Let be a commutative ring. From we can construct the category of -modules, which becomes a symmetric monoidal category when equipped with the tensor product of -modules. Now, whenever we have a monoidal operation (for example, the multiplication on a ring), it’s interesting to look at the invertible things with respect to that operation (for example, the group of units of a ring). This suggests the following definition.
Definition: The Picard group of is the group of isomorphism classes of -modules which are invertible with respect to the tensor product.
By invertible we mean the following: for there exists some such that the tensor product is isomorphic to the identity for the tensor product, namely .
In this post we’ll meander through some facts about this Picard group as well as several variants, all of which capture various notions of line bundle on various kinds of spaces (where the above definition captures the notion of a line bundle on the affine scheme ).
Proposition: An invertible module is finitely presented and projective.
Proof. If is invertible, then the functor is an autoequivalence of categories with inverse , and consequently it preserves all categorical properties; in addition, it sends to , so it follows that any categorical property of is also a categorical property of . In particular, since being projective is a categorical property (namely the property that is exact) and is projective, is also projective.
Less obviously, being finitely presented is also a categorical property: an -module is finitely presented iff preserves filtered colimits (that is, is a compact object).
We can avoid appealing to this fact with the following more hands-on argument. By assumption, . Let
be the element of representing . Then the map
is surjective, where denotes , as can be seen by setting . Surjective morphisms of -modules are precisely the epimorphisms, and this is a categorical property, so tensoring with we get an epimorphism , from which it follows that is finitely generated (by the elements ), and for a projective module this is equivalent to being finitely presented.
Proposition: Being invertible is preserved under extension of scalars. More precisely, if is a morphism of commutative rings, then the extension of scalars functor
sends invertible modules to invertible modules, and in fact induces a homomorphism .
Proof. It suffices to show that extension of scalars is a monoidal functor. More or less this boils down to having a natural isomorphism
But by the associativity of the tensor product, the RHS is just
and , so the conclusion follows using the commutativity of the tensor product of modules over commutative rings.
Theorem: The following conditions on an -module are equivalent.
- is invertible.
- is locally free of rank : that is, for every prime ideal of , the localization is a free -module of rank .
- The natural map is an isomorphism, where is the dual module. In this case .
Proof. 1) 2): Since localization is a special case of extension by scalars, remains invertible, hence is in particular finitely presented projective. We can give an independent argument that this is true as follows: since localization is exact, it preserves finite presentations, so is finitely presented. Since we have a tensor-hom adjunction
it follows that if is exact then so is , hence localization preserves projectivity.
But since is a local ring, it follows that must be free, and since rank is multiplicative under tensor product, must be free of rank : that is, we must have .
2) 3): being an isomorphism is a local property, so the natural map is an isomorphism iff its localizations are. But if is locally free of rank then for all .
3) 1): by definition.
The proposition shows in particular that invertibility is a local condition: is invertible iff is invertible for all . We still haven’t given any interesting examples of invertible modules, though.
The ideal class group
Proposition: Let be an integral domain with fraction field . Then every invertible module over is isomorphic to a fractional ideal of .
Proof. is projective, hence in particular torsion-free, so the natural inclusion is an embedding. Since is finitely generated, we can multiply the image of this embedding by the product of the denominators of the images of its generators in , and we conclude that an -multiple of the image of lands in as desired.
Proposition: Let be a Dedekind domain. Then a finitely generated module over is projective iff it is torsion-free.
Proof. Projectivity is a local property, so is projective iff is projective for all . If is torsion-free, then so is . Since the localizations are all DVRs, hence in particular PIDs, it follows by the structure theorem for finitely generated modules over a PID that each is free, hence in particular projective.
Proposition: Let be two fractional ideals of a Dedekind domain . Then .
Proof. WLOG are ideals. We want to show that the natural surjection
is an isomorphism. Since are fractional ideals, they are torsion-free, hence projective, hence flat, so embeds into . Any element in the kernel of the natural surjection above must therefore also be in the kernel of the natural map , but this natural map is an isomorphism.
Theorem: The Picard group of a Dedekind domain is canonically isomorphic to its ideal class group of invertible fractional ideals modulo principal invertible fractional ideals. In particular, is nontrivial iff is not a UFD.
Proof. By the above proposition, any invertible fractional ideal gives rise to an invertible module, and moreover multiplication of fractional ideals corresponds to tensor product of ideals. The kernel of this map consists of invertible fractional ideals which are isomorphic to the trivial module, which is precisely the principal invertible fractional ideals; hence we get an injection from the ideal class group to . We also showed that every invertible module comes from a fractional ideal, necessarily also invertible, so this injection is a surjection and hence a bijection.
Example. Let be the ring of integers of the number field . This is a Dedekind domain which is not a UFD because of the non-unique factorization
Here have norms respectively. An examination of the norm form reveals that has no elements of norm or , hence all four of these elements are irreducible. The factorization above refines to unique prime ideal factorizations
which gives and in the ideal class group. Because has norm and there exist no elements of of norm , we also know that in the ideal class group.
By the Minkowski bound, the ideal class group is generated by ideals of norm at most
and since is the only prime ideal lying over , the ideal class group must be generated by . Hence we compute the Picard group to be
This example turns out to be minimal in the sense that is the number field of smallest discriminant (in absolute value) whose ideal class group is nontrivial.
