Java - How to find the reciprocal of a BigDecimal?

Question

I'm trying to calculate digits of pi in java using the Chudnovsky algorithm. I'm using BigDecimal to be as precise as possible, but the formula is for 1/pi. I want to convert the value that I find to pi/1. I tried using Math.pow() and BigDecimal.pow() but couldn't figure it out. What would be the best way to find the reciprocal of a BigDecimal? Here's my code:

    public static void main(String[] args)
    {
      JFrame j = new JFrame("Pi");
      j.setSize(300,100);
      j.setVisible(true);
      JTextArea jt = new JTextArea();
      Container contain = new Container();
      FastPi fp = new FastPi();
      BigDecimal pi = new BigDecimal(0);
      for(long l=0; l<Long.MAX_VALUE; l++)
      {
        pi = BigDecimal.valueOf(12*(((Math.pow(-1,l)*fp.factorial(6*l)*
                                (545140134*l+13591409))/
                                (fp.factorial(3*l)*
                                Math.pow(fp.factorial(l),3)*
                                Math.pow(640320,3*l+3/2)))));
        contain = j.getContentPane();
        // tried to do pi^-1 below this comment
        jt.setText(String.valueOf(pi.pow(-1)) + " " + l);
        contain.add(jt);
      }
    }
    public long factorial(long f)
    {
      long result = 1;
      for(int i=2; i<=f; i++)
      {
        result *= i;
      }
      return result;
    }

Sorry I used L as a variable, it's colored different than the 1 though.

Answer 1

Divide 1 by your algorithm's result, preserving the scale of your result:

BigDecimal inversePi = yourAlgorithm();

BigDecimal pi = BigDecimal.ONE.divide(inversePi, inversePi.scale(), RoundingMode.HALF_UP);