1. Statista: Percentage of Households with Mobile Phones in the United Kingdom (UK) from 1996 to 2016 (2017).
https://www.statista.com/statistics/289167/mobile-phone-penetration-in-the-uk/
2. Pew Research Center: Mobile Fact Sheet (2017).
http://www.pewinternet.org/fact-sheet/mobile/
3. Smith, A.: Record Shares of Americans Now Own Smartphones, Have Home Broadband (2017).
http://www.pewresearch.org/fact-tank/2017/01/12/evolution-of-technology/
4. eMarketer: Czech Republic Just Tops Russia for Mobile Penetration (2017).
https://www.emarketer.com/Article/Czech-Republic-Just-Tops-Russia-Mobile-Penetration/1012047
5. Mehdipour, Y., Zerehkafi, H.: Mobile learning for education: benefits and challenges. Int. J. Comput. Eng. Res. 3(6), 93–101 (2013)