation on set A is called an equivalence relation if it is:
reflexive,
symmetric, and
Transitive
We use the notation
a  b

2. Example
Let R be a relation on set A, where A = {1, 2, 3, 4, 5} and R = {(1,1), (2,2), (3,3), (4,4), (5,5), (1,3), (3,1)}
Is R an equivalence relation?

3. Another example:
Let A={set of lines } and R is a relation R={(L1,L2):L1 is parallel to L2}

4. Representing Relations Using Matrices
Let R be a relation from A to B
A = {a1, a2, … }
B = {b1, b2, … }
The zero-one matrix representing the relation R has a 1 as its entry when ai is related to bj and a 0 in this position if ai is not related to bj.

5. Example Let R be a relation from A to B A={a, b, c} B={d, e}
R={(a, d), (b, e), (c, d)}
Find the relation matrix for R

6. Relation Matrix Let R be a relation from A to B A={a, b, c} B={d, e}
R={(a, d), (b, e), (c, d)}
Note that A is represented by the rows and B by the columns in the matrix.
Cellij in the matrix contains a 1 iff ai is related to bj.

7. Relation Matrices and Properties
Let R be a binary relation on a set A and let M be the zero-one matrix for R.
R is reflexive iff Mii = 1 for all i
R is symmetric iff M is a symmetric matrix

8. Relation Matrices and Properties

9. Example
Suppose that the relation R on a set is represented by the matrix MR. ú û ù ê ë é = 1 R M
Is R reflexive and symmetric?

10. Example Is R reflexive? Is R symmetric? ú û ù ê ë é = 1 MR M
Is R reflexive?
Is R symmetric?
All the diagonal elements = 1, so R is reflexive.
The lower left triangle of the matrix = the upper right triangle, so R is symmetric.

11. Let A={a1,a2,a3} and B={b1,b2,b3,b4,b5} which ordered pairs are in the relation R represented by the matrix ??

12. intersection
union

13. Find the union and intersection of zero_one matrices
The intersection
The union