With Default Namespace

import org.jdom.Element;
import org.jdom.Document;
import org.jdom.output.XMLOutputter;
import java.math.BigInteger;
import java.io.*;


public class UnprefixedFibonacci {

  public static void main(String[] args) {
   
    Element root 
     = new Element("math", "http://www.w3.org/1998/Math/MathML");	
  	      
    BigInteger low  = BigInteger.ZERO;
    BigInteger high = BigInteger.ONE;      
    
    for (int i = 0; i <= 25; i++) {
        
      Element mrow 
       = new Element("mrow", "http://www.w3.org/1998/Math/MathML");
      
      Element mi 
       = new Element("mi", "http://www.w3.org/1998/Math/MathML");
      mi.setText("f(" + i + ")"); 
      mrow.addContent(mi);
      
      Element mo 
       = new Element("mo", "http://www.w3.org/1998/Math/MathML");
      mo.setText("="); 
      mrow.addContent(mo);
      
      Element mn 
       = new Element("mn", "http://www.w3.org/1998/Math/MathML");
      mn.setText(low.toString());
      mrow.addContent(mn);

      BigInteger temp = high;
      high = high.add(low);
      low = temp;
      root.addContent(mrow);
      
    }
 
    Document doc = new Document(root);
    // serialize it into a file
    try {
      FileOutputStream out = new FileOutputStream("unprefixed_fibonacci.xml");
      XMLOutputter serializer = new XMLOutputter(); 
      serializer.output(doc, out);
      out.flush();	
      out.close();
    }
    catch (IOException e) {
      System.err.println(e);
    }

  }

}