1. L. von Ahn, M. Blum, J. Langford, CAPTCHA: using hard AI problems for security (2003)
2. M Naor. Verification of human in the loop or Identification via Turing Test, http://www.wisdom.weizmann.ac.il/~naor/PAPERS/human.ps.
3. US Patent no. 6195698. Method for selectively restricting access to computer systems, http://www.freepatentsonline.com/6195698.html.
4. E. Bursztein, M. Martin, J. Mitchell, in Proceedings of the 18th ACM Conference on Computer and Communications Security, Chicago, Illinois, USA. Text-based CAPTCHA strengths and weaknesses (2011)
5. I.J. Goodfellow, Y. Bulatov, J. Ibarz, S. Arnoud, V.D. Shet, Multi-digit number recognition from street view imagery using deep convolutional neural networks. CoRR abs/1312.6082 (2014)