Complexity

Volume 2018, Article ID 9073597, 11 pages

https://doi.org/10.1155/2018/9073597

## The Intuitionistic Fuzzy Linguistic Cosine Similarity Measure and Its Application in Pattern Recognition

^{1}School of Business, Central South University, Changsha, Hunan 410075, China^{2}Department of Mathematics, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China^{3}Hunan University of Commerce, Changsha, Hunan 410205, China

Correspondence should be addressed to Donghai Liu; moc.621@uiliahgnod

Received 7 December 2017; Revised 6 January 2018; Accepted 21 January 2018; Published 22 April 2018

Academic Editor: László T. Kóczy

Copyright © 2018 Donghai Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We propose the cosine similarity measures for intuitionistic fuzzy linguistic sets (IFLSs) and interval-valued intuitionistic fuzzy linguistic sets (IVIFLSs), which are expressed by the linguistic scale function based on the cosine function. Then, the weighted cosine similarity measure and the ordered weighted cosine similarity measure for IFLSs and IVIFLSs are introduced by taking into account the importance of each element, and the properties of the cosine similarity measures are also given. The main advantage of the proposed cosine similarity measures is that the decision-makers can flexibly select the linguistic scale function depending on the actual semantic situation. Finally, we present the application of the cosine similarity measures for intuitionistic fuzzy linguistic term sets and interval-valued intuitionistic fuzzy linguistic term sets to pattern recognition and medical diagnosis, and the existing cosine similarity measures are compared with the proposed cosine similarity measures by the illustrative example.

#### 1. Introduction

The fuzzy set was proposed by Zadeh [1] and has achieved a great success in various fields, which is considered to be an effective tool to solve the decision-making problems, pattern recognition, and fuzzy inference [2–4]. Since the fuzzy set was put forward, it was extended in different aspects. One of the generalizations of fuzzy set is intuitionistic fuzzy set (IFS), which was introduced by Atanassov [5]. A typical feature of IFS is that the membership relations are represented by the membership degree and nonmembership degree, respectively. However, due to the fuzziness and uncertainty in the multiple criteria decision-making problems, it is difficult to use the exact values to present qualitative evaluation. At this time, people often provide their opinions in linguistic term sets. In some practical decision-making problems, the decision-maker regards the linguistic information as the values of linguistic variables; that is to say, the values of the variables are not represented by numerical values but are represented by linguistic values, such as “good,” “better,” “fair,” “slightly worse,” and “poor.” Up to now, many people have studied the problem of linguistic multiple criteria decision-making, Herrera and Verdegay [6] proposed the linguistic assessments in group decision-making (GDM) problem in 1993, then Herrera et al. [7] proposed a consensus model for group decision-making based on linguistic evaluations information, and Herrera et al. [8] considered several group decision-making processes using linguistic ordered weighted averaging (LOWA) operator. Later, Xu [9] proposed a group decision-making method based on the uncertain linguistic ordered weighted geometric (LOWG) operators and the induced uncertain LOWG operators. Furthermore, Xu [10] presented the linguistic hybrid aggregation (LHA) operator and applied it to group decision-making.

However, in some practical decision-making problems, the decision-makers may have some indeterminacy in their linguistic evaluation; they cannot express their preferences by using only membership degree of a linguistic term. Then Wang et al. [11] proposed the intuitionistic fuzzy linguistic aggregation operators and applied them to multicriteria group decision-making problems. For example, is an intuitionistic fuzzy linguistic number (IFLN), 0.2 is the membership degree of the linguistic term , and is the nonmembership degree of the linguistic term . The intuitionistic fuzzy linguistic sets (IFLSs) have made great progress in describing linguistic information and to some extent it can be regarded as an innovative construct. The research on this field has been growing rapidly [12–15].

On the other hand, similarity measure is an important topic in the fuzzy set theory, and it is widely used in some fields [16–19], such as pattern recognition, medical diagnosis, and citation analysis. One of the important similarity measures is the cosine similarity measure, which is defined in vector space. Ye [20] proposed the cosine similarity measure and the weighted cosine similarity measure between IFSs. Zhou et al. [21] presented the intuitionistic fuzzy ordered weighted cosine similarity measure and applied it to the group decision-making problem about the choice of investment plan. Liu et al. [22] presented the interval-valued intuitionistic fuzzy ordered weighted cosine similarity (IVIFOWCS) measure and applied it to the investment decision-making. As far as we know, the study of cosine similarity measures of intuitionistic fuzzy set has not been discussed. In the following, we will propose the cosine similarity measures of IFLSs and IVIFLSs. The main characteristics of the cosine similarity measures that we can calculate are based on linguistic term set by the linguistic scale function. Linguistic scale function between IFLSs and IVIFLSs was introduced by Wang et al. [23], which was used to calculate the Hausdorff distance between hesitant fuzzy linguistic numbers (HFLNs); it can assign different semantic values to the linguistic terms under different circumstances and improve the flexibility of the proposed cosine similarity measures.

