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Fuzzy rank functions in the set of all binary systems
SpringerPlus volume 5, Article number: 1865 (2016)
Abstract
In this paper, we introduce fuzzy rank functions for groupoids, and we investigate their roles in the semigroup of binary systems by using the notions of right parallelisms and \(\rho\)shrinking groupoids.
Background
The notion of a fuzzy subset of a set was introduced by Zadeh (1965). His seminal paper has opened up new insights and applications in a wide range of scientific fields. Rosenfeld (1971) used the notion of a fuzzy subset to set down corner stone papers in several areas of mathematics. Mordeson and Malik (1998) published a remarkable book, Fuzzy commutative algebra, presented a fuzzy ideal theory of commutative rings and applied the results to the solution of fuzzy intersection equations. The book included all the important work that has been done on Lsubspaces of a vector space and on Lsubfields of a field.
In the study of groupoids \((X,*)\) defined on set X, it has also proven useful to investigate the semigroups \((Bin(X), \Box )\) where Bin(X) is the set of all binary systems (groupoids) \((X,*)\) along with an associative product operation \((X,*)\,\Box \,(X, \bullet ) = (X, \Box )\) such that \(x\,\Box \,y = (x*y)\,\bullet \,(y*x)\) for all \(x, y\in X\). Thus, e.g., it becomes possible to recognize that the leftzerosemigroup \((X,*)\) with \(x*y = x\) for all \(x, y\in X\) acts as the identity of this semigroup [see Kim and Neggers (2008)]. Fayoumi (2011) introduced the notion of the center ZBin(X) in the semigroup Bin(X) of all binary systems on a set X, and showed that a groupoid \((X,\bullet )\in ZBin(X)\) if and only if it is a locallyzero groupoid. Han et al. (2012) introduced the notion of hypergroupoids \((HBin(X),\Box )\), and showed that \((HBin(X), \Box )\) is a supersemigroup of the semigroup \((Bin(X), \Box )\) via the identification \(x \longleftrightarrow \{x\}\). They proved that \((HBin^*(X), \ominus , [\emptyset ])\) is a BCKalgebra.
In this paper, we introduce fuzzy rank functions for groupoids, and we investigate their roles in the semigroup of binary systems by using the notions of right parallelisms and \(\rho\)shrinking groupoids.
Preliminaries
Given a nonempty set X, we let \(Bin(X)\) denote the collection of all groupoids \((X,*)\), where \(*: X\times X\rightarrow X\) is a map and where \(*(x, y)\) is written in the usual product form. Given elements \((X,*)\) and \((X,\bullet )\) of \(Bin(X)\), define a product “\(\Box\)” on these groupoids as follows:
where
for any \(x, y\in X\). Using that notion, H. S. Kim and J. Neggers proved the following theorem.
Theorem 1
(Kim and Neggers 2008) \((Bin(X), \Box )\) is a semigroup, i.e., the operation “\(\Box\)” as defined in general is associative. Furthermore, the left zerosemigroup is the identity for this operation.
Fuzzy rank functions for groupoids
Given a groupoid \((X,*)\) in Bin(X), a map \(\rho : X\rightarrow [0, \infty )\) (or \(\rho : X\rightarrow [0, 1]\)) is said to be:

(i)
a (fuzzy) ranksubalgebra: \(\rho (x*y)\ge \min \{\rho (x), \rho (y)\}\),

(ii)
a (fuzzy) rankcosubalgebra: \(\rho (x*y) \le \max \{\rho (x), \rho (y)\}\),

(iii)
a (fuzzy) rankdfunction: \(\rho (x*y) =\max \{ \rho (x) \rho (y), 0\}\),

(iv)
a (fuzzy) symmetricrankfunction: \(\rho (x*y)= \max \{\rho (x)\rho (y), \rho (y) \rho (x)\}\,\hbox{for\,all}\,x,y \in X.\)
Note that if \(\rho : X\rightarrow [0, \infty )\) is a fuzzy subset of X, then it is a fuzzy ranksubalgebra as well. Thus these algebraic structures are special cases of the classes of fuzzy ranksubalgebras. Each of the types listed above serve as some idea of measure of size when the binary operation “\(*\)” is considered as corresponding very roughly to a (group) sum (product), a leftzerosemigroup, a dalgebra (BCKalgebra) or an absolute (value) difference. For the sake of being able to make comparisons in the behavior of interactions of ranktype and groupoid it seems a better idea to consider these simultaneously rather than study only isolated cases without considering common aspects as well as distinguishing ones.
