# Java program for Brun’s Constant and Twin Prime number

Twin Primes are the prime numbers with a difference of 2, e.g., (3, 5), (5, 7), (11, 13), (17, 19) … etc. The sum of reciprocals of the twin primes up to a limit, converges to sum known as Brun’s Constant.

Example : Say for Inputted number 13 and its twin primes are (3, 5), (5, 7), (11, 13) and its sum of reciprocals is (1/3 + 1/5) + (1/5 + 1/7) + (1/11 + 1/13) =  1.044022644022644

Write a program to create Primes which include methods:
boolean isPrime(int n) – Checks whether n is prime or not

And check twinPrime and also calculate Brun’s constant.

```import java.util.Scanner;
class BrunsConstant
{
static boolean isPrime(int n)
{
int c=0;
for(int i=1; i<=n; i++)
{
if(n%i==0)
c++;
}
if(c==2)
return true;
else
return false;
}

public static void main(String args[])
{
Scanner in = new Scanner(System.in);
System.out.println("Enter a number");
int n = in.nextInt();
double b=0;
System.out.println("Twin Primes");
for(int i=1; i<=n-2; i++)
{
//condition for twin prime
if(isPrime(i)&&isPrime(i+2))
{
System.out.println(i + "\t" + (i+2));
b += (double)1/i + (double)1/(i+2);
}
}
System.out.println("Value of Brun's Constant upto " +n+ " is : " +b);
}
}```