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Algebra I pp 253–278Cite as

Projective Spaces

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Abstract

Let V be a vector space of dimension (n + 1) over a field \(\mathbb{k}\). Besides the (n + 1)-dimensional affine space \(\mathbb{A}^{n+1} = \mathbb{A}(V )\), associated with V is the n- dimensional projective space \(\mathbb{P}_{n} = \mathbb{P}(V )\), called the projectivization of V. By definition, points of \(\mathbb{P}(V )\) are 1-dimensional vector subspaces in V, or equivalently, lines in \(\mathbb{A}(V )\) passing through the origin. To observe such points as usual “dots,” we have to use a screen, that is, an n-dimensional affine hyperplane in \(\mathbb{A}(V )\) that does not pass through the origin (see Fig. 11.1).

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Notes

  1. 1.

    See Sect. 6.5 on p. 142.

  2. 2.

    See Sect. 6.5 on p. 142.

  3. 3.

    Or just the infinity.

  4. 4.

    See Sect. 6.5.2 on p. 143.

  5. 5.

    These orientations will coincide when we overlap the planes by a continuous movement along the surface of the sphere.

  6. 6.

    It is infinite dimensional as soon V is infinite as a set.

  7. 7.

    Dotted things in (11.6) are not changed.

  8. 8.

    That is, two distinct points of \(\mathbb{P}(V )\).

  9. 9.

    Move x 1 2 to the right-hand side and divide both sides by x 2 + x 1.

  10. 10.

    See Proposition 3.5 on p. 50.

  11. 11.

    In Fig. 11.6, the tangent line through p crosses L at the point (0: 1: 0) lying at infinity.

  12. 12.

    See formula (11.17) on p. 268.

  13. 13.

    That is, all numbers p 1, p 2, p 3, p 4 are finite.

  14. 14.

    See Example 11.5 on p. 266.

  15. 15.

    That is, a statement about the annihilators of points and lines from Pappus’s theorem that holds in \(\mathbb{P}_{2}^{\times }\). It could start thus: “Given two distinct points and two triples of concurrent lines intersecting in these points….”

  16. 16.

    This is, lie in the same pencil.

  17. 17.

    See Example 11.6 on p. 266.

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Gorodentsev, A.L. (2016). Projective Spaces. In: Algebra I. Springer, Cham. https://doi.org/10.1007/978-3-319-45285-2_11

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