A strict order is a linear order which is not a linear relation. Strict orders are used in models of the real numbers which have infinitesimals, such as in the smooth real numbers in synthetic differential geometry, and thus where not every real number not greater than or less than zero is equal to zero.
A strict order is a binary relation $\lt$ on a set $S$ which is irreflexive, asymmetric, transitive, and a comparison:
for all $a \in S$, $\neg(a \lt a)$
for all $a \in S$ and $b \in S$, if $a \lt b$, then $\neg(b \lt a)$
for all $a \in S$, $b \in S$, and $c \in S$, if $a \lt b$ and $b \lt c$, then $a \lt c$
for all $a \in S$, $b \in S$, and $c \in S$, if $a \lt c$, then $a \lt b$ or $b \lt c$
The negation of a strict order is a preorder $a \leq b \coloneqq \neg(b \lt a)$, and thus has an equivalence relation defined as $a \approx b \coloneqq a \leq b \wedge b \leq a$. Every strict order which is also a linear relation is a linear order, whose negation is a partial order. Thus, in any foundations with quotient sets, every strict order can be made into a linear order by taking the quotient with respect to the equivalence relation $S/\approx$.
Strict orders are used in defining the smooth real numbers in:
Last revised on December 7, 2022 at 15:05:14. See the history of this page for a list of all contributions to it.