docs: private static double specificShift(int idx, int n, double[] d, double[] e) {
  
      double g = d[idx];
      double p = (d[idx + 1] - g) / (2.0 * e[idx]);
      double r = hypot(p, 1.0);
      if (p < 0) {
        r = -r;
      }
      d[idx] = e[idx] / (p + r);
      d[idx + 1] = e[idx] * (p + r);
  
      double h = g - d[idx];
      for (int i = idx + 2; i < n; i++) {
        d[i] -= h;
      }
      return h;
  
    }