Points to remember
A prime number has only two factors (1 and the number itself). For example: 2, 3, 5, 7, ............. 97.
The prime numbers less than 100 are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97.
3, 5, 7 is known as prime triplet.
The prime number with a difference of 2 are known as twin primes. For example: 3, 5; 5, 7; 11, 13; 17, 19; 29, 31; 41, 43; 59, 61 and 71, 73.
There are 8 pairs of twin primes below 100.
There are 25 prime numbers below 100.
Every composite number can be written as the product of two or more (not necessarily distinct) primes.
There are certain prime numbers whose digits when reversed are also form a prime number.
For example: 13, 31; 17, 71; 37, 73; 79, 97.
The sum of these pairs of numbers are multiple of 11.
13 + 31 = 44
17 + 71 = 88
37 + 73 = 110
79 + 97 = 176
The smallest even composite number is 4.
The smallest odd composite number is 9.
The smallest two digit odd prime number is 11.
The largest two digit odd prime number is 97
The number of composite numbers from 1 to 100 are 74 (1 is excluded as it is neither a prime nor a composite number)
The number of composite numbers between 1 and 100 are 73.
The last two digit composite number is 99.
The smallest single digit composite number is 4.
The largest single digit composite number is 9.
The only even prime is 2.
The first odd prime number is 3.