code: @SuppressWarnings("WeakerAccess")
      public static double pow2(double x) {
          final double exponent = Math.floor(x) - 1;
          x = x - exponent;
          double m = 3 - sqrt(7 - 3 * x);
          return Math.pow(2, exponent + 1) * m;
      }
docs: Computes an approximate value of 2^x. This is done as an exact inverse of #approxLog2 so
  that bin boundaries can be computed exactly.
  
  @param x The power of 2 desired.
  @return 2^x approximately.