Cone (algebraic geometry)
# Cone (algebraic geometry)

In algebraic geometry, a cone is a generalization of a vector bundle. Specifically, given a scheme X, the relative Spec

    $C=\operatorname {Spec} _{X}R$

of a quasi-coherent graded $O_X-algebra$ R is called the cone or affine cone of R. Similarly, the relative Proj

    $\mathbb {P} (C)=\operatorname {Proj} _{X}R$

is called the projective cone of C or R.

Note: The cone comes with the $\mathbb {G} _{m}$-action due to the grading of R; this action is a part of the data of a cone (whence the terminology).

## Examples

* If X = Spec k is a point and R is a homogeneous coordinate ring, then the affine cone of R is the (usual) affine cone over the projective variety corresponding to R.
* If $R=\bigoplus _{0}^{\infty }I^{n}/I^{n+1}$ for some ideal sheaf I, then $\operatorname {Spec} _{X}R$ is the normal cone to the closed scheme determined by I.
* If $R=\bigoplus _{0}^{\infty }L^{\otimes n}$ for some line bundle L, then $\operatorname {Spec} _{X}R$ is the total space of the dual of L.
* More generally, given a vector bundle (finite-rank locally free sheaf) E on X, if $R=Sym(E^*)$ is the symmetric algebra generated by the dual of E, then the cone $\operatorname {Spec} _{X}R$ is the total space of E, often written just as E, and the projective cone $\operatorname {Proj} _{X}R$ is the projective bundle of E, which is written as $\mathbb {P} (E)$.
* Let ${\mathcal {F}}$ be a coherent sheaf on a Deligne–Mumford stack X. Then let $C({\mathcal {F}}):=\operatorname {Spec} _{X}(\operatorname {Sym} ({\mathcal {F}})).$ For any $f:T\to X$, since global Spec is a right adjoint to the direct image functor, we have: $C({\mathcal {F}})(T)=\operatorname {Hom} _{{\mathcal {O}}_{X}}(\operatorname {Sym} ({\mathcal {F}}),f_{*}{\mathcal {O}}_{T})$; in particular, $C({\mathcal {F}})$ is a commutative group scheme over X.
* Let R be a graded ${\mathcal {O}}_{X}$-algebra such that $R_{0}={\mathcal {O}}_{X}$ and $R_{1}$ is coherent and locally generates R as $R_{0}$-algebra. Then there is a closed immersion

       $\operatorname {Spec} _{X}R\hookrightarrow C(R_{1})$
    given by $\operatorname {Sym} (R_{1})\to R$. Because of this, $C(R_{1})$ is called the abelian hull of the cone $\operatorname {Spec} _{X}R.$ For example, if $R=\oplus _{0}^{\infty }I^{n}/I^{n+1}$ for some ideal sheaf I, then this embedding is the embedding of the normal cone into the normal bundle.

### Computations

Consider the complete intersection ideal $(f,g_{1},g_{2},g_{3})\subset \mathbb {C} [x_{0},\ldots ,x_{n}]$ and let $X$ be the projective scheme defined by the ideal sheaf ${\mathcal {I}}=(f)(g_{1},g_{2},g_{3})$. Then, we have the isomorphism of ${\mathcal {O}}_{\mathbb {P} ^{n}}$-algebras is given by

    $\bigoplus _{n\geq 0}{\frac {{\mathcal {I}}^{n}}{{\mathcal {I}}^{n+1}}}\cong {\frac {{\mathcal {O}}_{X}[a,b,c]}{(g_{2}a-g_{1}b,g_{3}a-g_{1}c,g_{3}b-g_{2}c)}}$

## Properties

If $S\to R$ is a graded homomorphism of graded $O_X-algebras,$ then one gets an induced morphism between the cones:

    $C_{R}=\operatorname {Spec} _{X}R\to C_{S}=\operatorname {Spec} _{X}S$.

If the homomorphism is surjective, then one gets closed immersions $C_{R}\hookrightarrow C_{S},\,\mathbb {P} (C_{R})\hookrightarrow \mathbb {P} (C_{S}).$

In particular, assuming $R_0$ = $O_X,$ the construction applies to the projection $R=R_{0}\oplus R_{1}\oplus \cdots \to R_{0}$ (which is an augmentation map) and gives

    $\sigma :X\hookrightarrow C_{R}$.

It is a section; i.e., $X{\overset {\sigma }{\to }}C_{R}\to X$ is the identity and is called the zero-section embedding.

Consider the graded algebra R[t] with variable t having degree one: explicitly, the n-th degree piece is

    $R_{n}\oplus R_{n-1}t\oplus R_{n-2}t^{2}\oplus \cdots \oplus R_{0}t^{n}$.

Then the affine cone of it is denoted by $C_{R[t]}=C_{R}\oplus 1$. The projective cone $\mathbb {P} (C_{R}\oplus 1)$ is called the projective completion of $C_R.$ Indeed, the zero-locus t = 0 is exactly $\mathbb {P} (C_{R})$ and the complement is the open subscheme $C_R.$ The locus t = 0 is called the hyperplane at infinity.

## O(1)

Let R be a quasi-coherent graded $O_X-algebra$ such that $R_0$ = $O_X$ and R is locally generated as $O_X-algebra$ by $R_1.$ Then, by definition, the projective cone of R is:

    $\mathbb {P} (C)=\operatorname {Proj} _{X}R=\varinjlim \operatorname {Proj} (R(U))$

where the colimit runs over open affine subsets U of X. By assumption R(U) has finitely many degree-one generators $x_i's.$ Thus,

    $\operatorname {Proj} (R(U))\hookrightarrow \mathbb {P} ^{r}\times U.$

Then $\operatorname {Proj} (R(U))$ has the line bundle O(1) given by the hyperplane bundle ${\mathcal {O}}_{\mathbb {P} ^{r}}(1)$ of $\mathbb {P} ^{r}$; gluing such local O(1)'s, which agree locally, gives the line bundle O(1) on $\mathbb {P} (C)$.

For any integer n, one also writes O(n) for the n-th tensor power of O(1). If the cone $C=Spec_XR$ is the total space of a vector bundle E, then O(-1) is the tautological line bundle on the projective bundle P(E).

Remark: When the (local) generators of R have degree other than one, the construction of O(1) still goes through but with a weighted projective space in place of a projective space; so the resulting O(1) is not necessarily a line bundle. In the language of divisor, this O(1) corresponds to a Q-Cartier divisor.

