1. Automobile Inspection & Registration Information Association (2015) Car Ownership Rate in Each Prefecture. https://www.airia.or.jp/publish/statistics/trend.html [Accessed: June 10, 2018] (in Japanese).
2. Cabinet Office, Government of Japan (2017) Declining Birthrate White Paper, FY2016. http://www8.cao.go.jp/shoushi/shoushika/whitepaper/measures/w-2017/29webhonpen/index.html [Accessed: June 10, 2018] (in Japanese).
3. Center for Spatial Information Science, The University of Tokyo (2015) Geocoding service for CSV formatted file on WWW. http://newspat.csis.u-tokyo.ac.jp/geocode-cgi/geocode.cgi?action=start [Accessed: October 31, 2017] (in Japanese).
4. Council for Gender Equality of Cabinet Office, Government of Japan (2013) About day-care for sick children, the 8th conference of child and child-feeding support, November 25. http://www8.cao.go.jp/shoushi/shinseido/meeting/kodomo_kosodate/b_8/pdf/s2-2.pdf [Accessed: June 10, 2018] (in Japanese).
5. Dijkstra, E.W. (1959) A note on two problems in connexion with graphs. Numerische Mathematik, 1, 269-271.