Logic and Logical Philosophy Volume 26 (2017), 197–206 DOI: 10.12775/LLP.2016.025 Yaroslav Petrukhin NATURAL DEDUCTION FOR THREE-VALUED REGULAR LOGICS Abstract. In this paper, I consider a family of three-valued regular logics: the well-known strong and weak S. C. Kleene's logics and two intermediate logics, where one was discovered by M. Fitting and the other one by E. Komendantskaya. All these systems were originally presented in the semantical way and based on the theory of recursion. However, the proof theory of them still is not fully developed. Thus, natural deduction systems are built only for strong Kleene's logic both with one (A. Urquhart, G. Priest, A. Tamminga) and two designated values (G. Priest, B. Kooi, A. Tamminga). The purpose of this paper is to provide natural deduction systems for weak and intermediate regular logics both with one and two designated values. Keywords: natural deduction; regular logic; Kleene's logic; three-valued logic 1. Introduction Regular logics were first mentioned in the works of S. C. Kleene [4, 5] where he defined two three-valued regular logics (a strong and a weak one). A regular logic is understood as a logic which propositional connectives are regular. What is its regularity, and how it can be useful, are explained in [5] as follows: We conclude that, in order for the propositional connectives to be partial recursive operations (or at least to produce partial recursive predicates when applied to partial recursive predicates), we must choose tables for them which are regular, in the following sense: A given column (row) contains t in the u row (column), only if the column (row) consists entirely of t's; and likewise for f. [5, p. 334] Received May 27, 2016. Revised August 23, 2016. Published online August 29, 2016 © 2016 by Nicolaus Copernicus University 198 Yaroslav Petrukhin The values t and f are understood in a usual way, i.e., as "truth" and "falsehood", value u is understood as "not defined". For t, u, and f we will use 1, 1⁄2, and 0, respectively. Let At and Form be, respectively, the set of all propositional variables and the set of all formulas of the propositional language in language that is built with propositional variables and the propositional connectives: ¬, ∨, and ∧. All logics we will build in the set Form. Let us denote a truth-table f for a connective c by fc. In the three-valued case, there are only four regular conjunctions and disjunctions (see [6] for details).1 Depending on the choice of the number of designated values each collection