Example 2

(a)
Let \((X,*)\) be a leftzerosemigroup, i.e., \(x*y = x\) for all \(x, y\in X\). If \(\rho : X\rightarrow [0, \infty )\) is any map, then \(\rho (x*y) = \rho (x)\) for all \(x, y\in X\). It follows that \(\rho (x*y) \ge \min \{\rho (x), \rho (y)\}\) and \(\rho (x*y) \le \max \{\rho (x), \rho (y)\}\) for all \(x, y\in X\), which shows that every function \(\rho : X\rightarrow [0, \infty )\) is both a (fuzzy) ranksubalgebra and a (fuzzy) rankcosubalgebra.

(b)
Let \((X,*)\) be a leftzerosemigroup and let \(\rho : X \rightarrow [0, \infty )\) be a (fuzzy) rankdfunction. Then it is a zero function. In fact, if we assume that there exists an \(x_0\in X\) such that \(\rho (x_0)> 0\), then \(0< \rho (x_0) = \rho (x_0* y) =\max \{\rho (x_0)\rho (y), 0\}\) for all \(y\in X\). If we take \(y:=x_0\), then it leads to a contradiction.

(c)
Let \((X,*)\) be a leftzerosemigroup and let \(\rho : X \rightarrow [0, \infty )\) be a (fuzzy) symmetricrankfunction. Assume that there exists \(x_0\in X\) such that \(0< \rho (x_0)\). Since \(\rho\) is a (fuzzy) symmetricrankfunction, we have \(\rho (x) = \rho (x*y) = \max \{\rho (x) \rho (y), \rho (y)\rho (x)\}\) for all \(x, y\in X\). If we take \(x:= x_0, y:= y_0\), then \(0< \rho (x_0)= \rho (x_0* x_0) =\max \{ \rho (x_0)\rho (x_0), \rho (x_0)\rho (x_0)\}=0\), a contradiction. This shows that \(\rho\) is a zero function.
If \((X,*)\) is a rightzerosemigroup, i.e., \(x*y = y\) for all \(x, y\in X\), then any function \(\rho : X\rightarrow [0, \infty )\) is both a (fuzzy) ranksubalgebra and a (fuzzy) rankcosubalgebra of \((X,*)\), while if \(\rho\) is either a (fuzzy) rankdfunction or a (fuzzy) symmetricrankfunction, then it is a zero function.
A groupoid \((X,*)\) is said to be selective (Neggers 1976; Neggers and Kim 1996) if \(x*y\in \{x, y\}\) for all \(x, y\in X\). For example, every left(right)zerosemigroup is selective. Given a selective groupoid \((X,*)\), we may construct a digraph via \(x\rightarrow y \Leftrightarrow x*y =y\) for all \(x, y\in X\). Hence selective groupoids are interpretable as digraphs on the (vertex) set X.
Proposition 3
Let \((X,*)\) be a selective groupoid and let \(\rho : X\rightarrow [0, \infty )\) be a (fuzzy) rankdfunction of X. If \(x\in X\) such that \(\rho (x)>0\), then any vertex \(y (\not = x)\) in X with \(x\nrightarrow y\) has \(\rho (y)=0\).
Proof
Let \(y (\not = x)\) in X with \(x\nrightarrow y\). Since \((X,*)\) is selective and \(x\nrightarrow y\), we have \(x*y = x\). It follows from the fact that \(\rho\) is a rankdfunction that \(\rho (x) = \rho (x*y) = \max \{\rho (x)\rho (y), 0\}\) and hence that \(\rho (x) = \rho (x) \rho (y)\), proving that \(\rho (y)= 0\). \(\square\)
Theorem 4
Let \((X,*)\) be a selective groupoid and let \(\rho : X\rightarrow [0, \infty )\) be a (fuzzy) rankdfunction of X. If \(x\in X\) such that \(\rho (x)>0\), then there exists at most one vertex \(x\in X\) such that \(\rho (x)>0\).
