Is there a modal logic axiom corresponding to the condition that for every world w there is some world v such that vRw? (Reverse Seriality) Seriality on certain modal frames - that for every world w there is some world v such that wRv - corresponds to the axiom: $\square$P$\rightarrow$$\diamondsuit$P, assuming a standard interpretation of the relevant operators.
Consider what we might call the reverse seriality: that for every world w there is some world v such that vRw. Is there an axiom to which reverse seriality corresponds, assuming a standard interpretation of the operators?
I've observed any frame satisfying reflexivity, i.e. every w is such that wRw which corresponds to the axiom: $\square$P $\rightarrow$P, satisfies reverse seriality, but that's as far as I've thought it through.
 A: Expanding on the other answer, there is no such formula in the standard semantics.
Indeed, consider the following two models (here every world will be a $p$ world, where $p$ is the only atomic proposition):

*

*$\mathfrak{M}_1$ with exactly one world $w_1$ and the empty relation.


*$\mathfrak{M}_2$ with two worlds $x$ and $w_2$, such that $x$ sees $w_2$.
There is a bisimulation linking $w_1$ and $w_2$, so they satisfy the same modal formulas. But if there were a formula that expressed $\exists v . v R w$, then
$w_2$ would satisfy it while $w_1$ wouldn't! Thus no such formula exists.
If you really want to express this formula, you need to pass to a richer language. As the other answerer has said, the standard way to do this is by adding symbols that express moving backwards along relations. Often in this context we write $\overset{\rightarrow}{\square}$ and $\overset{\rightarrow}{\lozenge}$ instead of $\square$ and $\lozenge$. We also add symbols $\overset{\leftarrow}{\square}$ and $\overset{\leftarrow}{\lozenge}$ to represent moving backwards along the relation. That is:
$$w \models \overset{\rightarrow}{\square} \varphi \iff \forall v . wRv \to v \models \varphi$$
$$w \models \overset{\leftarrow}{\square} \varphi \iff \forall v . vRw \to v \models \varphi$$
It turns out the following axioms (plus Necessisitation for $\overset{\rightarrow}{\square}$ and $\overset{\leftarrow}{\square}$ and Modus Ponens) are sound and complete wrt the class of all frames with the above semantics:

*

*All classical tautologies


*$\overset{\rightarrow}{\square}(\varphi \to \psi) \to \overset{\rightarrow}{\square} \varphi \to \overset{\rightarrow}{\square} \psi$


*$\overset{\leftarrow}{\square}(\varphi \to \psi) \to \overset{\leftarrow}{\square} \varphi \to \overset{\leftarrow}{\square} \psi$


*$\varphi \to \overset{\rightarrow}{\square} \overset{\leftarrow}{\lozenge} \varphi$


*$\varphi \to \overset{\leftarrow}{\square} \overset{\rightarrow}{\lozenge} \varphi$
In this system, "reverse seriality" can be expressed as $\overset{\leftarrow}{\square}\varphi \to \overset{\leftarrow}{\lozenge}\varphi$.

I hope this helps ^_^
A: Since there is no other answer, I'd like to give a preliminary, possibly incomplete answer.
I am quite sure that you would need two modal operators to axiomatize the reverse seriality, as we need to talk about the 'forward' direction of the accessibility relation $R$ and its 'backward' direction $R^{-1}$.
To this end, I propose a temporal interpretation of the modal operators: Let

*

*$[X]\phi$ stand for "next, it will be necessary that $\phi$ holds",

*$\langle X\rangle \phi$ stand for "next, it will be possible that $\phi$ holds",

*$[P]\phi$ stand for "before, it was necessary that $\phi$ holds", and

*$\langle P \rangle \phi$ stand for "before, it was possible that $\phi$ holds.

Now, we can provide the following axiomatization
$$
\begin{align}
\text{for any tautology $\phi$ infer }&\vdash \phi\\
\text{from } \vdash \phi \to \psi \text{ and } \vdash \phi \text{ infer } &\vdash \psi \tag{mp}\\
\text{from } \vdash \phi \text{ infer } &\vdash [X]\phi \tag{Nec$_X$}\\
\text{from } \vdash \phi \text{ infer } &\vdash [P]\phi \tag{Nec$_P$}\\
&\vdash [X](\phi \to \psi) \to [X]\phi \to [X]\psi \tag{K$_X$}\\
&\vdash [P](\phi \to \psi) \to [P]\phi \to [P]\psi \tag{K$_P$}\\
&\vdash \phi \to [P]\langle X \rangle \phi \tag{C1}\\
&\vdash \phi \to \langle P \rangle \langle X \rangle \phi\tag{C2}
\end{align}
$$
That is, $X$ and $P$ are modal operators, as for both the necessitation rule $\text{Nec}$ and the axiom $\text{K}$ hold. The cancellation axiom $\text{C1}$ says that whenever $\phi$ holds, then before it was necessary that next $\phi$ will be possible. The second cancellation axiom $\text{C2}$ says that whenever $\phi$ holds, then before it was possible that next $\phi$ will be possible.
On the semantic side we have the equivalences

*

*$\omega\models [X]\phi$ if and only if for all $\omega'$ with $\omega R \omega'$ it holds $\omega'\models \phi$ and

*$\omega\models [P]\phi$ if and only if for all $\omega'$ with $\omega S \omega'$ it holds $\omega'\models \phi$

*(and corresponding versions for the duals $\langle X \rangle$ and $\langle P \rangle$)

It is easy to show that for any Kripke structure where $S$ is the inverse $R^{-1}$ the axiom $\text{C1}$ holds. Moreover, if $R$ is 'reverse serial', then also $\text{C2}$ has to hold.
I conjecture that this axiomatization is also complete for the class of Kripke structures with $S=R^{-1}$ and $R$ being 'reverse serial'.
I suspect that more elaborated analyses and also completeness results of 'reverse seriality' can be found in the literature on branching time modal logics with (infinite) past.
