1. [1] D. Lamey, "The evolution of technology: past, present and future", Discover Tech, 2018 [Online]. Available: http://www.discovertec.com/blog/the-evolution-of-technology(accessed on 2019-11-02)
2. [2] D. Murphy, "2.4 bn smartphone users in 2017, says eMarketer", Mobile Marketing Magazine, 2017 [Online]. Available: https://mobilemarketingmagazine.com/24bn-smartphone-users-in-2017-says-emarketer(accessed on 2019-01-03)
3. [3] "Number of smartphone users worldwide from 2014 to 2020 (In millions)", Statista [Online]. Available: https://www.statista.com/statistics/330695/number-of-smartphoneusers-worldwide/
4. [4] C. Low, "It's no longer enough for budget phones to just be cheap", CES 2017, 2017 [Online]. Available: https://www.engadget.com/2017-01-08-budget-phones-ces-2017.html(accessed on 2019-10-04)
5. [5] "Condiçoes de oferta e de utilizaçao de serviços de comunicaçoes eletrónicas da MEO" [Online]. Available: https://conteudos.meo.pt/meo/documentos/condicoes-oferta-servicos/mod-c1001276.pdf(accessed on 2019-10-25) [in Portuguese]