Proof
Assume that there are two vertices x and y in X such that \(\rho (x)>0, \rho (y)>0\). It follows that \(x\rightarrow y, y\rightarrow x\) by Proposition 3. Now \(x\rightarrow y\) implies that \(x*y= y\). Hence \(0 < \rho (y) =\rho (x*y) =\max \{\rho (x)\rho (y), 0\}= \rho (x)\rho (y)\). This shows that \(2\rho (y) = \rho (x)\). Similarly, \(y\rightarrow x\) implies \(2\rho (x) = \rho (y)\). Thus we obtain \(\rho (x) = 2\rho (y) = 4\rho (x)\), which implies \(\rho (x)=0\), a contradiction. \(\square\)
Proposition 5
If \((X,*)\) is a selective groupoid, then every (fuzzy) rankdfunction of \((X, *)\) is a zero function.
Proof
If \((X,*)\) is a selective groupoid, then \(x*x = x\) for all \(x\in X\). Since \(\rho\) is a (fuzzy) rankdfunction, we obtain \(\rho (x) = \rho (x*x) = \max \{\rho (x)\rho (x), 0\} = 0\), proving the proposition. \(\square\)
Given a map \(\rho : X\rightarrow [0, \infty )\), we define
Proposition 6
\((\sum _i(\rho ), \Box )\) is a subsemigroup of \((Bin(X), \Box )\) where \(i= 1, 2, 3\).
Proof
Let \((X,*), (X,\bullet )\in \sum _1(\rho )\). Then \(\rho (x*y)\ge \min \{\rho (x), \rho (y)\}\) and \(\rho (x\bullet y)\ge \min \{\rho (x), \rho (y)\}\) for all \(x, y\in X\). If we let \((X,\Box ):= (X,*)\,\Box \,(X,\bullet )\), then for any \(x, y\in X\), we have \(x\,\Box \,y = (x*y)\bullet (y*x)\). It follows that
This shows that \((X,*)\,\Box \,(X,\bullet )= (X, \Box )\in \sum \nolimits _1(\rho )\). Hence \((\sum \nolimits _1(\rho ), \Box )\) is a subsemigroup of \((Bin(X), \Box )\). Similarly, we may obtain that \((\sum \nolimits _i(\rho ), \Box )\) is also a subsemigroup of \((Bin(X), \Box )\) where \(i= 2, 3\). \(\square\)
Note that \((\sum \nolimits _4(\rho ), \Box )\) need not be a subsemigroup of \((Bin(X), \Box )\). Let \((X,*), (X,\bullet )\in \sum \nolimits _1(\rho )\) and let \((X,\Box ):= (X,*)\,\Box \,(X,\bullet )\). If we take \(x, y\in X\) such that \(\rho (x) > \rho (y)\), then we have
which shows that \((\sum \nolimits _4(\rho ), \Box )\) is not a subsemigroup of \((Bin(X), \Box )\).
Let \(\rho : X\rightarrow [0, \infty )\) be a map. A groupoid \((X,*)\) is said to have a \(\rho\)chain n if there exist \(x_1, \ldots , x_n\in X\) such that \(\rho (x_1)< \rho (x_2)< \cdots < \rho (x_n)\). We denote the \(\rho\)chain by \(\langle x_1, \ldots , x_n \rangle\). A groupoid \((X,*)\) is said to have the \(\rho\)height n if \(\langle x_1, \ldots , x_n \rangle\) is the largest maximal \(\rho\)chain in \((X,*)\).
Proposition 7
Let \(\rho : X\rightarrow [0, \infty )\) be a map and let \(Bin(X) = \sum _1(\rho )\). Then the \(\rho\)height of \((X,*)\) is ≥2 for any \((X,*)\in Bin(X)\).
Proof
Assume there exist \(x, y, z\in X\) such that \(\rho (x)< \rho (y) < \rho (z)\). Let \((X,*)\) be a groupoid such that \(x= y*z\). Then \((X,*) \in Bin(X) = \sum \nolimits _1(\rho )\). It follows that \(\rho (y*z) =\rho (x) < \rho (y) = \min \{\rho (y), \rho (z)\}\). This shows that \((X,*)\not \in \sum \nolimits _1(\rho ) = Bin(X)\), a contradiction. \(\square\)
Proposition 8
Let \(\rho : X\rightarrow [0, \infty )\) be a map and let \(Bin(X) = \sum _1(\rho )\). If \(\rho\) has two values \(a, b\in [0, \infty )\) with \(a< b\), then there exists uniquely an \(\widehat{x} \in X\) such that \(\rho (\widehat{x})= b\) and \(\rho (y) = a\) for all \(y\not = \widehat{x}\) in X.
