Penrose R (2005): The road to reality: a complete guide to the laws of the universe. London: Vintage Books
p. 47 The 3 basic kinds of uniform plane geometry, as illustrated by Escher using tessellations of angels and devils. z#4811
(a) Elliptic case (positive curvature), (b) Euclidean case (zero curvature), and (c) Hyperbolic case (negative curvature) – in the conformal representation (Escher's *Circle Limit IV*, to be compared with Fig. 2.11).
DE
Die 3 Grundtypen der einheitlichen ebenen Geometrie, wie sie von Escher anhand der Mosaike von Engeln und Teufeln dargestellt wurden.
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See also Cognitive Radars