Research Article | Open Access

# Roughness in Lattice Ordered Effect Algebras

**Academic Editor:**Bijan Davvaz

#### Abstract

Many authors have studied roughness on various algebraic systems. In this paper, we consider a lattice ordered effect algebra and discuss its roughness in this context. Moreover, we introduce the notions of the interior and the closure of a subset and give some of their properties in effect algebras. Finally, we use a Riesz ideal induced congruence and define a function in a lattice ordered effect algebra and build a relationship between it and congruence classes. Then we study some properties about approximation of lattice ordered effect algebras.

#### 1. Introduction

Quantum effects play a basic role in the foundations of quantum mechanics. In 1994, Foulis and Bennett introduced effect algebras for modeling unsharp measurement in a quantum mechanical system [1]. In the same year, Kôpka and Chovanec introduced equivalent structures called D-posets [2]. It is a generalization of many structures which arise in quantum physics [3] and in mathematical economics [4, 5], in particular, of orthomodular lattices in noncommutative measure theory and MV-algebras in fuzzy measure theory. Effect algebras were originally introduced as partial algebraic structures and after 1994, there have been a great number of papers concerning effect algebras [6–11]. They are a generalization of many structures which arise in quantum physics.

The theory of rough sets was first introduced by Pawlak [12] as a tool for dealing with granularity in knowledge. Rough set theory, a new mathematical approach to deal with inexact, uncertain, or vague knowledge, has recently received wide attention on the research areas in both of the real life applications and the theory itself. Rough set theory assumes that every object in a universe of discourse is linked with some form of characterizations. Objects described using the same characterizations are indiscernible. An elementary set in a rough set theory consists of all indiscernible objects and forms the smallest piece of knowledge about the universe. All the elementary sets form a partition of the universe. Any set formed from a union of elementary sets is called an observable, definable, or crisp set. Otherwise, it is called a rough (vague) set. Due to the granularity of knowledge, rough sets cannot be characterized by using available knowledge. Therefore with every rough set we associate two crisp sets, called its lower and upper approximation. Intuitively, the lower approximation of a set consists of all elements that surely belong to the set, whereas the upper approximation of the set constitutes all elements that possibly belong to the set. The difference of the upper and the lower approximation is a boundary region. It consists of all elements that cannot be classified uniquely to the set or its complement, by employing available knowledge. Thus any rough set, in contrast to a crisp set, has a nonempty boundary region. The lower and upper approximations form the most precise approximations of the given by crisp sets. It is well known that a partition induces an equivalence relation on a universe and vice versa. The properties of rough sets can thus be examined via either partitions or equivalence classes. Rough set theory is emerging as a powerful theory dealing with imperfect data. It is an expending research area which stimulates explorations on both real world applications and on the theory itself. It has found practical applications in many areas such as knowledge discovery, data analysis, approximate classification, and conflict analysis. The reader will find in [4, 5, 8, 12–25] the deep study of rough set theory. It soon invoked a natural question concerning possible connection between rough sets and algebras. Davvaz discussed the roughness of ring [4], Hv-group [13], Hv-module [14], hyperring [15], and so on. In [26], as a generalization of ideals in BCK-algebras, the notion of rough ideals is discussed.

Events of quantum logics do not describe “unsharp measurements” since unsharp measurements do not have a “yes-no” character. To include such events another algebraic structure was introduced by Foulis and Bennett (1994), called an effect algebra. Hence elements of an effect algebra can be regarded as (possibly) unsharp experimentally testable propositions about a physical system. A subset of can be regarded as (possibly) some unsharp experimentally testable propositions about a physical system. We think about a rough approximation of to be the set of propositions which is indiscernible with some propositions in . It means that two propositions and are observable simultaneously in terms of quantum measurement theory that , are indiscernible.

In this paper, we discuss the roughness of lattice ordered effect algebra and introduce the notions of the interior and the closure of a subset. We give some properties of the interior and the closure of a subset in lattice ordered effect algebras.

