1.Euclid The Elements. New York: Dover, 2nd edn. Translated with introduction and commentary by Thomas L. Heath. 2.Hilbert, David Foundations of Geometry. La Salle, Ill.: Open Court Publishing Co., 4th edn. Translated by Leo Unger. 3.Miller, Nathaniel. CDEG User’s Manual. Availablefrom 4.Miller, Nathaniel Euclid and his twentieth century rivals: Diagrams in the logic of Euclidean geometry. Stanford, CA: CSLI Press. This poster briefly describes the computer proof system CDEG, version 2.0. CDEG stands for “Computerized Diagrammatic Euclidean Geometry.” This computer proof system implements a diagrammatic formal system for giving diagram-based proofs of theorems of Euclidean geometry that are similar to the informal proofs found in Euclid’s Elements. It is based on the diagrammatic formal system FG, which is described in detail in the book Euclid and his Twentieth Century Rivals: Diagrams in the Logic of Euclidean Geometry. That book also describes an earlier version of CDEG; however, CDEG has evolved significantly since the publication of the book. In particular, a beta version of CDEG is now publicly available, and can be downloaded from the website given above. I encourage anyone who is interested in this research to download CDEG and to try it out for themselves. CDEG diagrams are based on the idea that all of the information contained in a diagram is reflected in the planar topology of the diagram, and two diagrams with the same planar topology are equivalent. (Two diagrams have the same planar topology if one can be stretched into the other while staying in the plane.) A second, more complicated diagram is shown below. This diagram, which occurs in the proof of Euclid’s first Proposition, shows two intersecting circles that contain an equilateral triangle. CDEG diagrams contain several numbered elements: * circles, which represent points; solid line segments, which represent straight lines; dotted line segments, which represent circles; A bold line around the edge, called the frame, which encloses our diagram; and rectangles, which label regions in the diagram. A sample diagram is shown below. This diagram shows two points, the segment connecting them, and the circle drawn on this segment. CDEG: Computerized Diagrammatic Euclidean Geometry 2.0 Nathaniel Miller University of Northern Colorado, Greeley, CO, USA CDEG Diagrams CDEG vs. Euclid’s Elements Selected References Note that different circles and lines are identified by the use of different colors. CDEG’s commands are closely related to Euclid’s Postulates, Common Notions, and Definitions from the Elements, so that most proofs from the Elements can be formalized using CDEG. The two tables shown here summarize these connections. Why a Computer System? CDEG is essentially a computer implementation of the formal system FG described in [14]. Why would we want to implement an existing formal system on a computer? 1.To demonstrate that this system really is completely formal and works as advertised. This worry is not just academic. A number of other published diagrammatic formal systems for Euclidean geometry that were not implemented on a computer have later turned out to be unsound, inconsistent, and did not work as their authors intended. A computer implementation shows that the system is fully formalized and gives the opportunity to check that it works as intended. 2.To make it available for other people to try out. Few people are going to take the time to try to understand exactly how a formal system works. A computer implementation makes it much more likely that they will try it out. If you’ve read this far, you may be interested enough to want to try it. I encourage you to try it out! 3. In order to explore exactly what it can prove. This system is intended to formalize Euclid’s proof methods from the Elements, and it should be able to duplicate most or all of the proofs in the first four books of the Elements. Without a working computer implementation, this claim is essentially unverifiable. With the computer implementation, verifying this is now an ongoing research project.