Canonical line bundle

From Wikipedia, the free encyclopedia

The canonical or tautological line bundle on a projective space appears frequently in mathematics, often in the study of characteristic classes. Note that there is possible confusion with the theory of the canonical class in algebraic geometry; for which reason the name tautological is preferred in some contexts. See also tautological bundle.

1 Definition
2 Facts
3 See also
4 References

[edit] Definition

Form the cartesian product $\mathbf{R} P^n\times\mathbf{R}^{n+1}$ , with the first factor denoting real projective n-space. We consider the subset

$E(\gamma^n):=\big\{(\{\pm\;x\},v)\in\mathbf{R}P^n\times\mathbf{R}^{n+1}:v=\lambda x,\;\lambda\in\mathbf{R}\big\}.$

We have an obvious projection map $\pi:E(\gamma^n)\to\mathbf{R}P^n$ , with $(\{\pm\;x\},v)\mapsto\{\pm\;x\}$ . Each fibre of $π$ is then the line through $x$ and $- x$ inside Euclidean (n+1)-space. Giving each fibre the induced vector space structure we obtain the bundle

$\gamma^n:=(E(\gamma^n)\to\mathbf{R}P^n),$

the canonical line bundle over $\mathbf{R}P^n$ .

[edit] Facts

$γ n$ is locally trivial but not trivial, for $n\geq 1$ .

In fact, it is straightforward to show that, for $n = 1$ , the canonical line bundle is none other than the well-known bundle whose total space is the Möbius band. For a full proof of the above fact, see ^[1].