# Succinctness of the fixed point theorem for provability logic

## Konstantinos Papafilippou

**Authors:** Konstantinos Papafilippou and David Fernández Duque

A classical result of provability logic is the fixed point theorem, proved independently by D. de Jongh and G. Sambin [3] with various proof methods for it ever since. Its statement is the following: Given a modal formula \(\phi(p)\) that is modalized for \(p\) — i.e. every occurrence of \(p\) in \(\phi\) occurs within the scope of a \(\Box\) — there is a formula \(\sigma\) without \(p\) occurring in it such that \(GL \vdash \phi(\sigma) \rightarrow \sigma\). In fact, this fixed point is unique under equivalence over GL. Sambin’s construction [3,4], gives a rough upper bound for succinctness of the fixed point \(\sigma\) relative to the original formula \(\phi\) of the scale of \(\mid\sigma\mid \leq n^{O(n)}\) where \(n = \mid\phi\mid\). However there was no known succinctness lower bound.

The methods that we use to obtain a succinctness lower bound are those of formula-size games that were developed in the setting of Boolean function complexity [1] and of first-order logic and some temporal logics [2]. By now, the formula-size games have been adapted to a host of modal logics and used to obtain lower bounds on modal formulas expressing properties of Kripke models. These methods work by selecting a formula \(\phi\) of a language \(\mathcal{L}\) and two sets of models \(\mathcal{A}, \mathcal{B}\) that are separable by \(\phi\). Then the game is setup and played with rules according to a language \(\mathcal{L}'\). Once the game is concluded, we obtain a formula \(\psi\) in \(\mathcal{L}'\) equivalent to \(\phi\) and the size of \(\psi\) can be calculated by a careful analysis of the game on the sets \(\mathcal{A}\) and \(\mathcal{B}\).

Let \(\mathcal L_\Diamond\) be the standard modal language (with an irreflexive modality) and \(\mathcal L_\dot{\Diamond}\) be the language which instead includes a reflexive modality as primitive. In the case of GL, P. Iliev and D. Fern'andez-Duque have derived an exponential (\(2^{O(n)}\)) succinctness lower bound for \(\mathcal{L}_{\dot{\Diamond}}\) over \(\mathcal{L}_\Diamond\) in GL where \(\dot{\Diamond}\phi =: \phi \vee \Diamond \phi\). The sequence of formulas they used were defined inductively as:

- \(\phi_1 = p_1\);
- \(\phi_{n+1} = \dot{\Diamond} (p_{n+1} \wedge \phi_n )\).

Their result can be easily reformulated to show a succinctness lower bound for formulas in \(\mathcal{L}_{\Diamond}\), thus giving the following:

**Theorem.**
There exists a sequence of formulas \((\psi_n)_{n<\omega}\) linear in \(n\) such that any fixed point in \(\mathcal L_\Diamond\) for \(\psi_n\) over \(\sf GL\) has size \(2^{O(n)}\).

We expand this succinctness lower bound in the following sense, we write formulas of \(\mathcal{L}\) whose fixed point in \(\mathcal{L}_{\Diamond\dot{\Diamond}}\) (i.e., the bi-modal logic with a reflexive and an irreflexive modality) is of the scale \(2^{O({n})}\). This is done with formulas expressing a kind of tree embeddability into our model.
With this, we may improve upon the above Theorem as follows.

**Theorem.**
There exists a sequence of formulas \((\gamma_n)_{n<\omega}\) linear in \(n\) such that any fixed point in \(\mathcal L_{\Diamond\dot{\Diamond}}\) for \(\gamma_n\) over \(\sf GL\) has size \(2^{O(n)}\).

## Bibliography

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