code: public void setSigma(double sigma)
      {
          if(sigma <= 0 || Double.isNaN(sigma) || Double.isInfinite(sigma))
              throw new IllegalArgumentException("Sigma must be a positive constant, not " + sigma);
          this.sigma = sigma;
          this.sigmaSqrd2Inv = 0.5/(sigma*sigma);
      }
docs: Sets the kernel width parameter, which must be a positive value. Larger
  values indicate a larger width
  
  @param sigma the sigma value