Grassmann bundle

In algebraic geometry, the Grassmann d-plane bundle of a vector bundle E on an algebraic scheme X is a scheme over X:

$p:G_d(E)\to X$

such that the fiber $p^{-1}(x)=G_d(E_x)$ is the Grassmannian of the d-dimensional vector subspaces of $E_x$. For example, $G_1(E)=\mathbb{P}(E)$ is the projective bundle of E. In the other direction, a Grassmann bundle is a special case of a (partial) flag bundle. Concretely, the Grassmann bundle can be constructed as a Quot scheme.

Like the usual Grassmannian, the Grassmann bundle comes with natural vector bundles on it; namely, there are universal or tautological subbundle S and universal quotient bundle Q that fit into

$0\to S\to p^{*}E\to Q\to 0$.

Specifically, if V is in the fiber p⁻¹(x), then the fiber of S over V is V itself; thus, S has rank r = d = dim(V) and ${\wedge }^{d}S$ is the determinant line bundle. Now, by the universal property of a projective bundle, the injection ${\wedge }^{r}S\to p^{*}({\wedge }^{r}E)$ corresponds to the morphism over X:

$G_d(E)\to \mathbb{P}({\wedge }^{r}E)$,

which is nothing but a family of Plücker embeddings.

The relative tangent bundle T_Gd(E)/X of G_d(E) is given by

$T_{G_d(E)/X}=\mathrm{Hom}(S,Q)=S^{\vee }\otimes Q,$

which morally is given by the second fundamental form. In the case d = 1, it is given as follows: if V is a finite-dimensional vector space, then for each line $l$ in V passing through the origin (a point of $\mathbb{P}(V)$), there is the natural identification (see Chern class#Complex projective space for example):

$\mathrm{Hom}(l,V/l)=T_l\mathbb{P}(V)$

and the above is the family-version of this identification. (The general care is a generalization of this.)

In the case d = 1, the early exact sequence tensored with the dual of S = O(-1) gives:

$0\to {𝒪}_{\mathbb{P}(E)}\to p^{*}E\otimes {𝒪}_{\mathbb{P}(E)}(1)\to T_{\mathbb{P}(E)/X}\to 0$,

which is the relative version of the Euler sequence.

References

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