#### 2. Basic Concepts and Properties of Effect Algebras and Approximation Spaces

##### 2.1. Effect Algebras

In this section, we recall some definitions and results which will be used in the sequel.

*Definition 1 (Foulis and Bennett [1]). *An effect algebra is a partial algebra with partial binary operation and two nullary operations 0 and 1 satisfying the following axioms.(E1) if is defined.(E2) if one side is defined.(E3)For every there exists a unique such that .(E4)If is defined then .

*Example 2 (Gudder [27]). *(1) A simple example of an effect algebra is , where is defined by , in which case .

(2) Another example is an -chain, , where if and only if for , , in which case .

(3) Let be a complex Hilbert space and let be the set of all bounded self-adjoint operators on . The positive cone in is the set of all that satisfy for all . We then write if . Letting 0 and 1 be the zero and identity operators, respectively, we have that and is a partially ordered Abelian group. It can be checked that is an effect algebra but not a Boolean effect algebra.

Having an effect algebra , we can introduce a partial order on : iff there exists . We denote iff . It is easy to check that is a well defined partial operation. In [7], a class of partial structures equivalent to effect algebra, so-called D-posets, was introduced independently. The axioms for D-posets are based on .

As usual, we denote by . Further, we denote iff exists iff iff . If is an effect algebra and is a lattice, then is called lattice ordered and it is said to be distributive iff, as a poset, it forms a distributive lattice. Every lattice ordered effect algebra satisfies the De Morgan law: . Lattice ordered effect algebras are called D-lattices in [2]. If is a bounded lattice with the orthosupplement operation satisfying the orthomodular law: for all , , then is said to be orthomodular lattice.

Lemma 3 (Foulis and Bennett [1]). *Let be an effect algebra, , .*(1)*,*(2)*,*(3)* and ,*(4)

*,*(5)

*(6)*

*and*,*,*(7)

*.*(8)

*(9)*

*and*,*,*(10)

*,*(11)

*and .*

Lemma 4 (Foulis and Bennett [1]). *Let be an effect algebra. If , with , then*(i)*,*(ii)*,*(iii)* and ,*(iv)

*(v)*

*and*,*,*(vi)

*.*

Lemma 5 (Foulis and Bennett [1]). *Let be an effect algebra. If and , then the following statements hold:*(i)*,*(ii)*,*(iii)*,*(iv)*.*

Lemma 6 (Riečanová [28]). *In an effect algebra , the following statements are satisfied.*(i)* For all , if , , and , then is defined.*(ii)

*(iii)*

*For all**, if**, then**if and only if**.*

*For all**,**if and only if*.*(iv)*

*Moreover, if**is lattice ordered, for all**, we have the following.**(v)*

*If**and**, then**.*

*If**,**, then**.**Definition 7 (Foulis and Bennett [1]). *A subset of an effect algebra is called a subeffect algebra of if and only if , is closed under , and for all , .

*Definition 8 (Gudder [29]). *Let be an effect algebra; an element is called sharp if . The set of all sharp elements is denoted by .

In [29] it has been shown that in a lattice ordered effect algebra , is a subeffect algebra that is an orthomodular lattice. Obviously, in an effect algebra , .

*Definition 9 (Greechie et al. [30]). *Let be an effect algebra. If , for , and , then is called principal.

*Definition 10 (Greechie et al. [30]). *An element of an effect algebra is called central, if(1) and are principal,(2)for every there are such that , and .

The center of the effect algebra is the set of all central elements of . is a subeffect algebra of the effect algebra and forms a Boolean algebra (see [30]).

*Definition 11 (Ma [10]). *Let be an effect algebra. A nonempty subset of is said to be an effect algebra ideal of , if the following conditions are satisfied: for all ,(i), implies ,(ii), implies .

Equivalently, a nonempty subset of the effect algebra is an effect algebra ideal iff it satisfies the condition that if is defined, then .

*Definition 12 (Jenca and Pulmannová [9]). *Let be an effect algebra and let be an ideal of . If for any , , , implies that there are , , with , one calls a Riesz ideal.