of connectives yields two different regular logics. First, we have two logics K3 and K 2 3 , respectively, for the matrixes 〈{1, 1⁄2, 0}, f¬, f∨, f∧, {1}〉 and 〈{1, 1⁄2, 0}, f¬, f∨, f∧, {1, 1⁄2}〉, where f¬ 1 0 1⁄2 1⁄2 0 1 f∨ 1 1⁄2 0 1 1 1 1 1⁄2 1 1⁄2 1⁄2 0 1 1⁄2 0 f∧ 1 1⁄2 0 1 1 1⁄2 0 1⁄2 1⁄2 1⁄2 0 0 0 0 0 Second, we have two logics Kw3 and K w2 3 , respectively, for the matrixes 〈{1, 1⁄2, 0}, f¬, f∨, f∧, {1}〉 and 〈{1, 1⁄2, 0}, f¬, f∨, f∧, {1, 1⁄2}〉, where f¬ is the same as for K3 and f∨ 1 1⁄2 0 1 1 1⁄2 1 1⁄2 1⁄2 1⁄2 1⁄2 0 1 1⁄2 0 f∧ 1 1⁄2 0 1 1 1⁄2 0 1⁄2 1⁄2 1⁄2 1⁄2 0 0 1⁄2 0 The logics K3 and K w 3 are called strong Kleene's logic and weak Kleene's logic, respectively. They were introduced by Kleene in [4] in 1938 (see also [5]). However, K3 appeared in [8] as a fragment of Łukasiewicz's logic Ł3 (1920), and K w 3 appeared in [1] as a fragment of Bochvar's logic B3 (1938) independently by [4]. Note that K 2 3 is also known as LP (Logic of Paradox) and was carefully studied by Priest [11]. A natural deduction system for K3 was first created by Urquhart [14]; later Priest [11] and Tamminga [12] independently obtained the same result. A natural deduction system for K2 3 was created by Priest [11] and later it was independently provided by Kooi and Tamminga [7]. 1 Four-valued regular logics are described in [13]. Natural deduction for three-valued regular logics 199 Thirdly, we have two logics K→ 3 and K→2 3 , respectively, for the matrixes 〈{1, 1⁄2, 0}, f¬, f∨, f∧, {1}〉 and 〈{1, 1⁄2, 0}, f¬, f∨, f∧, {1, 1⁄2}〉, where f¬ is the same as for K3 and f∨ 1 1⁄2 0 1 1 1 1 1⁄2 1⁄2 1⁄2 1⁄2 0 1 1⁄2 0 f∧ 1 1⁄2 0 1 1 1⁄2 0 1⁄2 1⁄2 1⁄2 1⁄2 0 0 0 0 Fourthly, we have two logics K← 3 and K←2 3 , respectively, for the matrixes 〈{1, 1⁄2, 0}, f¬, f∨, f∧, {1}〉 and 〈{1, 1⁄2, 0}, f¬, f∨, f∧, {1, 1⁄2}〉, where f¬ is the same as for K3 and f∨ 1 1⁄2 0 1 1 1⁄2 1 1⁄2 1 1⁄2 1⁄2 0 1 1⁄2 0 f∧ 1 1⁄2 0 1 1 1⁄2 0 1⁄2 1⁄2 1⁄2 0 0 0 1⁄2 0 K → 3 and K← 3 are called intermediate logics. The logic K→ 3 was first discovered by Fitting [2]. The logic K← 3 was discovered by Komendantskaya [6]. 2. Rules for natural deduction systems We will use the following rules of inference: (EFQ) A ¬A B (EM) A ∨ ¬A (¬¬I) A ¬¬A (¬¬E) ¬¬A A (∨I1) A A ∨ B (∨I2) B A ∨ B (∨I3) ¬A ∧ B A ∨ B (∨I4) A ∧ ¬B A ∨ B (∨I5) A ∧ B A ∨ B (∧I1) A B A ∧ B (∧I2) A ¬A A ∧ B (∧I3) B ¬B A ∧ B (∧E1) A ∧ B A (∧E2) A ∧ B B (∧E3) A ∧ B ¬A ∨ B (∧E4) A ∧ B A ∨ ¬B (¬∨I1) ¬A ∧ ¬B ¬(A ∨ B) (¬∨I2) A ∧ ¬A ¬(A ∨ B) (¬∨I3) B ∧ ¬B ¬(A ∨ B) (¬∨E1) ¬(A ∨ B) ¬A ∧ ¬B (¬∨E2) ¬(A ∨ B) ¬A ∨ B (¬∨E3) ¬(A ∨ B) A ∨ ¬B (¬∧I1) ¬A ∨ ¬B ¬(A ∧ B) (¬∧I2) ¬A ¬(A ∧ B) (¬∧I3) ¬B ¬(A ∧ B) 200 Yaroslav Petrukhin (¬∧I4) ¬A ∧ B ¬(A ∧ B) (¬∧I5) A ∧ ¬B ¬(A ∧ B) (¬∧E) ¬(A ∧ B) ¬A ∨ ¬B Moreover, we consider the following four versions of the proof construction rule (∨E): (∨E1) [A] [B] A ∨ B C C C (∨E2) [A] [¬A ∧ B] A ∨ B C C C (∨E3) [B] [A ∧ ¬B] A ∨ B C C C (∨E4) [A ∧ B] [A ∧ ¬B] [¬A ∧ B] A ∨ B C C C C where [X ] means that the assumption X is discharged Note that Rcl := {(EFQ), (EM), (¬¬I), (¬¬E), (∨I1), (∨I2), (∨E1), (∧I1), (∧E1), (∧E2), (¬∨I1), (¬∨E1), (¬∧I1), (¬∧E)} is a set of rules of a natural deduction system for classical logic. For natural deduction systems for K3 and K 2 3 are suitable sets Rcl \{(EM)} and Rcl\{(EFQ)}, respectively. Moreover, Rcl \{(EFQ), (EM)} is a set of inference rules for FDE (it was proven by Priest in [11]; for more detailed proof see [10]). The notion of a deduction of A from Γ in all natural deduction systems described in this paper is defined as a tree labeled with formulas. As an example, consider the following deduction of (A∧B) ∨ (A∧C) from A ∧ (B ∨ C) in the mentioned systems. A ∧ (B ∨ C) (∧E2) B ∨ C A ∧ (B ∨ C) (∧E1) A (1) [B] (∧I1) A ∧ B (∨I1) (A ∧ B) ∨ (A ∧ C) A ∧ (B ∨ C) (∧E1) A (2) [C] (∧I1) A ∧ C (∨I2) (A ∧ B) ∨ (A ∧ C) (∨E1)(1),(2) (A ∧ B) ∨ (A ∧ C) 3. Natural deduction system for K→ 3 A set of rules for K→3 is as follows: (EFQ), (¬¬I), (¬¬E), (∨I1), (∨I3), (∨E2), (∧I1), (∧E1), (∧E2), (¬∨I1), (¬∨E1), (¬∧I2), (¬∧I5), (¬∧E). Soundness follows by a simple routine check. Theorem 3.1 (Soundness). For any Γ ⊆ Form and any A ∈ Form: if Γ ⊢K→ 3 A then Γ |=K→ 3 A. Natural deduction for three-valued regular logics 201 For completeness proof Henkin's method is used. At that the notational conventions of [7, 12] are adopted. Definition 3.1. A set of formulas Γ is a nontrivial prime theory iff the following conditions are met: (Γ1) Γ 6= Form (non-triviality) (Γ2) Γ ⊢K→ 3 A iff A ∈ Γ (closure of ⊢K→ 3 ) (Γ3) if A ∨ B ∈ Γ then either A ∈ Γ or both ¬A ∈ Γ and B ∈ Γ (primeness) Definition 3.2. For all Γ ⊆ Form and A ∈ Form, e(A, Γ ) is a canonic valuation iff the following conditions hold: e(A, Γ ) =            1 ⇐⇒ A ∈ Γ and ¬A /∈ Γ 1⁄2 ⇐⇒ A /∈ Γ and ¬A 6∈ Γ 0 ⇐⇒ A /∈ Γ and ¬A ∈ Γ ∅ ⇐⇒ A ∈ Γ and ¬A ∈ Γ Note. For logics with two designated values conditions for 1⁄2 and ∅ are defined in a different way: e(A, Γ ) = { 1⁄2 ⇐⇒ A ∈ Γ and ¬A ∈ Γ ; ∅ ⇐⇒ A 6∈ Γ and ¬A /∈ Γ Lemma 3.1. For all Γ ⊆ Form and A, B ∈ Form: 1. e(A, Γ ) 6= ∅, 2. f¬(e(A, Γ )) = e(¬A, Γ ), 3. f∨(e(A, Γ ), e(B, Γ )) = e(A ∨ B, Γ ), 4. f∧(e(A, Γ ), e(B, Γ )) = e(A ∧ B, Γ ). Proof. Ad 1. Suppose e(A, Γ ) = ∅. Then A ∈ Γ , ¬A ∈ Γ . By the rule (EFQ), B ∈ Γ , that is Γ = Form. This contradicts to (Γ1). Therefore, e(A, Γ ) 6= ∅. Ad 2. If e(A, Γ ) = 1 then A ∈ Γ , ¬A /∈ Γ . By the rule (¬¬I), ¬¬A ∈ Γ . Hence, e(¬A, Γ ) = 0 = f¬(1) = f¬(e(A, Γ )). If e(A, Γ ) = 1⁄2 then A 6∈ Γ , ¬A 6∈ Γ . Suppose ¬¬A ∈ Γ . By the rule (¬¬E), A ∈ Γ . Contradiction. Consequently, ¬¬A 6∈ Γ . Hence e(¬A, Γ ) = 1⁄2 = f¬(1⁄2) = f¬(e(A, Γ )). If e(A, Γ ) = 0 then A 6∈ Γ , ¬A ∈ Γ . Suppose ¬¬A ∈ Γ . By the rule (¬¬E), A ∈ Γ . Contradiction. Consequently, ¬¬A /∈ Γ . Hence e(¬A, Γ ) = 1 = f¬(0) = f¬(e(A, Γ )). 202 Yaroslav Petrukhin Ad 3. If e(A, Γ ) = 1 and e(B, Γ ) = 1, then A ∈ Γ , ¬A /∈ Γ , B ∈ Γ , ¬B /∈ Γ . By the rule (∨I1), A ∨ B ∈ Γ . Let ¬(A ∨ B) ∈ Γ . By the rule (¬∨E1), ¬A∧¬B ∈ Γ . Applying the rules (∧E1) and (∧E2), get ¬A ∈ Γ and ¬B ∈ Γ . Contradiction. Hence ¬(A ∨ B) /∈ Γ . Consequently, e(A ∨ B, Γ ) = 1 = f∨(1, 1) = f∨(e(A, Γ ), e(B, Γ )). If e(A, Γ ) = 1⁄2 and e(B, Γ ) = 1, then A /∈ Γ , ¬A /∈ Γ , B ∈ Γ , ¬B /∈ Γ . Let A ∨ B ∈ Γ . By (Γ3), either A ∈ Γ or both ¬A ∈ Γ and B ∈ Γ . Since A /∈ Γ , so ¬A ∈ Γ and B ∈ Γ . But then ¬A ∈ Γ . Contradiction. A ∨ B /∈ Γ . Let ¬(A ∨ B) ∈ Γ . Applying the rules (¬∨E1), (∧E1), and (∧E2), get ¬A ∈ Γ , ¬B ∈ Γ . Contradiction. Hence ¬(A ∨ B) /∈ Γ . Consequently, e(A ∨ B, Γ ) = 1⁄2 = f∨(1⁄2, 1) = f∨(e(A, Γ ), e(B, Γ )). If e(A, Γ ) = 0 and e(B, Γ ) = 1, then A /∈ Γ , ¬A ∈ Γ , B ∈ Γ , ¬B 6∈ Γ . Applying the rules (∧I1) and (∨I3), get A ∨ B ∈ Γ . Let ¬(A ∨ B) ∈ Γ . Applying the rules (¬∨E1) and (∧E2), get ¬B ∈ Γ . Contradiction. Hence ¬(A ∨ B) /∈ Γ . Consequently, e(A ∨ B, Γ ) = 1 = f∨(0, 1) = f∨(e(A, Γ ), e(B, Γ )). The other cases are proved similarly. Ad 4. If e(A, Γ ) = 1 and e(B, Γ ) = 1, then A ∈ Γ , ¬A /∈ Γ , B ∈ Γ , ¬B 6∈ Γ . By the rule (∧I1), A ∧ B ∈ Γ . Let ¬(A ∧ B) ∈ Γ . By the rule (¬∧E), ¬A ∨ ¬B ∈ Γ . By (Γ3), either ¬A ∈ Γ or both ¬¬A ∈ Γ and ¬B ∈ Γ . Since ¬A /∈ Γ , so ¬¬A ∈ Γ and ¬B ∈ Γ . But then ¬B ∈ Γ . Contradiction. Hence ¬(A ∧ B) 6∈ Γ . Therefore, e(A ∧ B, Γ ) = 1 = f∧(1, 1) = f∧(e(A, Γ ), e(B, Γ )). If e(A, Γ ) = 1 and e(B, Γ ) = 0, then A ∈ Γ , ¬A 6∈ Γ , B 6∈ Γ , ¬B ∈ Γ . Let A ∧ B ∈ Γ . By the rule (∧E2), B ∈ Γ . Contradiction. A ∧ B 6∈ Γ . Applying the rules (∧I1) and (¬∧I5), get ¬(A ∧ B) ∈ Γ . Therefore, e(A ∧ B, Γ ) = 0 = f∧(1, 0) = f∧(e(A, Γ ), e(B, Γ )). If e(A, Γ ) = 0 and e(B, Γ ) = 1⁄2, then A /∈ Γ , ¬A ∈ Γ , B 6∈ Γ , ¬B 6∈ Γ . Let A ∧ B ∈ Γ . By the rule (∧E1), A ∈ Γ . Contradiction. A ∧ B /∈ Γ . By the rule (¬∧I2), ¬(A ∧ B) ∈ Γ . Therefore, e(A ∧ B, Γ ) = 0 = f∧(0, 1⁄2) = f∧(e(A, Γ ), e(B, Γ )). The other cases are proved similarly. By a structural induction on formulas, using Lemma 3.1 we obtain: Lemma 3.2. Let Γ be any nontrivial prime theory and vΓ be an arbitrary valuation such that vΓ (p) = e(p, Γ ), for any p ∈ At. Then for any A ∈ Form we have vΓ (A) = e(A, Γ ). Natural deduction for three-valued regular logics 203 Lemma 3.3 (Lindenbaum). For all Γ ⊆ Form, A ∈ Form, if Γ 0K→ 3 A, then there is Γ ∗ ⊆ Form such that (1) Γ ⊆ Γ ∗, (2) Γ ∗ 0K→ 3 A, and (3) Γ ∗ is a nontrivial prime theory. Proof. Let B1, B2, . . . be an enumeration of all formulas. Now define a sequence of sets of formulas Γ1, Γ2, . . . . Let Γ1 = Γ and Γn+1 = { Γn ⋃ {Bn+1} if Γn ⋃ {Bn+1} 0K→ 3 A, Γn+1 = Γn otherwise. We put Γ ∗ := ⋃ ∞ n=1 Γn. Then: (1) Follows from the definition of Γ ∗. (2) By a straightforward induction on i. Since Γ1 = Γ , so Γ1 0K→ 3 A. By the inductive assumption, Γi 0K→ 3 A. If Γi+1 = Γi, then Γi+1 0K→ 3 A. If Γi+1 6= Γi, then Γi+1 = Γi ∪{Bi+1}. Assume that Γi ∪{Bi+1} ⊢K→ 3 A. But then Γi+1 = Γi, by the definition of the sequence of Γ1, Γ2, . . . . Contradiction. Hence, Γi ∪ {Bi+1} 0K→ 3 A. Thus, if Γi+1 6= Γi, then Γi+1 0K→ 3 A. Clearly, that if it holds for Γi that Γi 0K→ 3 A, then Γ ∗ 0K→ 3 A. (3) We show the condition (Γ1)–(Γ3). (Γ1) Since Γ ∗ 0K→ 3 A, so it is obvious that Γ ∗ 6= Form. (Γ2) "⇒" Assume that Γ ∗ ⊢K→ 3 B. Then there is i such that B = Bi and for some Γi we have Γi ⊢K→ 3 Bi. Assume that Bi 6∈ Γi. Hence Γi−1 ∪ {Bi} ⊢K→ 3 A. But then Γ ∗ ⊢K→ 3 A, because Γi−1 ⊆ Γ ∗ and Γ ∗ ⊢K→ 3 B. Nonetheless, it was proved in (2) that Γ ∗ 0K→ 3 A. In this case Bi ∈ Γi. Therefore, if Γ ∗ ⊢K→ 3 B then B ∈ Γ ∗. "⇐" Suppose B ∈ Γ , Γ ∗ 0K→ 3 B. Then there is i such that B = Bi and for some Γi−1 we have Γi−1 ∪ {Bi} ⊢K→ 3 A. Since Γi−1 ⊆ Γ ∗, so Γ ∗ ∪ {Bi} ⊢K→ 3 A. From the latter and the fact that Γ ∗ 0K→ 3 A, obtain that Bi /∈ Γ ∗, that is B 6∈ Γ ∗. Contradiction. Therefore, Γ ∗ ⊢K→ 3 B. Thus, if B ∈ Γ ∗ then Γ ∗ ⊢K→ 3 B. (Γ3) To show (⋆): if B∨C ∈ Γ ∗ then either B ∈ Γ ∗ or both ¬B ∈ Γ ∗ and C ∈ Γ ∗, we first prove the following statements: (a) If B ∨ C ∈ Γ ∗ then either B ∈ Γ ∗ or ¬B ∧ C ∈ Γ ∗. (b) If B ∈ Γ ∗ or ¬B ∧ C ∈ Γ ∗, then either B ∈ Γ ∗ or both ¬B ∈ Γ ∗ and C ∈ Γ ∗. Suppose B∨C ∈ Γ ∗, but B /∈ Γ ∗ and ¬B∧C 6∈ Γ ∗. Since B∨C ∈ Γ ∗, so Γ ∗ ⊢K→ 3 B∨C; cf. (Γ2)). On the other hand, for some i and j we have: 204 Yaroslav Petrukhin B = Bi and ¬B ∧C = Bj ; Γi−1 ∪{Bi} ⊢K→ 3 A, and Γj−1 ∪{Bj} ⊢K→ 3 A. Moreover, Γi−1 ⊆ Γ ∗ and Γj−1 ⊆ Γ ∗. Then Γ ∗ ∪ {Bi} ⊢K→ 3 A and Γ ∗ ∪ {Bj} ⊢K→ 3 A. From the latter and the fact that Γ ∗ ⊢K→ 3 B ∨ C, by the rule (∨E2), we obtain that Γ ∗ ⊢K→ 3 A, but according to (2), Γ ∗ 0K→ 3 A. Contradiction. So the statement (a) is proved. Using the rules (∧E1) and (∧E2), it is simple to prove the statement (b). Moreover, using the transitivity and statements (a) and (b), we obtain (⋆). Theorem 3.2 (Completeness). For all Γ ⊆ Form and A ∈ Form: if Γ |=K→ 3 A then Γ ⊢K→ 3 A. Proof. Will be provided by contraposition. Let Γ 0K→ 3 A. Then, by Lemma 3.3, there is Γ ∗ ⊆ Form such that Γ ⊆ Γ ∗, Γ ∗ 0K→ 3 A, and Γ ∗ is a nontrivial prime theory. By Lemma 3.2, there is a valuation vΓ ∗ such that: vΓ ∗(B) = 1, for any B ∈ Γ , and vΓ ∗(A) 6= 1. But then Γ 6|=K→ 3 A. In the light of theorems 3.1 and 3.2 we obtain: Theorem 3.3 (Adequacy). For all Γ ⊆ Form and A ∈ Form: Γ ⊢K→ 3 A iff Γ |=K→ 3 A. 4. Natural deduction systems for K→ 3 -related logics For a natural deduction system of the logic K→2 3 we have the following set of rules: (EM), (¬¬I), (¬¬E), (∨I1), (∨I2), (∨E1), (∧I1), (∧I2), (∧E1), (∧E3), (¬∨I1), (¬∨I2), (¬∨E1), (¬∨E3), (¬∧I1), (¬∧E). For a natural deduction system of the logic K← 3 we have the following set of rules: (EFQ), (¬¬I), (¬¬E), (∨I2), (∨I4), (∨E3), (∧I1), (∧E1), (∧E2), (¬∨I1), (¬∨E1), (¬∧I3), (¬∧I4), (¬∧E). For a natural deduction system of the logic K←23 we have the following set of rules: (EM), (¬¬I), (¬¬E), (∨I1), (∨I2), (∨E1), (∧I1), (∧I3), (∧E2), (∧E4), (¬∨I1), (¬∨I3), (¬∨E1), (¬∨E2), (¬∧I1), (¬∧E). For a natural deduction system of the logic Kw 3 we have the following set of rules: (EFQ), (¬¬I), (¬¬E), (∨I3), (∨I4), (∨I5), (∨E4), (∧I1), (∧E1), (∧E2), (¬∨I1), (¬∨E1), (¬∧I1), (¬∧E). Natural deduction for three-valued regular logics 205 For a natural deduction system of the logic Kw2 3 we have the following set of rules: (EM), (¬¬I), (¬¬E), (∨I1), (∨I2), (∨I5), (∨E1), (∧I1), (∧I2), (∧I3), (∧E3), (∧E4), (¬∨I1), (¬∨I2), (¬∨I3), (¬∨E1), (¬∧I1), (¬∧E). Similarly as Theorem 3.3 we obtain: Theorem 4.1 (Adequacy). Let L be one of the following logics: K→2 3 , K ← 3 , K←2 3 , Kw 3 , Kw2 3 . Then for all Γ ⊆ Form and A ∈ Form: Γ ⊢L A iff Γ |=L A. Concluding remarks Thus, I have constructed natural deduction systems for weak and intermediate regular logics. Consequently, all three-valued regular logics (both with one and two designated values) are presented in the form of natural deduction systems. However, development of proof-search algorithms suitable for these