The rest of the paper is organized as follows. In Section 2, some basic concepts of LTSs, IFSs, IFLSs, and linguistic scale functions are briefly reviewed. In Section 3, we first introduce the cosine similarity measures between IFLSs and then discussed some related properties. Furthermore, the weighted cosine similarity measure between IFLSs, the ordered weighted cosine similarity measure between IFLSs, and the ordered weighted cosine similarity measure between IVIFLSs are analyzed. In Section 4, we give the application of the proposed cosine similarity measures between IFLSs and IVIFLSs on pattern recognition and medical diagnosis and then make comparison analysis with the existing cosine similarity measures. The conclusions are given in Section 5.

#### 2. Preliminaries

In this section, we will review and discuss some related basic concepts, including linguistic term sets (LTSs), intuitionistic fuzzy sets (IFSs), intuitionistic fuzzy linguistic sets (IFLSs), linguistic scale functions, the ordered weighted averaging (OWA) operator, and the cosine similarity measure between fuzzy sets.

##### 2.1. Linguistic Term Set

In some practical problems, the information expressed by the numerical values may bring inconvenience. At this time it is suitable to use linguistic term set to express information.

*Definition 1 (Herrera and Verdegay [6]). *Suppose that is a finite and totally ordered discrete term set, where represents a possible value for a linguistic variable; it satisfies the following characteristics:(1)(2)(3) if .For example, a set of seven terms could be given as follows: The discrete linguistic term cannot usually adapt to the aggregated results. In order to represent these results accurately, Xu [10] extended the discrete term set to the continuous term set , where is a sufficiently large positive integer.

##### 2.2. Intuitionistic Fuzzy Set

*Definition 2 (Atanassov [5]). *Given a fixed set , then an intuitionistic fuzzy set in is defined as where and represent the membership degree and nonmembership degree of to , respectively, and they satisfy the condition: .

For all , if , then is called the hesitancy degree of to .

On the basis of intuitionistic fuzzy set and linguistic term set, Wang et al. [11] presented the following intuitionistic fuzzy linguistic term set.

##### 2.3. Intuitionistic Fuzzy Linguistic Term Set

*Definition 3 (Wang et al. [11]). *Let be a fixed set and an intuitionistic fuzzy linguistic term set in is defined as where and are the membership function and nonmembership function of the element to , respectively, and

For all , let be the hesitancy function, which means the degree of hesitancy of to .

##### 2.4. Linguistic Scale Functions

One of the advantages of linguistic term set is that it can express uncertain information flexibly in practical problems, but if we use the subscript of linguistic terms directly in the process of operations, it may lose this advantage. The most important thing is to find effective tools to transform linguistic terms to numerical values. As we all know, linguistic scale function (Wang et al. [23]) is a mapping from linguistic term set to the real value . The linguistic scale function can assign different semantic values to the linguistic terms under different circumstances. In practice, the linguistic scale functions are very popular because they are very flexible and they can give more deterministic results based on different semantics.

*Definition 4 (Wang et al. [23]). *Let be a linguistic term. If is a numeric value between 0 and 1, then the linguistic scale function can be defined as follows: where . The linguistic scale function is strictly monotonously increasing function with respect to the subscript of ; in fact, the function value represents the semantics of the linguistic terms.

Now we introduce three kinds of linguistic scale functions as follows: The evaluation scale of the linguistic information expressed by is divided on average. For linguistic scale function , the absolute deviation between adjacent language sets will increase when we extend it from the middle of the given set of language to both ends. For linguistic scale function , the absolute deviation between adjacent language sets will decrease when we extend it from the middle of the given linguistic term set to both ends.

The above linguistic scale functions can be developed to (where is a nonnegative real number), which is a continuous and strictly monotonically increasing function.

##### 2.5. The OWA Operator

The ordered weighted averaging (OWA) operator (Yager [24]) is an aggregation operator that includes the minimum, the average, and the maximum as special cases, which is defined as follows.

*Definition 5 (Yager [24]). *An dimension OWA operator is a mapping OWA: that has an associated weighting vector with , such that where is the th largest of the arguments .

##### 2.6. Cosine Similarity Measure for Fuzzy Sets

*Definition 6 (Salton and Mcgill [25]). *Let ; assume and are two fuzzy sets; the cosine similarity measure between fuzzy sets and is defined as follows:The cosine similarity measure between fuzzy sets and satisfies the following properties:(1);(2);(3)for if , that is, , then .

#### 3. Cosine Similarity Measures for Intuitionistic Fuzzy Linguistic Term Sets

##### 3.1. Cosine Similarity Measure for Intuitionistic Fuzzy Linguistic Term Sets

At first, we will present cosine similarity measure for intuitionistic fuzzy linguistic term sets, which includes not only the membership degree and nonmembership degree of the IFLSs but also the linguistic scale function .

