# Advances in Discrete and Computational Geometry by Chazelle B., Goodman J.E., Pollack R. (eds.) By Chazelle B., Goodman J.E., Pollack R. (eds.)

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And can (with some eﬀort) be described as the connected component of the identity element in the set J−1 of invertible elements. The set of positive elements is called the positive cone Cone(J) of the formally real Jordan algebra J. Positive Cone Theorem. The positive cone C := Cone(J) of an n-dimensional formally real Jordan algebra J is an open regular convex cone in J ∼ = Rn that is self-dual with respect to the positive deﬁnite bilinear trace form σ(x, y) := tr(Vx•y ) = tr(Vx,y ). The linear operators Ux for x ∈ C generate a group G of linear transformations acting transitively on C.

Coordinates In the spirit of Descartes’s program of analytic geometry, we can introduce “algebraic coordinates” into any projective plane using a coordinate system, an ordered 4-point χ = {X∞ , Y∞ , 0, 1}. Here we interpret the plane as the completion of an aﬃne plane by a line at inﬁnity L∞ := X∞ ∨ Y∞ , with 0 as origin and 1 as unit point, X := 0 ∨ X∞ , Y := 0 ∨ Y∞ the X, Y axes, and U := 0 ∨ 1 the unit line. The coordinate set consists of the aﬃne points x of U , together with a symbol ∞. We introduce coordinates (coordinatize the plane) for the aﬃne points P , points at inﬁnity P∞ , aﬃne lines L, and line at inﬁnity L∞ via P → (x, y), P∞ = Y∞ → (n), P∞ = Y∞ → (∞), L L Y → [m, b], Y → [a], L∞ → [∞], where the coordinates of points are x = πX (P ) := P Y ∧ U, y = πY (P ) := P X ∧U, n = πY (1, n) = πY (P∞ ∨0)∧(1 Y ) , and the coordinates of lines are a = L∧U, b = πY (0, b) = πY L∧Y , m = πY (1, m) = πY (0 L)∧(1 Y ) .

The Freudenthal–Tits Magic Square Jacques Tits discovered in 1966 a general construction of a Lie algebra FT (C, J), starting from a composition algebra C and a Jordan algebra J of “degree 3,” which produces E8 when J is the Albert algebra and C the Cayley algebra. Varying the possible ingredients leads to a square arrangement that had been noticed earlier by Hans Freudenthal: The Freudenthal–Tits Magic Square: FT (C, J) C \ J R H3 (R) H3 (C) H3 (H) H3 (K) R 0 A1 A2 C3 F4 C 0 A 2 A 2 ⊕ A 2 A5 E6 H A1 C3 A5 A6 E7 K G 2 F4 E6 E7 E8 Some have doubted whether this is square, but no one has ever doubted that it is magic.