Bulletin of the Section of Logic
Volume 39:3/4 (2010), pp. 205–214
Norihiro Kamide
AN EMBEDDING-BASED COMPLETENESS PROOF FOR
NELSON’S PARACONSISTENT LOGIC
Abstract
It is known that a syntactical embedding theorem of Nelson’s paraconsistent logic
N4 into the positive intuitionistic logic LJ is useful to show the cut-elimination
and decidability theorems for N4. In this paper, a semantical embedding theorem of N4 into LJ is shown. An alternative proof of the Kripke-completeness
theorem for N4 is obtained by combining both the syntactical and semantical
embedding theorems. Thus, the completeness, cut-elimination and decidability
theorems can uniformly be obtained from these embedding theorems. A singleconsequence Kripke semantics for N4 is also addressed based on a modification
of the semantical embedding theorem.
1.
Syntactical embedding: A review
Nelson’s paraconsistent (four-valued) logic N4 (or equivalently N− ) [12],
which is a paraconsistent variant of Nelson’s constructive (three-valued)
logic N3 (or equivalently N) [6], has been studied by many researchers (see
e.g. [2,13] for detailed information on Nelson’s logics and their variations).
It was shown by Odintsov [7] that N3 is embeddable into N4. Cut-free
Gentzen-type sequent calculi for Nelson’s logics have been studied (e.g.
[8,13,4,5]), and Kripke semantics for Nelson’s logics have also been studied
(e.g. [10,11,13]). A translation function from N3 into intuitionistic logic has
been proposed and studied by Gurevich [3], Rautenberg [9] and Vorob’ev
[12]. A similar translation function from N4 into positive intuitionistic logic
(called here LJ) can also be obtained. By using such a translation function,
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Norihiro Kamide
a syntactical embedding theorem of N4 into LJ and the cut-elimination and
decidability theorems for N4 can easily be obtained (see e.g. [4,5,14] for
some related works).
In this section, the standard sequent calculi for the underlying logics
are presented, and the syntactical embedding theorem and its consequences
(i.e. cut-elimination and decidability) are reviewed. In the next section,
the standard Kripke semantics for the underlying logics are presented, and
a semantical embedding theorem, which is a new result of this paper, is
shown. An alternative embedding-based proof of the Kripke-completeness
theorem for N4 is obtained by combining both the syntactical and semantical embedding theorems. Thus, the completeness, cut-elimination and
decidability theorems can uniformly be obtained from these embedding
theorems. In the final section, a new single-consequence Kripke semantics
for N4 is introduced, and the equivalence between such a semantics and
the standard dual-consequence semantics is proved by modifying the proof
of the semantical embedding theorem.
Prior to the detailed discussion, the language used in this paper is introduced below. The usual propositional language with the strong negation
connective ∼ and without falsity and truth constants is used in this paper. Lower-case letters p, q, r, ... are used to denote propositional variables,
Greek lower-case letters α, β, γ, ... are used to denote formulas, and Greek
capital letters Γ, ∆, ... are used to represent finite (possibly empty) sets of
formulas. A sequent is an expression of the form Γ ⇒ γ. If a sequent S is
provable in a system L, then such a fact is denoted as L ` S.
Definition 1 (LJ). The initial sequents of LJ are of the form: p ⇒ p for
any propositional variable p.
The structural inference rules of LJ are of the form:
Γ ⇒ α α, Σ ⇒ γ
(cut)
Γ, Σ ⇒ γ
Γ⇒γ
(w − l).
α, Γ ⇒ γ
The logical inference rules of LJ are of the form:
Γ ⇒ α β, ∆ ⇒ γ
(→l)
α→β, Γ, ∆ ⇒ γ
α, β, Γ ⇒ γ
(∧l)
α ∧ β, Γ ⇒ γ
α, Γ ⇒ β
(→r)
Γ ⇒ α→β
Γ⇒α Γ⇒β
(∧r)
Γ⇒α∧β
An Embedding-Based Completeness Proof for Nelson’s Paraconsistent Logic 207
α, Γ ⇒ γ β, Γ ⇒ γ
(∨l)
α ∨ β, Γ ⇒ γ
Γ⇒α
(∨r1)
Γ⇒α∨β
Γ⇒β
(∨r2).
