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Disjoint Set

Disjoint Set

The word disjoint means non-joining or non-similar. Disjoint sets are those sets that do not have any common element among them. For example, you have Cadbury chocolates, and your friend has Amul chocolates. There is no common element among them.

Therefore, these two sets of chocolates are disjoint. Disjoint sets are mostly used in constructing structures to find any similar problem due to certain situations. If any problem comes common, engineers try to eliminate that problem.

Mathematically, a disjoint set is represented by the intersection of two sets. For example, set A has elements {a, b}, and set B has {c, d}. Then from the definition of a disjoint set, A ∩ B = 𝛟. It is read as A intersection B is equal to phi. 𝛟 denotes a null set (an empty set), which has no common elements of sets A and B.

Method to find two disjoint sets

 1. Find all the possible elements of various sets whose disjoint condition we need to check.
 2. Check if any element is common between these sets or not, i.e., find their intersection.
 3. If even a single element comes out common, then the sets are not disjoint.
 4. If no element comes out common, then the sets are said to be disjoint.
 5. Therefore, we can write them as A ∩ B = 𝛟.

For example, P = {4, 6}, Q = {8, 16} and R = {13, 19}. Let us check the disjoint condition in these sets.
We need to find the intersection of these sets.
P ∩ Q = {4, 6} ∩ {8, 16}
P ∩ Q = { }
And,
Q ∩ R = {8, 16} ∩ {13, 19}
Q ∩ R = { }
And then the last pair,
P ∩ R = {4, 6} ∩ {13, 19}
P ∩ R = 𝛟
From the above example, we find the sets P, Q and R are disjoint.

Pairwise disjoint sets

Consider a set A. A has two subsets, P and Q. If the elements of P and Q are not the same, then P and Q will be referred to as pairwise disjoint sets or mutually disjoint sets. To meet the condition of being a pairwise disjoint set, the following condition must be obeyed:

P ∩ Q = 𝛟, P, Q ∈ A, P ≠ Q

Properties of intersection

  • Commutative: A ∩ B = B ∩ A
  • Associative: A ∩ (B ∩ C) = (A ∩ B) ∩ C
  • A ∩ ∅ = ∅
  • A ∩ B ⊆ A
  • A ∩ A = A
  • A ⊆ B if and only if A ∩ B = A

Example: The elements of set A are {2,5,6}, and those of set B are {4,7,8}. Find out whether these sets are disjoint or not.

Solution:

We know, if the intersection of two sets is null, then those sets will be disjoint.

Therefore,

A ∩ B = {2,5,6} ∩ {4,7,8}

As we see, there is no common element in both sets.

So, A ∩ B = { }, which means A intersection B is null.

Hence, proved A and B are disjoint sets.

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