2 * 28 = 1 + 2 + 4 + 7 + 14 + 28 

import java.util.ArrayList; 
// 求解完美數(shù) 
public class PerfectNumber { 
  // 傳入一個值，求解至少多少個完美數(shù) 
  public static int[] lessThan(int number) { 
    int[] primes = Prime.findPrimes(number); 
 
    ArrayList list = new ArrayList(); 
     
    for(int i = 1; i <= number; i++) {  
      int[] factors = factor(primes, i);  
      if(i == fsum(factors))  
        list.add(new Integer(i)); 
    }  
 
    int[] p = new int[list.size()]; 
    Object[] objs = list.toArray();  
    for(int i = 0; i < p.length; i++) { 
      p[i] = ((Integer) objs[i]).intValue(); 
    } 
     
    return p; 
  } 
   
  // 分解因式 
  private static int[] factor(int[] primes, int number) {  
    int[] frecord = new int[number]; 
    int k = 0; 
     
    for(int i = 0; Math.pow(primes[i], 2) <= number;) {  
      if(number % primes[i] == 0) {  
        frecord[k] = primes[i];  
        k++;  
        number /= primes[i];  
      }  
      else  
        i++;  
    }  
 
    frecord[k] = number;  
 
    return frecord;  
  }  
 
  // 因式求和 
  private static int fsum(int[] farr) {  
    int i, r, s, q;  
 
    i = 0;  
    r = 1;  
    s = 1;  
    q = 1;  
 
    while(i < farr.length) {  
      do {  
        r *= farr[i];  
        q += r;  
        i++;  
      } while(i < farr.length - 1 && 
          farr[i-1] == farr[i]);  
      s *= q;  
      r = 1;  
      q = 1;  
    }  
 
    return s / 2;  
  } 
   
  public static void main(String[] args) { 
    int[] pn = PerfectNumber.lessThan(1000); 
    
    for(int i = 0; i < pn.length; i++) { 
      System.out.print(pn[i] + " "); 
    } 
     
    System.out.println(); 
  } 
}