Prime Numbers


A prime number is a whole number greater than 1, which is only divisible by 1 and itself. First few prime numbers are : 2 3 5 7 9 11 13 17 19 23 …..

Some interesting fact about Prime numbers

  1. 2 is the only even Prime number.
  2. every prime number can represented in form of 6n+1 or 6n-1, where n is natural number.
  3. 2, 3 are only two consecutive natural numbers which are prime too.
  4. Goldbach Conjecture: Every even integer greater than 2 can be expressed as the sum of two primes.
  5. GCD of a natural number with Prime is always one.
  6. 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
  7. Fermat’s Little Theorem: If n is a prime number, then for every a, 1 <= a < n,
    an-1 ≡ 1 (mod n)
    an-1 % n = 1 
  8. 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.

How we check whether a number is Prime or not?

  1. Naive solution.
    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.


    // A school method based C++ program to 
    // check if a number is prime
    #include <bits/stdc++.h>
    using namespace std;
    // function check whether a number 
    // is prime or not
    bool isPrime(int n)
        // Corner case
        if (n <= 1)
            return false;
        // Check from 2 to n-1
        for (int i = 2; i < n; i++)
            if (n % i == 0)
                return false;
        return true;
    // Driver Program to test above function
    int main()
        isPrime(11) ? cout << " true\n" : 
                      cout << " false\n";
        return 0;


    # A school method based Python3 program 
    # to check if a number is prime
    # function check whether a number 
    # is prime or not
    def isPrime(n):
        # Corner case
        if (n <= 1):
            return False
        # Check from 2 to n-1
        for i in range(2, n):
            if (n % i == 0):
                return False
        return True
    # Driver Program to test above function
    if isPrime(11):
        print ("true")
        print ("false")
    # This code is contributed by Sachin Bisht

    Time complexity :O(n)

  2. Efficient solutions

Algorithms to find all prime number smaller the N.

More problems related to Prime number

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