*Definition 13 (Jenca and Pulmannová [9]). *Let be an effect algebra and let be an ideal of . Define a binary relation on by if and only if there are , , such that .

Equivalently, if and only if there is , and , .

In particular, for each we have if and only if .

*Definition 14 (Jenca and Pulmannová [9]). *A binary relation on an effect algebra is called a congruence relation if the following conditions are satisfied:(i) is an equivalence relation,(ii), , , implies ,(iii)if , then there exists such that and .

Lemma 15 (Jenca and Pulmannová [9]). *Let be an ideal in an effect algebra . Then is a congruence if and only if is a Riesz ideal.*

This means that implies . The equivalence class of is denoted by . Note that if are Riesz ideals and , then .

*Definition 16 (Foulis and Bennett [1]). *Let , be effect algebras. A mapping is said to be(i)a morphism iff and with and ;(ii)a homomorphism iff is a morphism and with ;(iii)a monomorphism iff is a morphism and with .

A morphism is called a -morphism if whenever exists in . A morphism such that iff is a monomorphism. It is clear that and that for , with . In particular, (see [1]).

Lemma 17 (Ma [10]). *If is an effect algebra in which every element is principle, then for all , implies that .*

*Remark 18. *An MV-algebra is the same thing as a lattice ordered effect algebra in which disjoint pairs and a Boolean algebra is the same thing as a distributive lattice ordered effect algebra in which orthogonal pairs are disjoint pairs. Also, every MV-algebra is distributive.

##### 2.2. Approximation Spaces

Let denote a finite set and let be an indiscernibility relation on . is defined to be the pair and is called an approximation space. Intuitively, elements of a -equivalence class are to be regarded as indiscernible from one another.

*Definition 19. *For an approximation space , by a rough approximation in one means a mapping defined for every by where , . Here is called the interior (or the lower rough approximation) of , is called the closure (or the upper rough approximation) of , and is called the boundary of . is called the rough approximation of (or the rough set) in (see [4, 5, 13–17, 19–21, 23–25, 31, 32]).

For the sake of illustration, we give the following example (see [4]).

*Example 20. *Let be an approximation space, where and let be an equivalence relation with the following equivalence classes:

Let ; then and and so is a rough set.

Now, we give an example of the lower and upper approximation theory applied to effect algebras.

*Example 21. *Let consist of the six distinct elements , , , , , , where and are the only nonzero elements which are orthogonal to and , respectively. Furthermore, let . See the following table. Then is an effect algebra. Denote by the set and by the equivalence relation defined as follows:

, either or ,

Clearly is a congruence and and . Let ; then and .

Let be an approximation space and let be a subset of .(i)If , then is called a crisp set.(ii)If , then is called having an empty interior with respect to .(iii)We define to be the exterior of . If , then is called having an empty exterior with respect to .

Let be an approximation space. Clearly, and are definable with respect to . The family of all crisp sets is denoted by .

*Example 22. *In Example 20, is not a crisp set, has no empty interior, and has no empty exterior with respect to . In Example 21, is not a crisp set, but it has empty interior and empty exterior with respect to .

#### 3. Approximation in Effect Algebras

In this section, we present some properties of approximation spaces generally and approximation in effect algebras.

Let be an effect algebra, and let be a Riesz ideal of and let be a subset of . Then the sets , are called, respectively, the interior and closure (or the lower and upper approximations) of the set with respect to the ideal . When and is the equivalence relation defined in Definition 14, then we use the pair instead of the approximation space . Also, in this case we use the symbols and instead of and .

Lemma 23. *Let be an effect algebra, let be a Riesz ideal of , and let be a subset of . Then the following statements are equivalent:*(1)* is a crisp set;*(2)* iff ;*(3)* is a union of some equivalence classes.*

*Proof. *The proof is trivial.

The following properties of approximation spaces are well known and obvious. They are similar to Proposition 3.1 in [4] and Proposition 3.1.1 in [22].

