Page 2 - LN 5.1

Clearly, 1, 2 and 4 are the common factors of 12, and 16

 Common Multiples: A common multiple is number that is a multiple of

two or more numbers.
Ex- The common multiples of 3 and 4 are 0, 12, 24 ….
 Perfect numbers: A number for which the sum of all its factors is equal
to twice the number is called a perfect number. The number 28 is a

perfect number. Its factors are 1, 2, 4, 7, 14 and 28. Their sum is 56
which is twice 28.

 Twin primes: Twin primes are pairs of primes having a difference of 2.
For example, 3 and 5 are twin primes.
 Co-Prime Numbers: Two numbers that do not have a common factor
other than 1 are said to be co-prime.

 Examples: (2, 3), (4, 5), (3, 7) and (4, 9) are co-primes.
 Prime Numbers: The numbers other than 1 whose only factors are 1
and the number itself are called prime number.

Ex- 5 = 1 x 5, 23 = 1 x 23
Note: 2 is the only even prime number; all other prime numbers are
odd.

 Composite Numbers: It is defined as a number having more than two
factors.
Ex- Factors of 9 are 1, 3 and 9. Therefore it is a composite number.

Note:
 1 is neither prime nor composite number.
 Any whole number greater than 1 is either a prime or composite

number.