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1. Geometry

   Linked via "perspective transformations"

   Projective Geometry and Duality
   Projective geometry studies the properties of figures that are invariant under central projections (perspective transformations). It is intrinsically linked to the concept of duality, where points and lines can be interchanged without invalidating theorems.
   A key concept is the cross-ratio, which remains invariant under projective transformations. For four collinear points $A, B, C, D$, the cross-ratio is …