systems is left untouched and, hopefully, will stimulate further research. One more subject for future investigations could be study of proof theory of four-valued regular logics. Acknowledgments. I would like to thank an anonymous referee for helpful comments on the earlier version of this paper. References [1] Bochvar, D. A., "On a three-valued logical calculus and its application to the analysis of the paradoxes of the classical extended functional calculus", History and Philosophy of Logic 2 (1981): 87–112. English translation of Bochvar's paper of 1938. DOI: 10.1080/01445348108837023 [2] Fitting, M., "Kleene's three valued logics and their children", Fundamenta Informaticae 20 (1992): 113–131. [3] Karpenko, A. S., The Development of Many-Valued Logic (in Russian), LKI, 2010. [4] Kleene, S. C., "On a notation for ordinal numbers", The Journal of Symbolic Logic 3 (1938): 150–155. DOI: 10.2307/2267778 [5] Kleene, S. C., Introduction to Metamathematics, Sixth Reprint, WoltersNoordhoff Publishing and North-Holland Publishing Company, 1971. [6] Komendantskaya, E. Y., "Functional expressibility of regular Kleene's logics" (in Russian), Logical Investigations 15 (2009): 116–128. 206 Yaroslav Petrukhin [7] Kooi, B., and A. Tamminga, "Completeness via correspondence for extensions of the logic of paradox", The Review of Symbolic Logic 5 (2012): 720–730. DOI: 10.1017/S1755020312000196 [8] Łukasiewicz, J., "On three-valued logic", pages 87–88 in Selected Works, L. Borkowski (ed.), Amsterdam, North-Holland Publishing Company, 1997 (English translation of Łukasiewicz's paper of 1920). [9] Mendelson, E., Introduction to Mathematical Logic, Fourth Edition, Chapman & Hall, 1997. [10] Petrukhin, Y. I., "Correspondence analysis for first degree entailment", Logical Investigations 22, 1 (2016): 108–124. [11] Priest, G., "Paraconsistent logic", in Handbook of Philosophical Logic, Second Edition, Vol. 6, M. Gabbay and F. Guenthner (eds.), Dordrecht, Kluwer, 2002. DOI: 10.1007/978-94-017-0460-1_4 [12] Tamminga, A., "Correspondence analysis for strong three-valued logic", Logical Investigations 20 (2014): 255–268. [13] Tomova, N. E., "About four-valued regular logics" (in Russian), Logical Investigations 15 (2009): 223–228. [14] Urquhart, A., "Basic many-valued logic", in Handbook of Philosophical Logic, Second Edition, Vol. 2, M. Gabbay and F. Guenthner (eds.), Dordrecht, Kluwer, 2001. DOI: 10.1007/978-94-017-0452-6_4 Yaroslav Petrukhin Department of Philosophy Moscow State University Moscow, Russia yaroslav.petrukhin@mail.ru