Γ⇒α∨β
Definition 2 (N4). N4 is obtained from LJ by adding the initial sequents
of the form: ∼p ⇒ ∼p for any propositional variable p, and the logical
inference rules of the form:
α, Γ ⇒ γ
(∼∼l)
∼∼α, Γ ⇒ γ
α, ∼β, Γ ⇒ γ
(∼→l)
∼(α→β), Γ ⇒ γ
Γ⇒α
(∼∼r)
Γ ⇒ ∼∼α
Γ ⇒ α Γ ⇒ ∼β
(∼→r)
Γ ⇒ ∼(α→β)
∼α, Γ ⇒ γ ∼β, Γ ⇒ γ
(∼ ∧ l)
∼(α ∧ β), Γ ⇒ γ
Γ ⇒ ∼α
(∼ ∧ r1)
Γ ⇒ ∼(α ∧ β)
∼α, ∼β, Γ ⇒ γ
(∼ ∨ l)
∼(α ∨ β), Γ ⇒ γ
Γ ⇒ ∼β
(∼ ∧ r2)
Γ ⇒ ∼(α ∧ β)
Γ ⇒ ∼α Γ ⇒ ∼β
(∼ ∨ r).
Γ ⇒ ∼(α ∨ β)
The sequents of the form α ⇒ α for any formula α are provable in LJ
and N4.
Definition 3. We fix a set Φ of propositional variables and define the set
Φ0 := {p0 | p ∈ Φ} of propositional variables. The language LN 4 of N4 is
defined using Φ, →, ∧, ∨ and ∼. The language LLJ of LJ is obtained from
LN 4 by adding Φ0 and deleting ∼.
A mapping f from LN 4 to LLJ is inductively defined by:
1.
2.
3.
4.
5.
6.
for any p ∈ Φ, f (p) := p and f (∼p) := p0 ∈ Φ0 ,
f (α ◦ β) := f (α) ◦ f (β) where ◦ ∈ {→, ∧, ∨},
f (∼∼α) := f (α),
f (∼(α→β)) := f (α) ∧ f (∼β),
f (∼(α ∧ β)) := f (∼α) ∨ f (∼β),
f (∼(α ∨ β)) := f (∼α) ∧ f (∼β).
An expression f (Γ) denotes the result of replacing every occurrence of
a formula α in Γ by an occurrence of f (α).
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Theorem 4 (Syntactical embedding). Let Γ be a set of formulas in LN 4 , γ
be a formula in LN 4 , and f be the mapping defined in Definition 3. Then:
1. N4 ` Γ ⇒ γ iff LJ ` f (Γ) ⇒ f (γ),
2. N4 − (cut) ` Γ ⇒ γ iff LJ − (cut) ` f (Γ) ⇒ f (γ).
Corollary 5 (Cut-elimination). The rule (cut) is admissible in cut-free
N4.
Proof. Suppose N4 ` Γ ⇒ γ. Then, we have LJ ` f (Γ) ⇒ f (γ) by Theorem 4 (1), and hence LJ − (cut) ` f (Γ) ⇒ f (γ) by the cut-elimination
theorem for LJ. By Theorem 4 (2), we obtain the required fact N4 − (cut)
` Γ ⇒ γ. Q.E.D.
Corollary 6 (Decidability). N4 is decidable.
Proof. By decidability of LJ, for each α, it is possible to decide if f (α)
is provable in LJ. Then, by Theorem 4, N4 is decidable. Q.E.D.
2.