Proposition 24. *Let be an effect algebra and let be a Riesz ideal of . For the approximation space , for arbitrary subsets , and for each , one has*(1)*,*(2)* and are crisp sets,*(3)*, , and are crisp sets with respect to ,*(4)*if , then and ,*(5)*,*(6)*,*(7)*,*(8)*,*(9)*,*(10)*,*(11)*, ,*(12)*, .*

The rough complement of denoted by is defined by .

*Proof. *The proof is similar to the [31, Theorem 2.1] and [16, Proposition 4.1].

The following example shows that the equations in (8) and (9) of the Proposition 24 do not hold.

*Example 25. *Let . Consider the following table:

Then is a Boolean effect algebra with the order given by Figure 1. So is also a lattice ordered effect algebra. Now, we also can check that is a Riesz ideal. Let and be subsets of . Then the equivalence classes are , . Therefore, we have , , , , , , and so , .

*Example 26. *Let . Define , , and for all . We can easily see that is a lattice effect algebra (which is not Boolean). We also can check that is a Riesz ideal. Let and be subsets of . Then , , , . Therefore, we have , , , , , , and so , .

Let be a subset of an effect algebra and let be the annihilator of in defined by . Denote , where .

Proposition 27. *Let be a Riesz ideal of effect algebra and let be a subset of . Then*(1)*,*(2)*,*(3)*,*(4)*.*

*Proof. *(1) By Proposition 24, we have . It is easy to see that if then , so we can take and again by Proposition 24, we have . Similarly, we can prove (2), (3), and (4).

The following example shows that inclusion symbols in Proposition 27 may not be replaced by equal sign.

*Example 28. *Let be a lattice ordered effect algebra in Example 25. Let , be subsets of and let be a Riesz ideal of . It is easy to check that , , , . So we have , , , . Hence, , , .

Lemma 29. *Let , be two Riesz ideals of effect algebra such that and let be a subset of . Then *(i)*,*(ii)*.*

*Proof. *It is straightforward.

#### 4. Approximation in Lattice Ordered Effect Algebras

In this section, we define a function in a lattice ordered effect algebra and build a relationship between it and congruence classes. Then we study some properties about approximation in lattice ordered effect algebras.

We give some examples of lattice ordered effect algebras.

*Example 30. *(1) In Example 2 (1), is also a lattice ordered effect algebra.

(2) Let be a partially ordered abelian group with an operator “+”. Let with , and let . Then can be organized into a lattice ordered effect algebra by defining iff , in which case . In the effect algebra we have and the effect algebra partial order on coincides with the restriction to of the partial order on . An effect algebra of the form (or isomorphic to an effect algebra of this form) is called an interval effect algebra or, for short, an interval algebra.

(3) Effect algebra in Example 25 is a lattice ordered effect algebra.

In a lattice ordered effect algebra , we define .

Proposition 31. *Let be a lattice ordered effect algebra. Then the following properties hold for every :*(i)*,*(ii)* if and only if ,*(iii)*,*(iv)*, if .*

*Proof. *(i) and (ii) are obvious. (iii): , , by Lemma 5, we have . (iv): , , ; if , we have .

Lemma 32 (Jenca and Pulmannová [9]). *Let be a Riesz ideal in a lattice ordered effect algebra . Then is a lattice ideal and is a lattice congruence. Moreover for all , if and only if .*

From the lemma, we have that if is a Riesz ideal in a lattice ordered effect algebra , then for all , if and only if .

*Definition 33. *Let be an effect algebra and let be a subset of . Then is called convex if for every and , implies .

Proposition 34. *Let be a Riesz ideal of effect algebra . Then is convex for each .*

*Proof. *Given , take with and let . Since , we have , too. Hence, . That is . So is convex for each .

Lemma 35. *Let be a Riesz ideal of linear ordered effect algebra . If and , then for each and , .*

*Proof. *Suppose that there exist and such that . We show that it is a contradiction. First, let . So we obtain and by Proposition 34, which is a contradiction. Now let . Thus and by Proposition 34, which is a contradiction again. Therefore for each and , .

