code: public static double gammToSigma(double gamma)
      {
          if(gamma <= 0 || Double.isNaN(gamma) || Double.isInfinite(gamma))
              throw new IllegalArgumentException("gamma must be positive, not " + gamma);
          return 1/Math.sqrt(2*gamma);
      }
docs: Another common (equivalent) form of the RBF kernel is k(x, y) =
  exp(-&gamma;||x-y||<sup>2</sup>). This method converts the &gamma; value
  equivalent &sigma; value used by this class.
  @param gamma the value of &gamma;
  @return the equivalent &sigma; value