1. They Make Apps.
http://theymakeapps.com
2. Bushell D (2011) In search of the best CAPTCHA.
https://www.smashingmagazine.com/2011/03/in-search-of-the-perfect-captcha/
3. Adafruit Blog.
https://blog.adafruit.com
4. Shet V (2014) Are you a robot? Introducing “NoCAPTCHA reCAPTCHA”.
https://security.googleblog.com/2014/12/are-you-robot-introducing-no-captcha.html
5. Invisible reCAPTCHA.
https://www.google.com/recaptcha/intro/
comingsoon/invisible.html