# How to determine equivalence relation on a set of ordered pair

I'm new to discrete maths and struggling with Relations, need to test the following relations for equivalence on the set X also find an equivalence class if it is equivalence relation.

Ques 1. X = R, x ∼ y <=> 2y < x2 + 1

• For reflexivity

x ∼ x <=> 2x < x2 + 1

for x = 1, the statement is false.

=> It is not reflexive.

• For symmetry[updated]

x ∼ y <=> 2y < x2 + 1

y ∼ x <=> 2x < y2 + 1

for x = 1 and y = 5 in 2y < x2 + 1

10 < 2, which is false

=> It is not symmetric.

• For transitivity[updated]

x ∼ y <=> 2y < x2 + 1

y ∼ z <=> 2z < y2 + 1

x ∼ z <=> 2y < x2 + 1

similar to symmetry

=> It is not transitive.

Ques 2. X = 2N, A ∼ B <=> (A ∪ B)' = N

I'm not sure how to interpret X = 2N and check if it is an equivalence relation.

• For the symmetry question, have you experimented with some simple (small positive and negative integer) values for $x$ and $y$? For question 2, $2^N$ probably means the power set of $N$, that is, the collection of subsets of $N$. – Steve Kass Nov 26 '16 at 21:20
• On the first question for reflexivity, $x=1$ (and only $x=1$) fails. – Joffan Nov 26 '16 at 21:25
• I edited the reflexivity. I still have to prove symmetry and transitivity. So if there's even one value for which the statement is false, the relation is not symmetric? eg. for x = 1 and y = 5 in 2y < x^2 + 1 gives 10 < 2 which is false. – Archetype2142 Nov 26 '16 at 21:26
• That's correct; you only need one counterexample. – Joffan Nov 26 '16 at 21:29

For 2, $X$ is the set of subsets of $\{1,2,3,\ldots,N\}$. $A$ and $B$ are two of those subsets. They are related if the complement of their union is all of $\{1,2,3,\ldots,N\}$. First use DeMorgan's law on the complement of the union to get ??? Then check RST.
• $A'$ includes $5$ in your example, but you are correct that it is not reflexive. You should not say $A \sim A=$ some set, you should say $A' \cap A'$=\$ some set. The first is not a set, while the second is. – Ross Millikan Nov 27 '16 at 0:12