Semantical embedding
Definition 7. A Kripke frame is a structure hM, Ri satisfying the following conditions:
1. M is a nonempty set,
2. R is a reflexive and transitive binary relation on M .
Definition 8. A valuation |= on a Kripke frame hM, Ri is a mapping
from the set Φ of propositional variables to the power set 2M of M such
that for any p ∈ Φ and any x, y ∈ M , if x ∈ |= (p) and xRy, then y ∈
|= (p). We will write x |= p for x ∈ |= (p). This valuation |= is extended
to a mapping from the set of all formulas to 2M by:
1. x |= α→β iff ∀y ∈ M [xRy and y |= α imply y |= β],
2. x |= α ∧ β iff x |= α and x |= β,
3. x |= α ∨ β iff x |= α or x |= β.
The following hereditary condition holds for |=: for any formula α and
any x, y ∈ M , if x |= α and xRy, then y |= α.
An Embedding-Based Completeness Proof for Nelson’s Paraconsistent Logic 209
Definition 9. A Kripke model is a structure hM, R, |=i such that
1. hM, Ri is a Kripke frame,
2. |= is a valuation on hM, Ri.
A formula α is true in a Kripke model hM, R, |=i if x |= α for any x ∈ M ,
and is LJ-valid in a Kripke frame hM, Ri if it is true for every valuation
|= on the Kripke frame.
Definition 10. Paraconsistent valuations |=+ and |=− on a Kripke frame
hM, Ri are mappings from the set Φ of propositional variables to the power
set 2M of M such that for any ? ∈ {+, −}, any p ∈ Φ and any x, y ∈ M ,
if x ∈ |=? (p) and xRy, then y ∈ |=? (p). We will write x |=? p for x
∈ |=? (p). These paraconsistent valuations |=+ and |=− are extended to
mappings from the set of all formulas to 2M by:
1.
2.
3.
4.
5.
6.
7.
8.
x |=+
x |=+
x |=+
x |=+
x |=−
x |=−
x |=−
x |=−
α→β iff ∀y ∈ M [xRy and y |=+ α imply y |=+ β],
α ∧ β iff x |=+ α and x |=+ β,
α ∨ β iff x |=+ α or x |=+ β,
∼α iff x |=− α,
∼α iff x |=+ α,
α→β iff x |=+ α and x |=− β,
α ∧ β iff x |=− α or x |=− β,
α ∨ β iff x |=− α and x |=− β.
The hereditary condition holds for |=+ and |=− .
Definition 11. A paraconsistent Kripke model is a structure hM, R, |=+ ,
|=− i such that
1. hM, Ri is a Kripke frame,
2. |=+ and |=− are paraconsistent valuations on hM, Ri.
A formula α is true in a paraconsistent Kripke model hM, R, |=+ , |=− i if
x |=+ α for any x ∈ M , and is N4-valid in a Kripke frame hM, Ri if it is
true for every paraconsistent valuations |=+ and |=− on the Kripke frame.
Lemma 12. Let f be the mapping defined in Definition 3. For any paraconsistent Kripke model hM, R, |=+, |=− i, there exists a Kripke model hM, R, |=i
such that for any formula α and any x ∈ M ,
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Norihiro Kamide
1. x |=+ α iff x |= f (α),
2. x |=− α iff x |= f (∼α).
Proof. Let Φ be a set of propositional variables and Φ0 be the set
{p0 | p ∈ Φ} of propositional variables. Suppose that hM, R, |=+ , |=− i
is a paraconsistent Kripke model where |=+ and |=− are mappings from Φ
to the power set 2M of M , and that the hereditary condition w.r.t. p ∈ Φ
holds for |=+ and |=− . Suppose that hM, R, |=i is a Kripke model where |=
is a mapping from Φ ∪ Φ0 to 2M , and that the hereditary condition w.r.t.
p ∈ Φ ∪ Φ0 holds for |=. Suppose moreover that these models satisfy the
following conditions: for any x ∈ M and any p ∈ Φ,
1. x |=+ p iff x |= p,
2. x |=− p iff x |= p0 .
