Is 1991 a prime number?

The number 1991 has got multiple factors, that are capable of dividing the number completely without leaving any remainder. Thus, the number 1991 is a non-prime number. The factors of 1991 include 1, 11, 181, and 1991.

A number to be a prime number should follow the criteria, which is that it should not have factors more than 2. Here, 1991 has more than 2 factors, hence making it a composite number.

Given below are a few ways that can be used to find prime or composite numbers.

The different methods we can use to check if a number is a prime number are explained below.

1. Counting Divisors Method
2. Divisibility Test
3. Prime Number Chart
4. Prime Factorization

For the counting divisors method, it is to be checked whether the number is divisible by any numbers other than 1 and the number itself.

The counting divisors method for 1991 would simply be

Divisors of 1991 = 1, 11, 181, 1991
Number of divisors = 4

The number 1991 can be considered composite.

In the division method, we try to divide the number by any of the prime numbers. If we cannot, then it is considered a prime number.

In the divisibility method, the prime number only has 2 divisors, which are 1 and itself.

The divisors of 1991 are 1, 11, 181, and 1991.

Thus, 1991 consists of 4 factors that divide it completely without any remainder.

The prime number chart is the list of prime numbers starting from 2 to infinity.

The list of prime numbers under 100 are;
2, 3, 5, 7, 11, 13, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97.

1991 is not present in the list, it is not a prime number.

This method is only used for a non-prime number/composite number. Since 1991 is a composite number, the prime factorization for 1991 is:

Factors of 1991 = 11 × 181

Prime Number: A natural number greater than 1 that has exactly two distinct positive divisors: 1 and itself. Example: 2, 3, 5.

Composite Number: A number that has more than two divisors, meaning it can be divided by numbers other than 1 and itself. Example: 4, 6, 1991.

Divisibility Test: A method used to check if one number is divisible by another without leaving a remainder. It helps identify factors of a number.

Prime Factorization: The expression of a composite number as a product of prime numbers. Example: 1991 = 11 × 181.

Divisors: The numbers that divide a given number exactly, without leaving a remainder. The divisors of 1991 are 1, 11, 181, and 1991.