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Abstract. After the introduction of fuzzy sets by Zadeh, there have been a number of generaizations of this fundamental concept. The notion of bipolar-valued fuzzy sets introduced by Lee is one among them. In this paper, we apply the concept of a bipolarvalued fuzzy set to quasi-associative ideals in BCI-algebras. The notion of a bipolar fuzzy quasi-associative ideal of a BCI-algebra is introduced, and some related properties are investigated. Characterizations of a bipolar fuzzy quasi-associative ideal are given. Extension property for a bipolar fuzzy QA-ideal is established.

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A neutrosophic set is initiated by Smarandache, and it is a novel tool to deal with vagueness considering the truth, indeterminacy and falsity memberships satisfying the condition that their sum is less than 3. The concept of neutrosophic quadruple numbers was introduced by Florentin Smarandache. Using this idea, Jun et al. introduced the notion of neutrosophic quadruple B C K / B C I -numbers, and studied neutrosophic quadruple B C K / B C I -algebras. As a continuation of Jun et al.’s paper, the notion of implicative neutrosophic quadruple B C K -algebras is introduced, and several properties are investigated. Given a set Y, conditions for the neutrosophic quadruple Y-set N q ( Y ) to be a neutrosophic quadruple B C I -algebra are provided. Conditions for the neutrosophic quadruple Y-set N q ( Y ) to be an implicative neutrosophic quadruple B C K -algebra are provided. Given subsets I and J of a B C K -algebra Y, conditions for the neutrosophic quadruple ( I , J ) -set N q ( I , J ) to be an implicative ideal of the neutrosophic quadruple B C K -algebra N q ( Y ) are discussed.

The notions of𝒩-subalgebras and𝒩-closed ideals in BCH-algebras are introduced, and the relation between𝒩-subalgebras and𝒩-closed ideals is considered. Characterizations of𝒩-subalgebras and𝒩-closed ideals are provided. Using special subsets,𝒩-subalgebras and𝒩-closed ideals are constructed. A condition for an𝒩-subalgebra to be an𝒩-closed ideal is discussed. Given an𝒩-structure, the greatest𝒩-closed ideal which is contained in the𝒩-structure is established.