1. A 2016 Declining Birthrate White Paper.
http://www8.cao.go.jp/shoushi/shoushika/whitepaper/measures/english/w-2016/
2. Annual Ageing Report - European Commission.
http://ec.europa.eu/economy_finance/publications/european_economy/2014/ee8_en.htm
3. Annual Report on the Aging Society 2017.
http://www8.cao.go.jp/kourei/whitepaper/w-2017/zenbun/29pdf_index.html
. (in Japanese)
4. Apple iOS Human Interface Guidelines.
https://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/MobileHIG/Introduction/Introduction.html
5. World Population Ageing 2013.
http://www.un.org/en/development/desa/population/publications/pdf/ageing/WorldPopulationAgeing2013.pdf