Towards a Standard Completeness for a Probabilistic Logic on Infinite
Total Page:16
File Type:pdf, Size:1020Kb
Towards a standard completeness for a probabilistic logic on infinite-valued events Tommaso Flaminio Artificial Intelligence Research Institute (IIIA - CSIC) Campus de la Univ. Aut`onomade Barcelona s/n, 08193 Bellaterra, Spain. [email protected] Abstract. MV-algebras with internal states, or SMV-algebras for short, are the equivalent algebraic semantics of the logic SFP(L ; L)which allows to represent and reason about the probability of infinite-valued events. In this paper we will make the first steps towards establishing complete- ness for SFP(L ; L)with respect to the class of standard SMV-algebras, a problem which has been left open since the first paper on SMV-algebras was published. More precisely we will prove that, if we restrict our atten- tion to a particular, yet expressive, subclass of formulas, then theorems of SFP(L ; L)are the same as tautologies of a class of SMV-algebras that can be reasonably called \standard". Keywords. MV-algebras, states, internal states, standard completeness, probabilistic logic. 1 Introduction MV-algebras with an internal state, or SMV-algebras, have been introduced in [6] and they consist of an MV-algebra A (cf. [2]) and a unary map σ on A axiomatized so as to preserve some basic properties of a state of A (cf. [12, 5]), i.e., a [0; 1]-valued, normalized and finitely additive function. These structures form a variety, denoted by SMV, which is the equivalent algebraic semantics for the probabilistic logic SFP(L ; L) which permits to represent and reason about the probability of infinite-valued events. In that logic, in particular, events are described by formulas of the infinite-valuedLukasiewicz calculus (cf. [2]). From the perspective of reasoning about uncertainty, the interest ofLukasie- wicz events is twofold: whilst capturing properties of the world which are better described as gradual rather than yes-or-no, they also mimic bounded random variables. Indeed, anyLukasiewicz event e may be regarded as a [0; 1]-valued continuous function e∗ on a compact Hausdorff space (see [2, Theorem 9.1.5] and Section 2) and any state of e coincides with the expected value of e∗ (see [4, Remark 2.8] and Section 3). SMV-algebras provide a framework to treat a generalization of classical prob- ability theory in a universal-algebraic setting. However, notwithstanding their meaningful expressive power, several problem concerning their universal-algebraic properties are still open. Here, we will concentrate on the following issue: de- termining a proper subclass of SMV-algebras which generates SMV and which could be regarded as a class of \measure-theoretical" models. In other words, the above problem is to establishing a \standard completeness" for the logic SFP(L ; L). In particular we will show that, upon restricting our language to a special, yet sufficiently expressive subset of well-formed formulas, the logic SFP(L ; L) turns out to be sound and complete with respect to a prominent subclass of SMV-algebras which can be reasonably called \standard". Indeed, these standard SMV-algebras are grounded on the same MV-algebra of continuous real-valued functions and their internal states are obtained by integrating the continuous functions which correspond toLukasiewicz events by a regular, Borel (and hence σ-additive) probability measure. Although our main result only offers a partial solution to the problem of establishing standard completeness for SFP(L ; L), the idea behind its proof is promising and it suggests ways to tackle the general problem. Indeed, the class of formulas to which we will restrict our attention, called purely probabilistic formulas, form the ground probabilistic language upon which the all formulas of SFP(L ; L)are inductively defined. This paper is organizes as follows: in the next section we will recall some basic definitions and needed results aboutLukasiewicz logic and MV-algebras. Section 3 is dedicated to a brief introduction to generalized probability theory onLukasiewicz logic, i.e., state theory, while in Section 4 we will focus on SMV- algebras and the probabilistic logic SFP(L ; L). In Section 5 we will prove the main results of this paper and in particular that a purely probabilistic formula φ is a theorem of SFP(L ; L)iff φ holds in all standard SMV-algebras. 