Constructive Projective Geometry

Abstract: Of the great theories of classical mathematics, projective geometry, with its powerful concepts of symmetry and duality, has been exceptional in continuing to intrigue investigators. The challenge put forth by Errett Bishop (1928-1983), "Every theorem proved with nonconstructive methods presents a challenge: to find a constructive version, and to give it a constructive proof", motivates a large portion of current constructive work. This challenge can be answered by discovering the hidden constructive content of classical projective geometry. Here we briefly outline, with few details, recent constructive work on the real projective plane, and projective extensions of affine planes. Special note is taken of a number of interesting open problems that remain; these show that constructive projective geometry is still a theory very much in need of further effort. The Bishop-type constructive mathematics discussed in the present paper proceeds from a viewpoint well-nigh opposite that of either formal logic or recursive function theory.