Modal depth

Template:Short description Template:Singlesource In modal logic, the modal depth of a formula is the deepest nesting of modal operators (commonly $◻$ and $◊$). Modal formulas without modal operators have a modal depth of zero.

Definition

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

$MD(p)=0$, where $p$ is an atomic formula.

$MD(\mathrm{\top })=0$

$MD(\mathrm{\perp })=0$

$MD(\mathrm{\neg }\phi )=MD(\phi )$

$MD(\phi \wedge \psi )=max(MD(\phi ),MD(\psi ))$

$MD(\phi \vee \psi )=max(MD(\phi ),MD(\psi ))$

$MD(\phi \to \psi )=max(MD(\phi ),MD(\psi ))$

$MD(◻\phi )=1+MD(\phi )$

$MD(◊\phi )=1+MD(\phi )$

Example

The following computation gives the modal depth of $◻(◻p\to p)$:

$MD(◻(◻p\to p))=$

$1+MD(◻p\to p)=$

$1+max(MD(◻p),MD(p))=$

$1+max(1+MD(p),0)=$

$1+max(1+0,0)=$

$1+1=:2$

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 $M,w\models ◊◊\phi$, one needs to check whether there exists an accessible world $v$ for which $M,v\models ◊\phi$. If that is the case, one needs to check whether there is also a world $u$ such that $M,u\models \phi$ 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., $◻\phi$ holds for all $\phi$ in a world $w$ when $\mathrm{\forall }v\in W(w,v)\in ̸R$, where $W$ is the set of worlds and $R$ is the accessibility relation). To check whether $M,w\models ◻◻\phi$, 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.