2Lukasiewicz logic and MV-algebras The language of the infinite-valuedLukasiewicz calculusLis made of a countable set of propositional variables fq1; q2;:::g, the binary connective ⊕, the unary connective : and the constant ?. Formulas are defined by the usual inductive rules. Further useful connectives are definable as follows: > = :?; ' = :(:' ⊕ : ); ' ! = :' ⊕ ; ' ^ = ' (' ! ); ' _ = :(:' ^ : ); ' = :(' ! ); d('; ) = (' ! ) ⊕ ( ! '). We invite the reader to consult [8, 2] for an axiomatization ofLukasiewicz logic. For the sake of the present paper it is important to recall that this calculus has an equivalent algebraic semantics: the variety MV of MV-algebras. These are structures of the form A = (A; ⊕; :; ?) (the same language ofLukasiewicz logic), where (A; ⊕; ?) is a commutative monoid and, defining further operations as above, the following equations hold: x ⊕ > = > (x ! y) ! y = (y ! x) ! x. The algebraizability ofLukasiewicz logic with respect to MV implies thatLis sound and complete w.r.t. the class of MV-algebras. This means the following: for every MV-algebra A, define an A-valuation as a map v from the propositional variables qi's to A which extends to all formulas by truth functionality; then, a formula ' is a theorem ofLukasiewicz logic iff for every MV-algebra A and every A-valuation v, v(') = >. Furthermore, every formula ' on propositional variables q1; : : : ; qk can be regarded, on the algebraic side, as a term t(x1; : : : ; xk) on the same number of variables. For every MV-algebra A and for every term t, we denote by tA the interpretation (or semantics) of t in A. In every MV-algebra the relation x ≤ y iff x ! y = > determines a lattice- order which coincides with the one given by the operations ^ and _. If in A the order ≤ is linear, we will say that A is an MV-chain. The following collects two typical examples of MV-algebras which will play a central role in this paper. Example 1. (1) The standard MV-algebra [0; 1]MV has support on the real unit interval [0; 1] and for all x; y 2 [0; 1], x ⊕ y = minf1; x + yg, :x = 1 − x and ? = 0. The semantics, in [0; 1]MV , of theLukasiewicz connectives we defined above is as follows: > = 1; x y = maxf0; x + y − 1g; x ! y = minf1; 1 − x + yg; x ^ y = minfx; yg; x _ y = maxfx; yg; x y = maxf0; x − yg; d(x; y) = jx − yj (the usual Euclidean distance). Chang's theorem [1] shows that MV is generated by [0; 1]MV , whenceLukasie- wicz logic is complete w.r.t. to [0; 1]MV . (2) For every k 2 !, let F(k) be the set of functions f : [0; 1]k ! [0; 1] which are continuous, piecewise linear and such that each piece has integer coefficient, i.e., the set of k-variable McNaughton functions [2]. The point-by-point applica- tion of the operations of [0; 1]MV makes F(k) an MV-algebra which coincides, up to isomorphism, with the free MV-algebra over k-variables. For every formula ofLukasiewicz language, we will denote by f its corresponding McNaughton function. The free MV-algebra over infinitely-many generators will be henceforth denoted by F(!). Let A and B be two MV-algebras. An MV-homomorphism is a function h : A ! B such that, adopting without danger of confusion the same symbols for the operations of both algebras: (1) h(?) = ?; (2) h(x⊕y) = h(x)⊕h(y); (3) h(:x) = :h(x). If S is a subset of A, a map h : S ! B is a partial homomorphism provided that the above conditions (1-3) holds for the elements of S. Injective (partial) homomorphisms are called (partial) embeddings. (Partial) embeddings of A to B will be denoted by A ,! B (A ,!p B, respectively). Let A be an MV-algebra and let S be a subset of A. We denote hSiA the MV- subalgebra of A generated by S. In particular, if S is finite, for every b 2 hSiA, A there are a1; : : : ; an 2 S and an n-ary term t such that b = t (a1; : : : ; an). Lemma 1. (1) For every MV-chain A and for every finite subset S of A, there p exists a partial embedding hS : S,! [0; 1]MV . (2) Let A; B be MV-algebras, let S be a finite subset of A. Every partial homo- ^ morphism hS : S ⊆ A ! B extends to a homomorphisms hS : hSiA ! B. Proof. (1) Immediately follows from Gurevich-Kokorin theorem (see for instance [8, Theorem 1.6.17]). (2) For every element b 2 hSiA, there is an term t and a1; : : : ; an 2 S such that A ^ b = t (a1; : : : ; an). Define hS : hSiA ! B as follows: ^ ^ A B h(b) = h(t (a1; : : : ; an)) = t (h(a1); : : : ; h(an)): It is easy to see that h^ commutes with the operations of A and B, whence it is an MV-homomorphism. ut An ideal of an MV-algebra A is a subset I of A such that: (1) ? 2 I; (2) if a 2 I and b 2 I, then a ⊕ b 2 I; (3) if a 2 I and b ≤ a, then b 2 I. Every ideal I of A determines a congruence ΘI of A: (a; b) 2 ΘI iff d(a; b) 2 I. For every MV-algebra A and every ideal I of A, we denote by A=I the quotient of A by the congruence ΘI .