Prime Number

Prime number-
An integer p which is not 0 or ± 1 and is divisible by no integer except ±1 and itself is called Prime number. Donzager stated “Upon looking at prime numbers one has the feeling of being in presence of one the inexplicable phenomena one site of creation.”

2 is the only prime number. There is no perfect technique which can tell us immediately the numbers of prime between two numbers. Though Erastothenes, a great Greek mathematician suggested a method to find the primes between two numbers called Sieve of Erastothenes.

Erastothenes(276-195BC) gave a golden rule though simple it is time consuming which states “First write down the number from 2 to N. Remove all the multiples of 2, 3 and continue the process until all the multiple of primes not greater than √N has been removed.”

Suppose we have to find the primes below 30, first we find the square root of 30 which is 5.477.so we need to remove the entire multiple up to primes 5.


2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Hence the primes below 30 are 2,3,5,7,11,13,17,19,23 and,29.


Properties of Primes:-
i) Every natural number greater than 1 has at least one prime.

ii) For every integer a, a^p –a is always divisible by prime p

p 2 2 2 2 3 3 3 3 5 5 5 5
a 2 3 4 5 2 3 4 5 2 3 4 5
a^p-a 2 6 12 20 6 24 60 120 30 240 1020 3120

iii) If N is a prime number then 1+ (N-1)! Is always divisible by N. This is called Wilson theorem.
For N = 2, 1+ (2-1)! = 2 us divisible by 2.
iv) Every odd integers is the sum of a prime and a power of 2, this was claimed in 1848 by De Polignac.
55 = 47+23

v) Every even number except 2 is the sum of two prime numbers.
e.g. 8 = 3 + 5, 16 = 13 + 3 ,60 = 13 + 47 etc. This is called Goldbach conjecture.

vi) Every even integer greater than 4 can be written as the sum of two odd prime numbers. 4 = 2 + 2 = 1 + 3, 6 = 3 + 3 = 1 + 5 …

vii) There is always at least one prime number between n and 2n-2 provided n is greater than 3. If n = 4, 2n-2 = 6 then obviously 5 lies in between 4 and 6. This conjecture was stated by Bertrand (1822-1903).

Interesting Facts

1. A pair of prime numbers is said to be a twin prime pair if the two numbers differ by any 2.
i.e. (3,5)(5,7)(11,13)(17,19)(29,31)(41,43)(59,61)(71,73)etc.
All the twin primes are of the form 6n-1, 6n+1.

2. Between 9,999,900 to 10, 000, 000, there are only 9 prime numbers.
9,999,901; 9,999,907; 9,999,929; 9,999,931 ;9,999,937 ;9,999,943 ;9,999,971 ;9,999,973 ;9,999,991.
But in the next 100 integer from 10,000,000 to 10, 000,100 there are only two primes 10, 000, 019 and 10,000,079.

3. (p,p+2,p+4)is called prime triplet if all numbers are primes

4. The largest known prime number is of 6, 320, 430, digits and was found by Michael Shafer in Dec 2003. It would need 1400 to 1500 pages to write.

5. A gap of 803 composite numbers exits between primes 90874329411493 and 90874329412297 which was found in 1989 by J.Yong and A.Poster



Dr. Rajesh Kumar Thakur