*Definition 7. *Let and be two IFLSs in , and let be a linguistic scale function. Then the cosine similarity measure for intuitionistic fuzzy linguistic term sets between and can be defined as follows: where The cosine similarity measure between IFLSs and satisfies the following properties:(1);(2);(3)for if , that is, , and , then .

*Proof. *Properties (1), (2), and (3) are obvious; here we omit the proof of property.

If we consider the weight of different element , now we introduce the intuitionistic fuzzy linguistic weighted cosine similarity measure , which can be defined as follows.

*Definition 8. *Let and be two IFLSs in , and let be a linguistic scale function. Then the weighted cosine similarity measure for intuitionistic fuzzy linguistic term sets between and can be defined as follows: where is the weight of , and .

*Remark 9. *For all , if we take , then the weighted cosine similarity measure is reduced to the cosine similarity measure .

Based on the idea of the OWA operator, we present the intuitionistic fuzzy ordered weighted cosine similarity measure between intuitionistic fuzzy linguistic term sets and as follows.

*Definition 10. *Let and be two IFLSs in , and let be a linguistic scale function. Then the ordered weighted cosine similarity measure for intuitionistic fuzzy linguistic term sets between and can be defined as follows:where the associated weighting vector with , and is any permutation of , such that is a similarity measure that uses the cosine similarity measure for IFLS in the OWA operator and the linguistic scale function is also applied.

*Example 1.* Let be the linguistic term set, and assume two intuitionistic fuzzy linguistic term sets are , . If the linguistic scale function (), by (10), we can get

If , then the weighted cosine similarity measure

If , the ordered weighted cosine similarity measure

##### 3.2. Cosine Similarity Measure for Interval-Valued Intuitionistic Fuzzy Linguistic Term Sets

In the intuitionistic fuzzy linguistic set , represents the membership degree of the element to , and represents the nonmembership degree of the element to ; they are all precise values in But in some circumstances, it is difficult to provide the precise membership degree and nonmembership degree of the element to . Atanassov and Gargov [26, 27] proposed the interval-valued intuitionistic fuzzy linguistic term set (IVIFLS), and the definition of interval-valued intuitionistic fuzzy linguistic term set is given as follows.

*Definition 11. *Let be a fixed set and an interval-valued intuitionistic fuzzy linguistic term set in is defined as where and are the interval membership degree and the interval nonmembership degree of the element to , respectively, and

Based on the cosine similarity measure for intuitionistic fuzzy linguistic term sets and , we present the cosine similarity measure for interval-valued intuitionistic fuzzy linguistic term sets and as follows.

*Definition 12. *Let and be two IVIFLSs in , and let be a linguistic scale function. Then the cosine similarity measure for IVIFLSs between and can be defined as follows: where The cosine similarity measure between IVIFLSs and also satisfies the following properties:(1);(2);(3)for if , that is, , and , then .

Next we go on studying the weighted cosine similarity measure between IVIFLSs; it can be defined as follows.

*Definition 13. *Let and be two IVIFLSs in , is the weight of , and . Assume be a linguistic scale function, then the weighted cosine similarity measure between IVIFLSs and can be defined as follows: where

*Remark 14. *For all , if we take , then the weighted cosine similarity measure is reduced to the cosine similarity measure .

*Remark 15. *For all , if , then the weighted cosine similarity measure is reduced to the weighted cosine similarity measure .

*Remark 16. *For all , if and , then the weighted cosine similarity measure is reduced to the cosine similarity measure for IFLSs.

Similarly, we also apply the OWA operator and the cosine similarity measure for IVIFLS to present the interval-valued intuitionistic fuzzy ordered weighted cosine similarity measure between the interval-valued intuitionistic fuzzy linguistic term sets and as follows.

*Definition 17. *Let and be two IVIFLSs in , is the weight of , and . Assume be a linguistic scale function, then the ordered weighted cosine similarity measure between IVIFLSs and can be defined as follows:where and is any permutation of , such that

#### 4. Applications of the Cosine Similarity Measure

In this section, we will apply the cosine similarity measures of IFLSs and IVIFLSs to pattern recognition and medical diagnosis.

##### 4.1. Intuitionistic Fuzzy Cosine Similarity Measure for Pattern Recognition

*Example 2.* Let , the linguistic term set . We consider some known patterns , which are represented by the IFLSs as follows:

If an unknown pattern , , , in order to classify the pattern in , , , , we can calculate the weighted cosine similarity measure between and , respectively. The best is derived by . Assume the weight of is and let the linguistic scale function , and by applying (10), (12), and (14), we obtain the cosine similarity measures between and , and the results are shown in Table 1.