1. K Appel, and W Haken: Every planar map is four colourable: Part I: discharging. Illinois Journal of Mathematics, 21: 429–490, 1977.
2. C Arcelli, and G Sanniti di Baja: Endoskeleton and exoskeleton of digital figures, an effective procedure. In V Cappellini and R Marconi, editors, Advances in Image Processing and Pattern Recognition, pages 224–228, North Holland, Amsterdam, 1986.
3. H Asada, and M Brady: The curvature primal sketch. IEEE Transactions on Pattern Analysis and Machine Intelligence, 8 (1): 2–14, 1986.
4. H H Atkinson, Gargantini, I, and T R S Walsh: Counting regions, holes and their nesting level in time proportional to the border. Computer Vision, Graphics, and Image Processing, 29: 196–215, 1985.
5. J Babaud, A P Witkin, M Baudin, and R O Duda: Uniqueness of the Gaussian kernel for scale-space filtering. IEEE Transactions on Pattern Analysis and Machine Intelligence, 8 (1): 26–33, 1986.