If is a subset of lattice ordered effect algebra , it is easy to check that for every if we have , then .

Theorem 36. *Let be a lattice ordered effect algebra and let be a Riesz ideal of . Let be a subset of . Then*(i)*,*(ii)*.*

*Proof. *(i) Let , so by Lemma 3 and . Therefore, . Hence there exists . By Lemma 32 we have and . By Proposition 31, and , so and this implies that . Conversely, let , so . Suppose that ; then . Similarly, and this proves part (i).

(ii) Let , so by Lemma 3. Therefore, . Hence for each , we have and hence by Proposition 31. This implies that . So . Conversely, let , so . Suppose that ; then by Proposition 31. Hence . By Lemma 3 and again, we have and this proves part (ii).

Theorem 37. *Let be a lattice ordered effect algebra and let be a subeffect algebra of . Then*(i)* is a subeffect algebra of ,*(ii)*if is an ideal and , then is a subeffect algebra of .*

*Proof. *(i) Since , then . For , there exists such that ; then . Since , so , therefore . Let and , then there are , , such that and . Since is subeffect algebra of , we have . Note that and ; we have and hence . That is, .

(ii) Let . First we prove and . For all , by Definition 13, we have , , , thus , so , therefore . Similarly, can be proved. Then we prove that implies . For all , we have . It follows that , then . Therefore . Let and . For all , by Definition 13, we have , , , . This yields that , thus . Therefore .

By the above theorem, we note that and are subeffect algebras of .

*Remark 38. *Let be a lattice ordered effect algebra. Then(1) is a subeffect algebra of ,(2) is a subeffect algebra of .

We define a Boolean effect algebra to be an effect algebra such that and an orthomodular effect algebra to be a lattice ordered effect algebra in which every element is principal.

Proposition 39. *Let be a Riesz ideal of a linear ordered effect algebra and let be a convex subset of . Then and are convex.*

*Proof. *Let and . We divide the proof into three cases. First let , we have for some . By Proposition 34, is convex, so that . It follows that ; therefore . Next let and . Then . Since , . So . Similarly for and , we can prove that and we omit it. Finally let , , and . Now we show . So let . By Lemma 35, for all and we have . Since , we have . Note that is convex; we obtain and hence . It follows that . This shows that is convex.

Now, let and . We divide the proof into three cases. First let ; we have for some . By Proposition 34, is convex, so that . It follows that and ; therefore . Hence . Next let and . Then . Since , then and hence . Therefore . Similarly for and , we can prove that . Now let , , and . We must show . Since , we have and . So there exist and . By Lemma 35, we have . So since is convex. This implies that and hence is convex.

Proposition 40. *Let be a lattice ordered effect algebra, and two Riesz ideals of , and a subset of .*(i)*If is an ideal of and , then ,*(ii)*If is crisp with respect to or , then .*

*Proof. *(i) By Lemma 29, . Conversely, assume . Then and hence . Now, let , so . Because is an ideal and , so . Hence we have . On the other hand, by Lemma 6, we have and so . This means that , which implies . Similarly, we can obtain and so . Therefore .

(ii) First, assume that is crisp with respect to , so . Therefore, we have . Also by Lemma 29, , and it proves the theorem.

#### 5. Approximation in Orthomodular Effect Algebras and Boolean Effect Algebras

We define a Boolean effect algebra to be an effect algebra such that and an orthomodular effect algebra to be a lattice ordered effect algebra in which every element is principal.

Let and be subsets of . We define and define the downset .

Proposition 41. *Let be an orthomodular effect algebra and let be a Riesz ideal of , and let , be subsets of . Then .*

*Proof. *Let , so . Thus there exists and consequently there exist and such that . On the other hand, since , by Definition 13, there are , , such that . By Lemma 32, , and , hence .

