El Ahmad, A. S., Yan, J. & Tayara, M. (2011). The Robustness of Google CAPTCHAs (). School of Computer Science, Newcastle University, UK .

Melnik, S., Gubarev, A., Long, J. J., Romer, G., Shivakumar, S., Tolton, M. & Vassilakis, T. (2010). Dremel: interactive analysis of web-scale datasets. Proceedings of the VLDB Endowment, 3, 330--339.