Klein quadric

In mathematics, the lines of a 3-dimensional projective space, S, can be viewed as points of a 5-dimensional projective space, T. In that 5-space, the points that represent each line in S lie on a quadric, Q known as the Klein quadric.

If the underlying vector space of S is the 4-dimensional vector space V, then T has as the underlying vector space the 6-dimensional exterior square Λ2V of V. The line coordinates obtained this way are known as Plücker coordinates.

These Plücker coordinates satisfy the quadratic relation
 * $$p_{12} p_{34}+p_{13}p_{42}+p_{14} p_{23} = 0 $$

defining Q, where
 * $$p_{ij} = u_i v_j - u_j v_i$$

are the coordinates of the line spanned by the two vectors u and v.

The 3-space, S, can be reconstructed again from the quadric, Q: the planes contained in Q fall into two equivalence classes, where planes in the same class meet in a point, and planes in different classes meet in a line or in the empty set. Let these classes be C and C′. The geometry of S is retrieved as follows:
 * 1) The points of S are the planes in C.
 * 2) The lines of S are the points of Q.
 * 3) The planes of S are the planes in C′.

The fact that the geometries of S and Q are isomorphic can be explained by the isomorphism of the Dynkin diagrams A3 and D3.