A class of illusion in which an object which is physically
unrealizable is apparently depicted.
More than 100 papers have been written about impossible figures (Kulpa 1987), and Escher made extensive use of them in some of his well-known drawings.
Cerf, C. "A Family of Impossible Figures Studied by Knot Theory." http://www.mi.sanu.ac.yu/vismath/cerf/.Cowan,
T. M. "The Theory of Braids and the Analysis of Impossible Figures."
J. Math. Psych.11, 190-212, 1974.Cowan, T. M. "Supplementary
Report: Braids, Side Segments, and Impossible Figures." J. Math. Psych.16,
254-260, 1977.Cowan, T. M. "Organizing the Properties of Impossible
Figures." Perception6, 41-56, 1977.Cowan, T. M.
and Pringle, R. "An Investigation of the Cues Responsible for Figure Impossibility."
J. Exper. Psych. Human Perception Performance4, 112-120, 1978.Cowan,
T. M. "Turning the Penrose Triangle Inside Out." J. Math. Psych.26,
252-262, 1982.Ernst, B. Adventures
with Impossible Figures. Stradbroke, England: Tarquin, 1987.Harris,
W. F. "Perceptual Singularities in Impossible Pictures Represent Screw
Dislocations." South African J. Sci.69, 10-13, 1973.Fineman,
M. The
Nature of Visual Illusion. New York: Dover, pp. 119-122, 1996.Jablan,
S. "Impossible Figures." http://members.tripod.com/~modularity/impos.htm.Kulpa,
Z. "Are Impossible Figures Possible?" Signal Processing5,
201-220, 1983.Kulpa, Z. "Putting Order in the Impossible."
Perception16, 201-214, 1987.Rakov, D. "Impossible
Objects." http://www.rakov.de/ARTE.HTM.Rausch,
J. "Impossible Objects." http://www.johnrausch.com/PuzzleWorld/cat/io000.htm.Robinson,
J. O. "Impossible Figures." In The
Psychology of Visual Illusion. New York: Dover, p. 176, 1998.Sugihara,
K. "Classification of Impossible Objects." Perception11,
65-74, 1982.Terouanne, E. "Impossible Figures and Interpretations
of Polyhedral Figures." J. Math. Psych.27, 370-405, 1983.Terouanne,
E. "On a Class of 'Impossible' Figures: A New Language for a New Analysis."
J. Math. Psych.22, 24-47, 1983.Thro, E. B. "Distinguishing
Two Classes of Impossible Objects." Perception12, 733-751, 1983.Wilson,
R. "Stamp Corner: Impossible Figures." Math. Intell.13,
80, 1991.