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Saturday, 20 August 2016

Chapter 6 Exercise 28, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

**6.28 (Mersenne prime) A prime number is called a Mersenne prime if it can be written in the form 2p - 1 for some positive integer p. Write a program that finds all Mersenne primes with p … 31 and displays the output as follows:

p    2^p –1
--------
2    3
3    7
5    31
...

public class ProgrammingExercise6_28 {
 
 public static void main(String[] args) {
  // Printing heading
 
  System.out.printf("%-5s%8s\n", "p", "2^p-1");
 
  for (int i = 0; i < 31; i++) {
 
   int prime = mersennePrime(i);
 
   if (prime != 0) {
    System.out.printf("%-5d%8d\n", i, prime);
   }
 
  }
 
 }
 
 // Return Mersenn prime. Return 0 if n does not generate a prime number
 public static int mersennePrime(int n) {
 
  int prime = (int) Math.pow(2, n) - 1;
  if (isPrime(prime)) {
   return prime;
  }
 
  return 0;
 
 }
 
 /** Check whether number is prime */
 public static boolean isPrime(int number) {
  for (int divisor = 2; divisor <= number / 2; divisor++) {
   if (number % divisor == 0) { // If true, number is not prime
    return false; // number is not a prime
   }
  }
 
  return true; // number is prime
 }
 
}

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