Kang Won Lee and  Jung Won Lee,  Passenger Flow Characteristics of Seoul Metropolitan Subway Network.   journal   = {International Journal of Industrial and Manufacturing Engineering}, [online]. World Academy of Science, Engineering and Technology.
    May 2018, vol. 139(7). 891 - 894
    [viewed 18 April 2021]. Available from: https://publications.waset.org/pdf/10009191.