Then, the lemma is proved by (simultaneous) induction on the complexity of α.
• Base step:
Case α ≡ p ∈ Φ: For (1), we obtain: x |=+ p iff x |= p iff x |= f (p) (by
the definition of f ). For (2), we obtain: x |=− p iff x |= p0 iff x |= f (∼p)
(by the definition of f ).
• Induction step:
Case α ≡ β ∧ γ: For (1), we obtain: x |=+ β ∧ γ iff x |=+ β and
x |=+ γ iff x |= f (β) and x |= f (γ) (by induction hypothesis for 1) iff
x |= f (β) ∧ f (γ) iff x |= f (β ∧ γ) (by the definition of f ). For (2), we
obtain: x |=− β ∧ γ iff x |=− β or x |=− γ iff x |= f (∼β) or x |= f (∼γ) (by
induction hypothesis for 2) iff x |= f (∼β) ∨ f (∼γ) iff x |= f (∼(β ∧ γ)) (by
the definition of f ).
Case α ≡ β ∨ γ: Similar to the above case.
Case α ≡ β→γ: For (1), we obtain: x |=+ β→γ iff ∀y ∈ M [xRy and
y |=+ β imply y |=+ γ] iff ∀y ∈ M [xRy and y |= f (β) imply y |= f (γ)]
(by induction hypothesis for 1) iff x |= f (β)→f (γ) iff x |= f (β→γ) (by the
definition of f ). For (2), we obtain: x |=− β→γ iff x |=+ β and x |=− γ
iff x |= f (β) and x |= f (∼γ) (by induction hypothesis for 1 and 2) iff
x |= f (β) ∧ f (∼γ) iff x |= f (∼(β→γ)) (by the definition of f ).
Case α ≡ ∼β: For (1), we obtain: x |=+ ∼β iff x |=− β iff x |= f (∼β)
(by induction hypothesis for 2). For (2), we obtain: x |=− ∼β iff x |=+ β iff
x |= f (β) (by induction hypothesis for 1) iff x |= f (∼∼β) (by the definition
of f ). Q.E.D.
An Embedding-Based Completeness Proof for Nelson’s Paraconsistent Logic 211
Lemma 13. Let f be the mapping defined in Definition 3. For any Kripke
model hM, R, |=i, there exists a paraconsistent Kripke model hM, R, |=+ ,
|=− i such that for any formula α and any x ∈ M ,
1. x |= f (α) iff x |=+ α,
2. x |= f (∼α) iff x |=− α.
Proof. Similar to the proof of Lemma 12. Q.E.D.
Theorem 14 (Semantical embedding). Let f be the mapping defined in
Definition 3. For any formula α, α is N4-valid iff f (α) is LJ-valid.
Proof. By Lemmas 12 and 13. Q.E.D.
Theorem 15 (Completeness). For any formula α, N4 ` ⇒ α iff α is
N4-valid.
Proof. N4 ` ⇒ α iff LJ ` ⇒ f (α) (by Theorem 4) iff f (α) is LJ-valid
(by the Kripke completeness theorem for LJ) iff α is N4-valid (by Theorem
14). Q.E.D.
3.
Single-consequence semantics
Some modifications of Lemmas 12 and 13 give an alternative “single-consequence” semantics for N4, which has a single valuation |=∗ instead of the
dual valuations |=+ and |=− .
Definition 16. Let Φ be the set of propositional variables and Φ∼ be the
set {∼p | p ∈ Φ}. A single paraconsistent valuation |=∗ on a Kripke frame
hM, Ri is a mapping from Φ ∪ Φ∼ to 2M such that for any p ∈ Φ ∪ Φ∼ and
any x, y ∈ M , if x ∈ |=∗ (p) and xRy, then y ∈ |=∗ (p). We will write
x |=∗ p for x ∈ |=∗ (p). The single paraconsistent valuation |=∗ is extended
to a mapping from the set of all formulas to 2M by:
1.
