SUBSET OF  NULL SET 

If null set is a super set, then it has only one subset. That is { }.

More clearly, null set is the only subset to itself. But it is not a proper subset.

Because, { } = { }.

Therefore, A set which contains only one subset is called null set.

Apart from the stuff "Subset of null set", let us know some other important stuff about subsets of a set.

Subset of a Set

A set X is a subset of set Y if every element of X is also an element of Y.

In symbol we write

x ⊆ y

Reading Notation :

Read ⊆ as "X is a subset of Y" or "X is contained in Y".

Read  as "X is a not subset of Y" or "X is not contained in Y".

Proper Subset

A set X is said to be a proper subset of set Y if X ⊆ Y and X ≠ Y.

In symbol, we write X ⊂ Y.

Reading Notation :

Read X ⊂ Y as "X is proper subset of Y".

The figure given below illustrates this.

Power Set

The set of all subsets of A is said to be the power set of the set A.

Reading Notation :

The power set of A is denoted by P(A).

Super Set

A set X is said to be a proper subset of set Y if X ⊆ Y and X ≠ Y.

In symbol, we write X ⊂ Y.

Here,

Y is called super set of X

Formula to Find Number of Subsets

If A is the given set and it contains n number of elements, we can use the following formula to find the number of subsets.

Number of subsets = 2

Formula to find the number of proper subsets :

Number of proper subsets = 2n - 1

Cardinality of Power Set

We already know that the set of all subsets of A is said to be the power set of the set A and it is denoted by P(A).

If A contains "n" number of elements, then the formula for cardinality of power set of A is

n[P(A)] = 2n

Note :

Cardinality of power set of A and the number of subsets of A are same.

Null Set is a Subset or Proper Subset

Null set is a proper subset for any set which contains at least one element.

For example, let us consider the set A = {1}.

It has two subsets. They are { } and {1}.

Here null set is proper subset of A. Because null set is not equal to A.

Solved Problems

Problem 1 :

Let A = {1, 2, 3, 4, 5} and B = {5, 3, 4, 2, 1}. Determine whether B is a proper subset of A.

Solution :

If B is the proper subset of A, every element of B must also be an element of A and also B must not be equal to A. 

In the given sets A and B, every element of B is also an element of A. But B is equal A.

Hence, B is the subset of A, but not a proper subset. 

Problem 2 :

Let A = {1, 2, 3, 4, 5} and B = {1, 2, 5}. Determine whether B is a proper subset of A.

Solution :

If B is the proper subset of A, every element of B must also be an element of A and also B must not be equal to A. 

In the given sets A and B, every element of B is also an element of A.

And also But B is not equal to A.

Hence, B is a proper subset of A.

Problem 3 :

Let A = {1, 2, 3, 4, 5} find the number of proper subsets of A.

Solution :

Let the given set contains "n" number of elements.

Then, the formula to find number of proper subsets is

= 2n - 1

The value of n for the given set  A is 5.

Because the set A = {1, 2, 3, 4, 5} contains five elements.

Number of proper subsets 25 - 1

= 32 - 1

= 31

Hence, the number of proper subsets of A is 31.

Problem 4 :

Let A = {1, 2, 3} find the power set of A.

Solution :

We know that the power set is the set of all subsets.

Here, the given set A contains 3 elements.

Then, the number of subsets = 23 = 8.

Therefore, 

P(A) = {{1}, {2}, {3}, {1, 2}, {2, 3}, {1, 3}, {1, 2, 3}, { }}

Problem 5 :

Let A = {a, b, c, d, e} find the cardinality of power set of A.

Solution :

The formula for cardinality of power set of A is given below. 

n[P(A)] = 2ⁿ

Here n stands for the number of elements contained by the given set A. 

The given set A contains five elements. So n = 5.

Then, we have

n[P(A)] = 25

n[P(A)] = 32

Hence, the cardinality of the power set of A is 32.

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