Modal depth
# Modal depth

In modal logic, the modal depth of a formula is the deepest nesting of modal operators (commonly $\Box$ and $\Diamond$). Modal formulas without modal operators have a modal depth of zero.

## Definition

Modal depth can be defined as follows. Let $MD(\phi )$ be a function that computes the modal depth for a modal formula $\phi$:

    $MD(p)=0$, where $p$ is an atomic formula.
    $MD(\top )=0$
    $MD(\bot )=0$
    $MD(\neg \varphi )=MD(\varphi )$
    $MD(\varphi \wedge \psi )=max(MD(\varphi ),MD(\psi ))$
    $MD(\varphi \vee \psi )=max(MD(\varphi ),MD(\psi ))$
    $MD(\varphi \rightarrow \psi )=max(MD(\varphi ),MD(\psi ))$
    $MD(\Box \varphi )=1+MD(\varphi )$
    $MD(\Diamond \varphi )=1+MD(\varphi )$

## Example

The following computation gives the modal depth of $\Box (\Box p\rightarrow p)$:

    $MD(\Box (\Box p\rightarrow p))=$
    $1+MD(\Box p\rightarrow p)=$
    $1+max(MD(\Box p),MD(p))=$
    $1+max(1+MD(p),0)=$
    $1+max(1+0,0)=$
    $1+1=:2$

## Modal depth and semantics

The modal depth of a formula indicates 'how far' one needs to look in a Kripke model when checking the validity of the formula. For each modal operator, one needs to transition from a world in the model to a world that is accessible through the accessibility relation. The modal depth indicates the longest 'chain' of transitions from a world to the next that is needed to verify the validity of a formula.

For example, to check whether $M,w\models \Diamond \Diamond \varphi$, one needs to check whether there exists an accessible world $v$ for which $M,v\models \Diamond \varphi$. If that is the case, one needs to check whether there is also a world $u$ such that $M,u\models \varphi$ and $u$ is accessible from $v$. We have made two steps from the world $w$ (from $w$ to $v$ and from $v$ to $u$) in the model to determine whether the formula holds; this is, by definition, the modal depth of that formula.

The modal depth is an upper bound (inclusive) on the number of transitions as for boxes, a modal formula is also true whenever a world has no accessible worlds (i.e., $\Box \varphi$ holds for all $\varphi$ in a world $w$ when $\forall v\in W\ (w,v)\not \in R$, where $W$ is the set of worlds and $R$ is the accessibility relation). To check whether $M,w\models \Box \Box \varphi$, it may be needed to take two steps in the model but it could be less, depending on the structure of the model. Suppose no worlds are accessible in $w$; the formula now trivially holds by the previous observation about the validity of formulas with a box as outer operator.

