code: private double calculateFunctionU( Coordinate p_i, Coordinate p_j ) {
          double distance = p_i.distance(p_j);
          return functionU(distance);
      }
docs: Calculates U function where distance = ||p<sub>i</sub>, p<sub>j</sub>|| (from source points)
  
  @param p_i p_i
  @param p_j p_j
  @return log(distance)*distance<sub>2</sub> or 0 if distance = 0