Converses of transitivity
Let $\prec$ be a binary relation. Two logically-equivalent ways to express transitivity are \begin{align} & a \prec b \to \forall x (x \prec a \to x \prec b) \\ & a \prec b \to \forall x (b \prec x \to a \prec x) \end{align} They have two distinct converses: [1] \begin{align} & \forall x (x \prec a \to x \prec b) \to a \prec b & \text{A} \\ & \forall x (b \prec x \to a \prec x) \to a \prec b & \text{B} \end{align}
The only reference I've found to them is [2], which calls them "converse transitivity from below" and "converse transitivity from above", adding:
Converse transitivity from below and converse transitivity from above are not necessarily the same, as the spouse/sibling example shows, and it is possible to construct other examples in which one or the other or both conditions do not hold.
My questions are the following:
- Do properties A and B have other names in the literature?
- Are they discussed in greater detail elsewhere in the literature?
- Is there a name for a binary relation that satisfies transitivity and both of its converses?
- What are some common, general mathematical structures that satisfy the above?
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Note that both imply reflexivity.
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John Randolph Lucas. §9.9 Converse Transitivity. Conceptual Roots of Mathematics. Routledge, 2002-09-11. Pages 253-254.
Solution 1:
Both of these converses are in fact just equivalent to reflexivity. You have already observed that they imply reflexivity, since the hypothesis of either converse is always true if $a=b$. Conversely, suppose $\prec$ is reflexive and for all $x$, $x\prec a$ implies $x\prec b$. In particular, we know that $a\prec a$ by reflexivity, and therefore $a\prec b$. The other converse works similarly.