A prime number is a natural number greater than 1, which is only divisible by 1 and itself. First few prime numbers are : 2 3 5 7 11 13 17 19 23 …..
Some interesting fact about Prime numbers
- Two is the only even Prime number.
- Every prime number can be represented in form of 6n+1 or 6n-1 except the prime number 2 and 3, where n is a natural number.
- Two and Three are only two consecutive natural numbers that are prime.
- Goldbach Conjecture: Every even integer greater than 2 can be expressed as the sum of two primes.
- Wilson Theorem: Wilson’s theorem states that a natural number p > 1 is a prime number if and only if
(p - 1) ! ≡ -1 mod p OR (p - 1) ! ≡ (p-1) mod p
- Fermat’s Little Theorem: If n is a prime number, then for every a, 1 <= a < n,
an-1 ≡ 1 (mod n) OR an-1 % n = 1
- Prime Number Theorem: The probability that a given, randomly chosen number n is prime is inversely proportional to its number of digits, or to the logarithm of n.
- Lemoine’s Conjecture: Any odd integer greater than 5 can be expressed as a sum of an odd prime (all primes other than 2 are odd) and an even semiprime. A semiprime number is a product of two prime numbers. This is called Lemoine’s conjecture.
How we check whether a number is Prime or not?
A naive solution is to iterate through all numbers from 2 to n-1 and for every number check if it divides n. If we find any number that divides, we return false.
Time Complexity: O(n)
Using Recursion :
Recursion can also be used to check if a number between 2 to n – 1 divides n. If we find any number that divides, we return false.
Time Complexity :O(N), Space Complexity :O(N)
- Efficient solutions
Algorithms to find all prime numbers smaller than the N.
- Sieve of Eratosthenes
- Sieve of Eratosthenes in 0(n) time complexity
- Segmented Sieve
- Sieve of Sundaram
- Bitwise Sieve
- Recent Articles on Sieve!
More problems related to Prime number
- Find two distinct prime numbers with a given product
- Print all prime numbers less than or equal to N
- Recursive program for prime number
- Find two prime numbers with a given sum
- Find the highest occurring digit in prime numbers in a range
- Prime Factorization using Sieve O(log n) for multiple queries
- Program to print all prime factors of a given number
- Least prime factor of numbers till n
- Prime factors of LCM of array elements – GeeksforGeeks
- Program for Goldbach’s Conjecture
- Prime numbers and Fibonacci
- Composite Number
- Recent Articles on Prime Numbers!
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