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 ).