Proof
Let \(\widehat{x} \in X\) such that \(\rho (\widehat{x}) = b > a\). If \(y\in X\) such that \(\widehat{x}\not = y\), then \(\rho (y) = a\). In fact, if \(\rho (y)=b\), then we may take a groupoid \((X,*)\) in Bin(X) such that \(\rho (\widehat{x} * y) = a\), since \(Bin(X) = \sum\nolimits _1(\rho )\). It follows that \(a = \rho (\widehat{x} * y) \ge \min \{ \rho (\widehat{x}), \rho (y)\}= b\), a contradiction. We claim that such an \(\widehat{x}\) is unique. Assume that there are two elements \(x^{\prime }, \widehat{x}\) in X such that \(\rho (\widehat{x}) = b = \rho (x^{\prime })\). A groupoid \((X,\bullet )\) satisfying \(\rho (\widehat{x}\,\bullet \, x^{\prime }) = a\) may then be selected. It follows that \(a= \rho (\widehat{x}\,\bullet \, x^{\prime }) \ge \min \{ \rho (\widehat{x}), \rho (x^{\prime })\}= b\), a contradiction. \(\square\)
Proposition 9
Let \((X,*)\in Bin(X)\) and let \(\rho : X\rightarrow [0, \infty )\) be a (fuzzy) rankdfunction of X. Then

(1)
\(Ker(\rho ) \not = \emptyset\),

(2)
if \(\rho (x) \le \rho (y)\) then \(x*y \in Ker(\rho )\),

(3)
\(Ker(\rho )\) is a right ideal of \((X,*)\).
Proof
(1) Given \(x\in X\), \(\rho (x*x) = \max \{\rho (x)\rho (x), 0\} = 0\). It follows that \(x*x\in Ker(\rho )\). (2) If \(\rho (x) \le \rho (y)\) then \(\rho (x*y) = \max \{\rho (x)\rho (y), 0\}= 0\) and hence \(x*y\in Ker(\rho )\). (3) Given \(x\in Ker(\rho ), y\in X\), we have \(\rho (x*y) = \max \{\rho (x)\rho (y), 0\} = \max \{0\rho (y), 0\} = 0\), which shows \(x*y\in Ker(\rho )\). \(\square\)
Right parallelisms
Let \(\rho _1, \rho _2\) be mappings from X to \([0, \infty )\). The map \(\rho _1\) is said to be right parallel to \(\rho _2\) if \(\rho _1(a)\le \rho _1(b)\) implies \(\rho _2(a)\le \rho _2(b)\), and we denote it by \(\rho _1\, \, \rho _2\).
Proposition 10
If \(\rho _1\) is a (fuzzy) ranksubalgebra of \((X,*)\) and \(\rho _1  \rho _2\), then \(\rho _2\) is also a (fuzzy) ranksubalgebra of \((X,*)\).
Proof
If \(\rho _1\) is a ranksubalgebra of \((X,*)\), then \(\rho _1(x*y)\ge \min \{\rho _1(x),\) \(\rho _1(y)\}\) for all \(x, y\in X\). Without loss of generality, we let \(\rho _1(x*y) \ge \rho _1(x)\). Since \(\rho _1 \rho _2\), we obtain \(\rho _2(x*y) \ge \rho _2(x)\). It follows that \(\rho _2(x*y)\ge \min \{\rho _2(x), \rho _2(y)\}\). \(\square\)
Given maps \(\rho _i: X\rightarrow [0, \infty )\), we define
for all \(x\in X\).
Note that \(\rho _1 + \rho _2\) need not be a (fuzzy) ranksubalgebra of \((X,*)\) even though \(\rho _1\) and \(\rho _2\) are (fuzzy) ranksubalgebras of \((X,*)\). In fact,
and
In the real numbers, it is not always true that \(\min \{a, b\} + \min \{c, d\} \ge \min \{a+c, b+d\}\), which shows that \(\rho _1 + \rho _2\) need not be a (fuzzy) ranksubalgebra of \((X,*)\).
Proposition 11
Let \(\rho _1, \rho _2\) be mappings from X to \([0, \infty )\). If \(\rho _1\) is a constant for all \(x\in X\) and if \(\rho _1 \rho _2\), then \(\rho _2\) is also a constant function on X.