Proposition 42. *Let be an orthomodular effect algebra and let be a Riesz ideal of , and let , be subsets of . Then .*

*Proof. *Suppose that , so there exist and such that . Now, let ; thus by by Definition 13, there are , , such that . By Lemma 32, we have and . Hence , this implies that .

The following example shows that we can not replace the inclusion symbol by equal sign in Proposition 42.

*Example 43. *Consider , as the lattice-ordered effect algebra in Example 26. Let , be the subsets of and let be the Riesz ideal of . We have , and . Therefore and . This shows that .

In what follows, that is, Propositions 44, 45, 47, 49, Examples 46 and 51, and Corollary 48, is a Boolean effect algebra.

Proposition 44. *Let be a Boolean effect algebra and let , be two Riesz ideals of . Then is also a Riesz ideal of .*

*Proof. *Let and ; then , , where , , . Since , then . So is an ideal. Let ; then , where , . For , and , we have . Taking , , we have and , . Since is a distributive lattice ordered effect algebra and is an orthomodular lattice, it follows that . Therefore, . Hence, is a Riesz ideal.

Proposition 45. *Let be a Boolean effect algebra, and let , be two Riesz ideals of and let be a subset of . Then .*

*Proof. *Assume that is a subset of . Let , so . Thus there exists which implies and ; hence by Definition 13, there exist , , , such that and also there exist , such that . Then . On the other hand by Lemma 32, we have and . Therefore, .

The following example shows that the symbol inclusion in Proposition 45 can be proper.

*Example 46. *Consider as the Boolean effect algebra in Example 25. It is easy to check that , are two ideals of . And obviously, is an ideal of too. We have , , , , , , , . Assume ; then , , , . So .

Proposition 47. *Let be a Boolean effect algebra, , two Riesz ideals of , and a subset of . Then . Furthermore, if , then one obtains .*

*Proof. *Let , so . Since and , hence .

Now, suppose that . There exist and such that . Suppose that , so there exist , , such that . Also, there are and such that . Then . By Lemma 32, we have and . This implies that .

From the above proposition, we have the following corollary.

Corollary 48. *Let be a Boolean effect algebra, and let , be two Riesz ideals of and let be a lattice ideal of . Then one has .*

*Proof. *By Proposition 47, we have . Since is an ideal, we can show easily, . Assume , so from Proposition 47. That is, . It yields that . Therefore, .

Proposition 49. *Let be a lattice ordered effect algebra and a Boolean effect algebra. If is a Riesz ideal of and is a monomorphism, then is a Riesz ideal of .*

*Proof. *First, we prove that is an ideal. Let and , where . Since is a monomorphism, then . So is an ideal of . Next, we prove that is a Riesz ideal. For any , let , , . Take , ; then and , . Since is a distributive lattice ordered effect algebra, and is an orthomodular lattice, it follows that . Therefore, . Hence, is a Riesz ideal of .

Theorem 50. *Let , be lattice ordered effect algebras, let be a morphism, and let be the kernel of .*(1)*If is a subset of , then .*(2)*Assume that is a monomorphism and is a Boolean effect algebra. If is a subset of and is a Riesz ideal of containing , then .*

*Proof. *(1) Since , it follows that . Conversely, let ; then there exists such that . So we have . Let . We have and , so there exist , , such that . Since is a morphism, we get that . It follows that , which proves the theorem.

(2) Let . Then there exists such that . Since , suppose , so we have and . Hence there exist , , such that . Since a monomorphism, then and . Therefore, .

Conversely, let , so . Suppose ; it implies that and . Since is a monomorphism, there exists such that and there exist and such that and , so and it implies that . Hence . So and by Lemma 32. Now, we conclude that , so . This proves the theorem.

In order to well illustrate the above theorem, we give the following example.

*Example 51. *Consider , as the lattice ordered effect algebra in Example 25. Also, let . For all , define , ; otherwise is undefined. Then is a Boolean effect algebra.

The map is given by , , , . We can easily check that is a monomorphism. Since , so , ,