Grothendieck topos

A Grothendieck topos is a category equivalent to the sheaves on a site for a small Grothendieck topology. It is an elementary topos with all small colimits and a small generating family. Small disjoint coproducts, effective quotients and the set of subobjects of any fixed object allow many constructions by unions.

A topos classifies a geometric theory when geometric morphisms into it from any Grothendieck topos correspond naturally to internal models of that theory. Pulling back one generic model gives the corresponding model. For a finitary algebraic theory, the classifying topos is the covariant functor category on finitely presented models.

In the sheaf topos for the Zariski coverage on finitely presented rings, the universal local ring is represented by the polynomial ring $\mathbb{Z}[t]$. At the stage of a finitely presented ring $A$, its section set is $A$, and restriction along $A\to B$ is that ring map. It is internally local because the localizations at $x$ and $1-x$ cover each stage. Its square-zero infinitesimals are represented by $\mathbb{Z}[\epsilon ]/({\epsilon }^2)$. Stage rings need not themselves be local or reduced; their variation supplies the internal local and infinitesimal semantics.

For $D=\{d\in R:d^2=0\}$ in the generic local ring, every internal map $D\to R$ has a unique affine expression $d\mapsto a+bd$. At the stage $A$, such maps are elements of $A[\epsilon ]/({\epsilon }^2)$, which is freely $A\oplus A\epsilon$ as an $A$-module. This decomposition is natural under ring maps and proves the isomorphism. Testing only on ordinary fields loses the generic infinitesimal and would give an incorrect uniqueness argument.

Finite strict total orders and increasing injections, including the empty order, give a presheaf classifier for strict total orders. Trichotomy locally merges two finite sorted tuples, identifying equal entries. A strict map from a chain is injective, so parallel maps admitting the same realization are already equal. Reflexification gives a geometric morphism to the classifying topos of linear orders; pulling back weak totality gives exactly strict trichotomy.

With empty objects allowed, finite totally ordered sets and all weak order-preserving functions give a presheaf classifier for weak total orders. Finite tuples in a model can locally be merged into a sorted tuple using finitely many totality disjunctions. Equalizing two maps between finite chains amounts to collapsing the convex intervals forced equal. These two constructions give the flatness conditions without assuming decidable equality.

Let $\mathbf{FinSPos}$ contain finite strict partial orders and strict order-preserving functions. Its covariant set-valued functor category classifies the strict theory. A finite presentation with a strict cycle is contradictory; every other finite diagram gives a finite strict partial order. Equalizing finitely many element identifications either gives such an order or forces a forbidden cycle, so the internal model-to-flat-functor construction remains valid. The reflexification of a strict partial order induces a surjective geometric morphism to the weak-order classifier, but not a geometric embedding because strict maps exclude collapse of comparable elements.

Let $\mathbf{FinPos}$ contain finite partially ordered sets and all order-preserving functions, including the empty object. The covariant functor category $[\mathbf{FinPos},\mathbf{Set}]$ classifies weak partial orders. Finite positive presentations reduce to finite preorders with mutually comparable cycles identified. An internal model $M$ gives a flat functor $P\mapsto \mathrm{Hom}(P,M)$ on ${\mathbf{FinPos}}^{\mathrm{op}}$; conversely the corresponding filtered presentation by finite models recovers its carrier and order relation.

Each geometric quotient theory is classified by the corresponding subtopos of the original classifying topos. Extra axioms impose extra covers on the geometric syntactic topology. Conversely additional definable covers give the corresponding deductively closed quotient. Under Morita equivalence of geometric theories, transporting a subtopos produces a corresponding quotient of the other theory, with equivalent classifiers. Stronger axioms correspond to smaller subtoposes under inclusion.

Geometric theories are Morita-equivalent when their classifying toposes are equivalent. This gives equivalent internal model categories pseudonaturally in every Grothendieck topos. A common classifying topos transports intrinsic invariants between different presentations; agreement of set-valued model categories alone is insufficient. The duality between geometric quotients and subtoposes transfers geometric theory extensions along such an equivalence.

On the opposite of finitely presented commutative rings, cover the zero ring by the empty family and cover $A$ by $A/(a),A/(b)$ whenever $ab=0$. The generated topology classifies nontrivial integral domains, with generic model the sheafified tautological ring. It is not subcanonical: at $\mathbb{Z}/4$, the covering quotient to $\mathbb{Z}/2$ identifies the distinct sections $0$ and $2$ of the polynomial-ring representable.

In the generic domain, negation of simultaneous invertibility of finitely many elements implies that one is zero. At a ring stage, localize at their product $t$. All become units, so a tuple satisfying the negation makes the localized stage empty. The empty-cover criterion for the domain-classifying site then says $A[1/t]=0$, hence $t$ is nilpotent. Repeated zero-product covers force one factor to vanish locally. The two-variable property conversely forces the integral-domain axiom in any nontrivial internal ring.

For a finitely presented ring $A$, its sheafified representable is initial exactly when $A$ is the zero ring. The zero ring has a generating empty cover. Every nonzero ring maps to a field by quotienting by a maximal ideal, and that set-based domain gives a point at which the representable has a section, precluding initiality.

Adding coherent or geometric axioms to a theory determines a coverage on a site for its classifying topos. The antecedent presentation is covered by presentations where its consequent disjuncts and witnesses hold. Sheafification imposes those axioms on the generic model. An inconsistent antecedent gets an empty cover.

In $[{\mathbb{T}}_{fp},\mathbf{Set}]$, the generic model assigns to each finitely presented algebra its underlying set at each sort, with the algebraic operations pointwise. It is covariant on algebra homomorphisms. Pullback along a geometric morphism yields its classified internal model.