# Order Theory/The Grothendieck axioms

AB3, AB3*, AB5 and AB5* are actually due to von Neumann, but Grothendieck's name stuck.

Definition (AB3):

A preordered class $(S,\leq )$ is said to satisfy the third Grothedieck axiom AB3 iff for all families $(x_{\alpha })_{\alpha \in A}$ indexed over a set $A$ , a least upper bound $\bigvee _{\alpha \in A}x_{\alpha }$ exists.

Definition (AB3*):

A preordered class $(S,\leq )$ is said to satisfy the dual third Grothedieck axiom AB3* iff for all families $(x_{\alpha })_{\alpha \in A}$ indexed over a set $A$ , a greatest lower bound $\bigwedge _{\alpha \in A}x_{\alpha }$ exists.

Definition (AB5):

A class ${\mathcal {B}}$ of preordered sets is said to satisfy the fifth Grothendieck axiom AB5 if and only if for all $(S,\leq )\in {\mathcal {B}}$ and all families of elements $(x_{\alpha })_{\alpha \in A}$ of elements of $S$ and each element $y\in S$ , we have

$\left(\bigvee _{\alpha \in A}x_{\alpha }\right)\wedge y=\bigvee _{\alpha \in A}(x_{\alpha }\wedge y)$ Definition (AB5*):

A class ${\mathcal {B}}$ of preordered sets is said to satisfy the dual fifth Grothendieck axiom AB5* if and only if for all $(S,\leq )\in {\mathcal {B}}$ and all families of elements $(x_{\alpha })_{\alpha \in A}$ of elements of $S$ and each element $y\in S$ , we have

$\left(\bigwedge _{\alpha \in A}x_{\alpha }\right)\vee y=\bigwedge _{\alpha \in A}(x_{\alpha }\vee y)$ Definition (AB6):

A class ${\mathcal {B}}$ of preordered sets is said to satisfy the sixth Grothendieck axiom AB6 if and only if for all $(S,\leq )\in {\mathcal {B}}$ 