Proof
Straightforward. \(\square\)
Note that any map \(\rho _1\) is right parallel to \(\rho _2\) if \(\rho _2\) is a constant function. The ‘right parallel’ relation “” is reflexive and transitive, but it is not an antisymmetric, i.e.,  is a quasi order, but not a partial order on \(\{\rho  \rho : X\rightarrow [0, \infty )\) : a map \(\}\).
For example, we let \(X:=[0, \infty )\) and let \(\rho _1: X\rightarrow [0, \infty )\) be the identity map, and let \(\rho _2: X\rightarrow [0, \infty )\) be a map defined by \(\rho _2(x):= e^ x\). Then \(\rho _1(a)\le \rho _1(b)\) if and only if \(\rho _2(a)\le \rho _2(b)\) for all \(a, b\in X\), i.e., \(\rho _1 \rho _2, \rho _2 \rho _1\), but \(\rho _1 \not = \rho _2\).
Proposition 12
If \(\rho _1\) is a (fuzzy) ranksubalgebra of \((X,*)\) and \(\rho _1 \rho _2\), then \(\rho _1\,\bullet \,\rho _2\) is also a (fuzzy) ranksubalgebra of \((X,*)\).
Proof
If \(\rho _1\) is a ranksubalgebra of \((X,*)\), then \(\rho _1(x*y)\ge \min \{\rho _1(x),\) \(\rho _1(y)\}\) for all \(x, y\in X\). If we assume \(\rho _1(x*y)\ge \rho _1(x)\), then
which proves the proposition. \(\square\)
Corollary 13
If \(\rho _1\) is a (fuzzy) rankcosubalgebra of \((X,*)\) and \(\rho _1 \rho _2\), then \(\rho _1\,\bullet \,\rho _2\) and \(\rho _2\) are also (fuzzy) rankcosubalgebras of \((X,*)\).
Proof
The proofs are similar to Propositions 10 and 12. \(\square\)
In the above note, we mentioned that \(\rho _1 + \rho _2\) need not be a ranksubalgebra of \((X,*)\). Using the notion of the right parallelism, we prove the following.
Proposition 14
If \(\rho _i\) is a ranksubalgebra of \((X,*)\) \((i= 1,2)\) and if \(\rho _1 \rho _2\), then \(\rho _1 + \rho _2\) is also a ranksubalgebra of \((X,*)\).
Proof
Since \(\rho _1\) is a ranksubalgebra of \((X,*)\), we have \(\rho _1(x*y)\ge \min \{\rho _1(x), \rho _1(y)\}\) for all \(x, y\in X\). Without loss of generality, we let \(\rho _1(x*y)\ge \rho _1(x)\). Since \(\rho _1 \rho _2\), we obtain \(\rho _2(x*y)\ge \rho _2(x)\). It follows that \(\rho _1(x*y) + \rho _2(x*y) \ge \rho _1(x) + \rho _2(x)\) and hence \((\rho _1 + \rho _2)(x*y) \ge \min \{ (\rho _1 + \rho _2)(x), (\rho _1 + \rho _2)(y)\}\), proving the proposition. \(\square\)
Corollary 15
If \(\rho _i\) is a rankcosubalgebra of \((X,*)\) \((i= 1,2)\) and if \(\rho _1 \rho _2\), then \(\rho _1 + \rho _2\) is also a rankcosubalgebra of \((X,*)\).
Proof
Similar to Proposition 14. \(\square\)
Proposition 16
Let \(X:={\mathbf{R}}\) be the set of all real numbers. Let \(\rho _1, \rho _2\) be mappings from X to \([0, \infty )\) and let \(\rho _2(x):=x\) for all \(x\in X\). If \(\rho _1 \rho _2\), then \(\rho _1\) is strictly increasing.
Proof
Assume that there are \(x, y\in X\) such that \(x<y, \rho _1(x) \ge \rho _1(y)\). Since \(\rho _1 \rho _2\), we obtain \(x=\rho _2(x)\ge \rho _2(y) = y\), a contradiction. \(\square\)
Theorem 17
If \(\rho _i\) is a rankdfunction of \((X,*)\) \((i= 1,2)\) and if \(\rho _1 \rho _2\), then \(\rho _1 + \rho _2\) is also a rankdfunction of \((X,*)\).
