[1] reCAPTCHAの無茶振りとその理由 - しろもじメモランダム (5/21/2012, 6:26:32 AM 版) <http://d.hatena.ne.jp/mashabow/20110930/1317400512>

[2] Google Online Security Blog: Are you a robot? Introducing “No CAPTCHA reCAPTCHA” (1/21/2015, 5:46:51 PM 版) <http://googleonlinesecurity.blogspot.jp/2014/12/are-you-robot-introducing-no-captcha.html>