nLab line object


This entry discusses line objects, their multiplicative groups and additive groups in generality. For the traditional notions see at affine line.


Topos Theory

topos theory



Internal Logic

Topos morphisms

Extra stuff, structure, properties

Cohomology and homotopy

In higher category theory




In linear algebra

In linear algebra over a field kk, the line is the field kk regarded as a vector space over itself. More generally, a line is a vector space isomorphic to this, i.e. any 1-dimensional kk-vector space.

The real line \mathbb{R} models the naive intuition of the geometric line in Euclidean geometry. See also at complex line.

In many contexts of modern mathematics, however, line implicitly refers to the complex line \mathbb{C} (which as a real vector space is the complex plane!). For instance this is the line usually meant when speaking of line bundles.

Over an algebraic theory

We discuss here how in the context of spaces modeled on duals of algebras over an algebraic theory TT, there is a canonical space 𝔸 T\mathbb{A}_T which generalizes the real line \mathbb{R}.


For TT (the syntactic category of) any Lawvere theory we have that Isbell conjugation

(𝒪Spec):TAlg opSpec𝒪Sh(C) (\mathcal{O} \dashv Spec)\colon T Alg^{op} \stackrel{\overset{\mathcal{O}}{\leftarrow}}{\underset{Spec}{\to}} Sh(C)

relates TT-algebras to the sheaf topos over duals TCTAlg opT \hookrightarrow C \subset T Alg^{op} of TT-algebras, for CC a small full subcategory with subcanonical coverage.

By the free T-algebra adjunction

(F TU T):TAlgU TF TSet (F_T \dashv U_T)\colon T Alg \stackrel{\overset{F_T}{\leftarrow}}{\underset{U_T} {\to}} Set

we have the free TT-algebra F T(*)TAlgF_T(*) \in TAlg on a single generator.


The TT-line object is

𝔸 TSpecF T(*)Sh(C). \mathbb{A}_T \coloneqq Spec F_T(*) \in Sh(C) \,.

The additive group object

For 𝒜𝒷\mathcal{Ab} the Lawvere theory of abelian groups, say that a morphism ab:𝒜𝒷Tab\colon \mathcal{Ab} \to T of Lawvere theories is an abelian Lawvere theory. Algebras over abelian Lawvere theories have underlying abelian groups

(ab *ab *):TAlgab *ab *Ab. (ab_* \dashv ab^*)\colon T Alg \stackrel{\overset{ab_*}{\leftarrow}}{\underset{ab^*}{\to}} Ab \,.

For TT an abelian Lawvere theory, by its underlying abelian group we have that 𝔸 T\mathbb{A}_T inherits the structure of an abelian group object in Sh(C)Sh(C). Write

𝔾 TAb(Sh(C)) \mathbb{G}_T \in Ab(Sh(C))

for this group object on 𝔸 T\mathbb{A}_T.

The multiplicative group object


For 𝔸 T\mathbb{A}_T a line object, write

(𝔸 T ×𝔸 T)Sh(C) (\mathbb{A}_T^\times \hookrightarrow \mathbb{A}_T) \in Sh(C)

be the maximal subobject of the line on those elements that have inverses under the multiplication 𝔸 T×𝔸 T𝔸 T\mathbb{A}_T \times \mathbb{A}_T \to \mathbb{A}_T.

This is called the multiplicative group of the line object, often denoted 𝔾 m\mathbb{G}_m.

The group of roots of unity

See at roots of unity.


See also analytic affine line.



For RR a ring and H et n(,)H^n_{et}(-,-) the etale cohomology, 𝔾 m\mathbb{G}_m the multiplicative group of the affine line; then

  • H et 0(R,𝔾 m)=R ×H^0_{et}(R, \mathbb{G}_m) = R^\times (group of units)

  • H et 1(R,𝔾 m)=Pic(R)H^1_{et}(R, \mathbb{G}_m) = Pic(R) (Picard group: iso classes of invertible RR-modules)

  • H et 2(R,𝔾 m)=Br(R)H^2_{et}(R, \mathbb{G}_m) = Br(R) (Brauer group Morita classes of Azumaya RR-algebras)


The notion of a line object over general abelian Lawvere theories has been considered in

in the context of function algebras on ∞-stacks.

In a monoidal category

Given a monoidal category CC, one may define a line object in CC to be an object LL such that the tensoring functor L:CC- \otimes L : C \to C has an inverse.

moduli spaces of line n-bundles with connection on nn-dimensional XX

nnCalabi-Yau n-foldline n-bundlemoduli of line n-bundlesmoduli of flat/degree-0 n-bundlesArtin-Mazur formal group of deformation moduli of line n-bundlescomplex oriented cohomology theorymodular functor/self-dual higher gauge theory of higher dimensional Chern-Simons theory
n=0n = 0unit in structure sheafmultiplicative group/group of unitsformal multiplicative groupcomplex K-theory
n=1n = 1elliptic curveline bundlePicard group/Picard schemeJacobianformal Picard groupelliptic cohomology3d Chern-Simons theory/WZW model
n=2n = 2K3 surfaceline 2-bundleBrauer groupintermediate Jacobianformal Brauer groupK3 cohomology
n=3n = 3Calabi-Yau 3-foldline 3-bundleintermediate JacobianCY3 cohomology7d Chern-Simons theory/M5-brane
nnintermediate Jacobian

Last revised on May 26, 2014 at 04:15:39. See the history of this page for a list of all contributions to it.