10 modtab

Lecture Notes on
Modal Tableaux
15-816: Modal Logic
Andr´ Platzer
e

Lecture 10
Februrary 18, 2010

1 Introduction to This Lecture
The Hilbert calculus for modal logic from the last lectures is incredibly sim-
ple, but it is not entirely simple to find a proof in it. In this lecture, we in-
troduce a modal tableau calculus that is more amenable to systematic proof
construction and automated theorem proving.
Tableaux calculi for modal logic can be found in the work of Fitting
[Fit83, Fit88] and the manuscript by Schmitt [Sch03].

2 The Petite Modal Zoo
In previous lectures, we have mainly seen the propositional modal logic
S4 and its Hilbert-style axiomatization. This is, by far, not the only modal
logic of interest. The minimal (normal) modal logic is modal logic K. The
axiomatisation of K is a subset of the axioms of S4 and the same proof rules
of S4; see Figure 1. In fact, normal modal logics share the same proof rules
(MP and G) and mostly differ in the choice of axioms.
Extensions of logic K are shown in Figure 2.

3 Modal Tableaux
For proving formulas in propositional modal logic, we develop a tableau
calculus. Tableaux often give very intuitive proof calculi. Here we choose
prefix tableaux, where every formula on the tableau has a prefix σ, which

L ECTURE N OTES F EBRURARY 18, 2010

L10.2 Modal Tableaux

(P) all propositional tautologies

(K) (φ → ψ) → ( φ → ψ)

φ φ→ψ
(MP)
ψ
φ
(G)
φ

Figure 1: Modal logic K

T is system K plus (T) φ→φ
S4 is system T plus (4) φ→ φ

Figure 2: Some other modal logics

is a finite sequence of natural numbers. In addition, every formula on the
tableau has a sign Z ∈ {F, T } that indicates the truth-value we currently
expect for the formula in our reasoning. That is, a formula in the modal
tableaux is of the form
σZA

where the prefix σ is a finite sequence of natural numbers, the sign Z is in
{F, T }, and F is a formula of modal logic. At this point, we understand a
prefix σ as a symbolic name for a world in a Kripke structure.

Definition 1 (K prefix accessibility) For modal logic K, prefix σ is accessible
from prefix σ if σ is of the form σn for some natural number n.

For every formula of a class α with a top level operator and sign (T or
F for true and false) as indicated, we define two successor formulas α1 and
α2 :
α α1 α2 β β1 β2
TA ∧ B TA TB TA ∨ B TA TB
FA ∨ B FA FB FA ∧ B FA FB
FA → B TA FB TA → B FA TB
F ¬A TA TA T ¬A FA FA

For the following cases of formulas we define one successor formula


Modal Tableaux L10.3

ν ν0 π π0
T A TA T ♦A T A
F ♦A F A F A FA
Every combination of top-level operator and sign occurs in one of the
above cases. Tableau proof rules by those classes are shown in Figure 3. A
tableau is closed if every branch contains some pair of formulas of the form
σT A and σF A. A proof for modal logic formula consists of a closed tableau
starting with the root 1F A.

σα σβ σν σπ
(α) (β) (ν ∗ ) 1 (π) 2
σα1 σβ1 σβ2 σ ν0 σ π0
σα2
1
σ accessible from σ and σ occurs on the branch already
2
σ is a simple unrestricted extension of σ, i.e., σ is accessible from σ and no other prefix
on the branch starts with σ

Figure 3: Tableau proof rules for QML

The tableau rules can also be used to analyze F A → ♦A as follows:
1 F A → ♦A (1)
1 T A (2) from 1
1 F ♦A (3) from 1
stop
No more proof rules can be used because the modal formulas are ν rules,
which are only applicable for accessible prefixes that already occur on the
branch. If we drop this restriction, we can continue to prove and close the
tableau:
1 F A → ♦A (1)
1 T A (2) from 1
1 F ♦A (3) from 1
1.1 TA (4) from 2
1.1 FA (5) from 3
∗
But this is bad news, because the formula A → ♦A that we set out to
prove in the first place is not even valid in K. Consequently, the side condi-
tion on the ν rule is necessary for soundness!
As an example proof in K-tableaux we prove A ∧ B) → (A ∧ B):



1 F ( A ∧ B) → (A ∧ B) (1)
1 T A∧ B (2) from 1
1 F (A ∧ B) (3) from 1
1 T A (4) from 2
1 T B (5) from 2
1.1 FA ∧ B (6) from 3

1.1 F A (7) from 6 1.1 F B (8) from 6
1.1 T A (9) from 4 1.1 T B (10) from 5
∗ 7 and 9 ∗ 10 and 8

Let us prove the converse (A ∧ B) → ( A ∧ B) in K-tableaux:

1 F (A ∧ B) → ( A ∧ B) (1)
1 T (A ∧ B) (2) from 1
1 F A∧ B (3) from 1

1 F A (4) from 3 1 F B (5) from 3
1.1 FA (6) from 4 1.1 FB (10) from 5
1.1 TA ∧ B (7) from 2 1.1 TA ∧ B (11) from 2
1.1 TA (8) from 7 1.1 TA (12) from 11
1.1 TB (9) from 7 1.1 TB (13) from 11
∗ 6 and 8 ∗ 10 and 13

Let us try to prove (A ∨ B) → A∨ B:

1 F (A ∨ B) → A∨ B (1)
1 T (A ∨ B) (2) from 1
1 F A∨ B (3) from 1
1 F A (4) from 3
1 F B (5) from 3
1.1 FA (6) from 4
1.2 FB (7) from 5
1.1 TA ∨ B (8) from 2
1.2 TA ∨ B (9) from 2

1.1 T A (10) from 8 1.1 T B (11) from 8 1.2 T A (12) from 9 1.2 T B (13) from 9
∗ 10 and 6 open open ∗ 13 and 7

This tableau does not close but remains open, which is good news because
the formula we set out to prove is not valid in K.


Modal Tableaux L10.5

Exercises
Exercise 1 Prove or disprove using modal tableaux: ♦(A ∧ B) → ♦A ∧ ♦B.

Exercise 2 Are the side conditions on the preﬁxes for the ν ∗ -rule and the π-rule
necessary or not? Prove or disprove each case.

Exercise 3 Use a tableau procedure to prove or disprove the formulas

A→ ( A ∨ B)

and
A↔ A
in the modal logic S4. Explain your solution.

Exercise 4 Use a tableau procedure to prove or disprove the formula

♦A → ♦ A

in the modal logic S4. Explain your solution and which difﬁculties exist in com-
parison to classical propositional cases.



References
[Fit83] Melvin Fitting. Proof Methods for Modal and Intuitionistic Logic. Rei-
del, 1983.

[Fit88] Melvin Fitting. First-order modal tableaux. J. Autom. Reasoning,
4(2):191–213, 1988.

[Sch03] Peter H. Schmitt. Nichtklassische Logiken. Vorlesungsskriptum
a ¨
Fakult¨ t fur Informatik , Universit¨ t Karlsruhe, 2003.
a


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