1. Kato, N., Suzuki, M., Omachi, S., Aso, H., Nemoto, Y.: A handwritten character recognition system using directional element feature and asymmetric Mahalanobis distance. IEEE Trans. on Pattern Analysis and Machine Intelligence 21(3), 258–262 (1999)
2. Lu, Y.: Machine Printed Character Segmentation-An Overview. Pattern Recognition 28(1), 67–80 (1995)
3. von Ahn, L., Blum, M., Langford, J.: Telling humans and computers apart (automatically), CMU Tech. Report CMUCS-02-117 (2002)
4. Lecture Notes in Computer Science;L. Ahn von,2003
5. Mori, G., Malik, J.: Recognizing Objects in Adversarial Clutter: Breaking a Visual CAPTCHA. In: Proc. IEEE Conf. Computer Vision and Pattern Recognition, vol. 1, pp. 134–141 (2003)