What Is A Composite Number?



A positive integer or a whole number that has at least three factors is a composite number. Of the three factors, two of them can be 1 and the number itself. A composite number can be differentiated from a prime number by finding the number of factors. A prime number has 2 factors which is one and the number itself whereas a composite number has at least three factors. It is essential to note that a positive integer can either be a prime number or a composite number and these are mutually exclusive.
Example: 4 is a composite number as it is a positive integer that has 2, 4 and 1 as factors.

Types Of Composite Numbers:
Perfect numbers
: It is a positive number which is equal to the sum of divisors. Ex: 6 is a perfect number as the divisors are 1,2, and 3. Which equals to 1+2+3 = 6
Another example is 28 and its divisors are 1,2,4, 7 and 14. And adding these 1+2+4+7+14 = 28 which is a perfect number.
Abundant numbers: It is a positive integer where the sum of factors is greater than the number. Ex: 12 is an abundant number as the factors 1,2,3,4,6 is > 12. Other abundant numbers are 18, 20, 24, 36 and 40.
Deficient Numbers: These are numbers that are opposite to the abundant numbers. The sum of factors is less than the number. Ex: 14, its factors are 1,2, and 7 and the sum of these 1+2+7 are less than 14.
Amicable Numbers: These are paired numbers where the sum of divisors not including the number itself is equal to the other number in the pair. Since they are always in pairs, these numbers are called amicable pairs or partners.
Ex: 220 and 284
The divisors of 220 are 1,2,4,5,10,11,21,22,44,55 and 110. Adding these gives 284
Divisors of 284 are 1,2,4,7,71, and 142 and add to 284

How To Find If The Number Is Composite Or Not?
The easiest way to find if the number is composite or not is through divisibility test. This test lets you find if a number is divisible by another without doing a lot of calculation. Check if the number can be divided by 2,3,5,7,11 and 13. Based on whether the number is odd or even you can decide on the factor to be chosen. Start with factor 2 if the number is even, 3 if the number is odd and with 5 if the number ends with 0 or 5. If using these factors the number cannot be divided then it is mostly a prime number.
Ex: 187
187 is an odd number and hence 2 is ruled out. Since the number does not end with 0 or 5, 5 is also not possible. 3 and 7 produces a decimal number as the answer. But 187/11 = 17 and thus the factors are 1,11, 17 and 187 making it a composite number.

Examples Of Non-Composite Numbers
5,2, and 7 are non-composite numbers.

5 has factors 1 and 5 which makes it a prime number.
2 has factors of 1 and 2 which is also a prime number
7 factors are 1 and 7 and are a prime number.