2.
3.
4.
5.
x |=∗
x |=∗
x |=∗
x |=∗
x |=∗
α→β iff ∀y ∈ M [xRy and y |=∗ α imply y |=∗ β],
α ∧ β iff x |=∗ α and x |=∗ β,
α ∨ β iff x |=∗ α or x |=∗ β,
∼∼α iff x |=∗ α,
∼(α→β) iff x |=∗ α and x |=∗ ∼β,
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Norihiro Kamide
6. x |=∗ ∼(α ∧ β) iff x |=∗ ∼α or x |=∗ ∼β,
7. x |=∗ ∼(α ∨ β) iff x |=∗ ∼α and x |=∗ ∼β.
The hereditary condition holds for |=∗ .
Definition 17. A single paraconsistent Kripke model is a structure hM, R, |=∗ i
such that
1. hM, Ri is a Kripke frame,
2. |=∗ is a single paraconsistent valuation on hM, Ri.
A formula α is true in a single paraconsistent Kripke model hM, R, |=∗ i if
x |=∗ α for any x ∈ M , and is S-valid in a Kripke frame hM, Ri if it is
true for every single paraconsistent valuation |=∗ on the Kripke frame.
Lemma 18. For any paraconsistent Kripke model hM, R, |=+ , |=− i, there
exists a single paraconsistent Kripke model hM, R, |=∗ i such that for any
formula α and any x ∈ M ,
1. x |=+ α iff x |=∗ α,
2. x |=− α iff x |=∗ ∼α.
Proof. Let Φ be a set of propositional variables and Φ∼ be the set
{∼p | p ∈ Φ}. Suppose that hM, R, |=+ , |=− i is a paraconsistent Kripke
model where |=+ and |=− are mappings from Φ to the power set 2M of
M , and that the hereditary condition w.r.t. p ∈ Φ holds for |=+ and |=− .
Suppose that hM, R, |=∗ i is a single paraconsistent Kripke model where |=∗
is a mapping from Φ ∪ Φ∼ to 2M , and that the hereditary condition w.r.t.
p ∈ Φ ∪ Φ∼ holds for |=∗ . Suppose moreover that these models satisfy the
following conditions: for any x ∈ M and any p ∈ Φ,
1. x |=+ p iff x |=∗ p,
2. x |=− p iff x |=∗ ∼p.
Then, the lemma is proved by (simultaneous) induction on the complexity of α. The proof is similar to the proof of Lemma 12. Q.E.D.
Lemma 19. For any single paraconsistent Kripke model hM, R, |=∗ i, there
exists a paraconsistent Kripke model hM, R, |=+ , |=− i such that for any
formula α and any x ∈ M ,
1. x |=∗ α iff x |=+ α,
2. x |=∗ ∼α iff x |=− α.
An Embedding-Based Completeness Proof for Nelson’s Paraconsistent Logic 213
Proof. Similar to the proof of Lemma 18. Q.E.D.
Theorem 20 (Equivalence). For any formula α, α is N4-valid iff α is
S-valid.
Proof. By Lemmas 18 and 19. Q.E.D.
Theorem 21 (Completeness). For any formula α, N4 ` ⇒ α iff α is
S-valid.
Proof. By Theorems 20 and 15. Q.E.D.
Acknowledgements. I would like to thank an anonymous referee and
Professor J. Michael Dunn for their valuable comments. This research
was supported by the Alexander von Humboldt Foundation and by the
Japanese Ministry of Education, Culture, Sports, Science and Technology,
Grant-in-Aid for Young Scientists (B) 20700015.
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Waseda Institute for Advanced Study
1-6-1 Nishi Waseda, Shinjuku-ku Tokyo 169-8050, JAPAN
e-mail: [email protected]