Example. Let be the ring of functions on a smooth affine curve
in the complex plane, and let denote its projective closure in the complex projective plane. Then ideals of can be identified with effective divisors on , and principal ideals of can be identified with effective principal divisors. Since meromorphic functions on are quotients of functions in , it follows that is canonically isomorphic to the divisor class group of , which is closely related to the divisor class group of , which is in turn very well-understood and which we will turn to later. The relationship is the following: restriction of divisors gives a surjection
(a priori it only gives a surjection on divisors, but since and have the same meromorphic functions, the natural map on divisors respects the quotient by principal divisors). The kernel of this map clearly contains the subgroup of generated by the points in , and in fact it must be precisely this subgroup: if is a divisor in the kernel, then is the divisor of some function on (that is, some element of ), but extends to a meromorphic function on and hence has a principal divisor whose restriction to is precisely . If there are points in , then we have an exact sequence
As we’ll see later, if has genus then
where denotes the Jacobian and denotes a torus of (real) dimension . In particular, is uncountable as soon as , and hence its quotient by the image of is nontrivial as soon as .
The topological Picard groups
The characterization of invertible modules as locally free modules of rank suggests that invertible modules over a commutative ring should be thought of as (modules of sections of) line bundles on . This idea is strongly supported by variants of the Serre-Swan theorem, such as the following.
Theorem: Let be a compact Hausdorff space and let resp. be the ring of continuous real-valued resp. complex-valued functions on . Assigning a real resp. complex vector bundle on its module of continuous sections gives an equivalence of monoidal categories between real resp. complex vector bundles on and finitely presented projective modules over resp. .
Corollary: resp. is canonically isomorphic to the abelian group of topological real resp. complex line bundles on , which is in turn canonically isomorphic to resp. .
This theorem suggests a natural definition of the real resp. complex Picard groups of an arbitrary space , not necessarily compact Hausdorff, namely the group of isomorphism classes of real resp. complex line bundles on .
Example. Let ; this is arguably the simplest example of a space with a nontrivial real line bundle over it. Since , there are exactly two (isomorphism classes of) real line bundles over , one trivial and one nontrivial. The nontrivial line bundle is the Möbius bundle. Its -module of continuous sections can be identified with the function space
where itself is thought of as the function space
and the module structure is given by pointwise multiplication. (So here we are thinking of as the quotient .)
Example. Let ; this is arguably the simplest space with a nontrivial complex line bundle over it. Since , there are countably many (isomorphism classes of) complex line bundles over , all of which are powers of a single generator.
Thinking of as the complex projective line , there are two choices for such a generator, one given by the tautological bundle which assigns to a point in the complex line in it represents, and the other given by its dual ; the other line bundles are given by , where if is negative then as expected.
The bundles are important in algebraic geometry because their spaces of algebraic (or equivalently, holomorphic) sections are precisely the homogeneous polynomials of degree . Their -modules of continuous sections can be identified with the function spaces
where itself is thought of as the function space
and, as above, the module structure is given by pointwise multiplication. (So here we are thinking of as the quotient .)
We can make the construction for look more like the construction for by thinking of as the real projective line and exhibiting it as the quotient of by the action of .
The algebraic and analytic Picard groups
The discussion of above blurred the distinction between topological, holomorphic, and algebraic line bundles, so it’s worth making the distinction in general.
To talk about topological vector bundles on a space only requires that it be equipped with a topology. To talk about holomorphic vector bundles requires that be equipped with the structure of a complex manifold. Finally, to talk about algebraic vector bundles requires that be equipped with the structure of a scheme, e.g. might be a complex variety. We can talk about all three on a smooth complex variety. On all three notions coincide, but in general they all differ.
The GAGA principle implies that the classifications of holomorphic and algebraic vector bundles on a smooth projective complex variety coincide; in particular, the classifications of holomorphic and algebraic line bundles on coincide. However, it is not true in general that these classifications also coincide with the classification of topological line bundles, although it is true for the complex projective spaces . In general, if is a complex manifold, then the exponential sheaf sequence
gives rise to a long exact sequence in sheaf cohomology
where turns out to be the Picard group of holomorphic line bundles on and the connecting homomorphism to sends such a line bundle to its first Chern class , which completely determines the underlying topological line bundle but not its holomorphic structure.
The classifications of holomorphic and topological line bundles on coincide iff this connecting homomorphism is an isomorphism. By exactness, this is guaranteed if , which in particular holds if is a Stein manifold (e.g. a smooth affine variety) by Cartan’s theorem B. More generally, the Oka-Grauert principle asserts that the classifications of holomorphic and topological vector bundles on a Stein manifold coincide.
But and are both nontrivial in general, so we can’t expect the holomorphic and topological classifications to agree in general. And because the holomorphic and topological classifications agree on smooth affine varieties, a smooth affine variety on which the algebraic and topological classifications disagree also shows that the algebraic and holomorphic classifications disagree in general.
Example. On a smooth projective curve of genus , the divisor class group turns out to be naturally isomorphic to the Picard group, with the first Chern class map corresponding to the degree map
under the isomorphism given by pairing with the fundamental class. Hence the degree zero divisor class group is naturally isomorphic to the Picard group of line bundles with vanishing first Chern class. This group measures the difference between the holomorphic / algebraic and the topological classifications of line bundles on .
An inspection of the long exact sequence associated to the exponential sequence shows that this group is in turn isomorphic to the quotient
which is one description of the Jacobian. As we saw previously, , and we know that . This exhibits as a complex torus (at least provided that we show that the image of in is a lattice). In particular, once , the holomorphic / algebraic and topological classifications of line bundles on disagree.
Example. As when we were discussing Dedekind domains, let be an elliptic curve minus a point. Then is a smooth affine variety, hence in particular a Stein manifold, so the classifications of holomorphic and topological line bundles agree: both Picard groups are isomorphic to , which vanishes since , being topologically a torus minus a point, deformation retracts onto its -skeleton. But the classification of algebraic line bundles is given by the divisor class group, which we saw earlier was uncountable. In particular, the holomorphic and algebraic classifications of line bundles on disagree.