Proof
Given \(x, y\in X\), we have four cases: (i) \(\rho _1(x) \ge \rho _1(y), \rho _2(x) \ge \rho _2(y)\); (ii) \(\rho _1(x) \ge \rho _1(y), \rho _2(x) < \rho _2(y)\); (iii) \(\rho _1(x) < \rho _1(y), \rho _2(x) \ge \rho _2(y)\); (iv) \(\rho _1(x)< \rho _1(y), \rho _2(x) < \rho _2(y)\). Since \(\rho _1 \rho _2\), the cases (ii) and (iii) are removed. For the case (i), we have \(\rho _1(x*y) =\rho _1(x) \rho _1(y)\) and \(\rho _2(x*y)=\rho _2(x) \rho _2(y)\). It follows that
For the case (iv), we have \(\rho _1(x*y) = 0 = \rho _2(x*y)\). It follows that \((\rho _1+ \rho _2)(x*y) = \rho _1(x*y) + \rho _2(x*y)=0 =\max \{(\rho _1+\rho _2)(x)(\rho _1+\rho _2)(y), 0\}\), proving the theorem. \(\square\)
Note that if \(\rho _1 \rho _2, \rho _2  \rho _3\), then \(\rho _1  \rho _2 + \rho _3\), and if \(\rho _1  \rho _2, \rho _1  \rho _3\), then \(\rho _1  \rho _2 + \rho _3\).
\(\rho\)Shrinking groupoids
A groupoid \((X,*)\) is said to be \(\rho\)shrinking if \(\rho : X\rightarrow [0, \infty )\) is a map satisfying the condition:
for all \(x, y\in X\).
Example 18
Let \(X:=[0, \infty )\) and let \(x*y:= x +y\) for all \(x, y\in X\). If we define \(\rho (x):=x\) for all \(x\in X\), then \(\rho (x*y)\ge \max \{\rho (x), \rho (y)\}\). It follows that \((e^{\rho })(x*y) = e^{x}e^{y}\le \min \{e^{x}, e^{y}\}= \min \{ (e^{\rho })(x),\) \((e^{\rho })(y)\}\), which shows that \((X,*)\) is \(e^{\rho }\)shrinking.
Theorem 19
If \((X,*)\) is both \(\rho _1\)shrinking and \(\rho _2\)shrinking and if \(\rho _1 \rho _2\), then \((X,*)\) is both \((\rho _1 + \rho _2)\)shrinking and \(\rho _1\,\bullet \,\rho _2\)shrinking.
Proof
Since \((X,*)\) is \(\rho _1\)shrinking, we have \(\rho _1(x*y) \le \min \{\rho _1(x),\) \(\rho _1(y)\}\) for all \(x, y\in X\). The condition \(\rho _1  \rho _2\) implies that \(\rho _2(x*y) \le \min \{\rho _2(x), \rho _2(y)\}\). Hence
Without loss of generality, we may assume \(\rho _1(x)\le \rho _1(y)\). Then \(\rho _2(x)\le \rho _2(y)\), since \(\rho _1  \rho _2\). It follows that \((\rho _1+ \rho _2)(x*y) \le \rho _1(x) + \rho _2(y) = (\rho _1+ \rho _2)(x)\). Hence \((X,*)\) is \((\rho _1 + \rho _2)\)shrinking. Similarly, if we assume \(\rho _1(x)\le \rho _1(y)\), then
which shows that \((X,*)\) is \(\rho _1\,\bullet \, \rho _2\)shrinking. \(\square\)
Proposition 20
If \((X,*)\) and \((X,\bullet )\) are \(\rho\)shrinking and if \((X,\Box ):=(X,*)\,\Box \,(X,\bullet )\), then \((X,\Box )\) is also \(\rho\)shrinking.
Proof
If \((X,\Box ):=(X,*)\,\Box \,(X,\bullet )\), then for all \(x, y\in X\), we have
showing that \((X,\Box )\) is \(\rho\)shrinking. \(\square\)
Proposition 20 shows that the collection of all \(\rho\)shrinking groupoids forms a subsemigroup of \((Bin(X), \Box )\).
Given maps \(\rho : X\rightarrow [0, \infty )\) and \(\sigma : Y\rightarrow [0, \infty )\), we define a map \([\rho , \sigma ]: X\times Y\rightarrow [0, \infty )\) by \([\rho , \sigma ](x):= \rho (x) + \sigma (y)\) as a sort of “inner product” ranking. Given groupoids \((X,*)\) and \((Y,\bullet )\), we define a Cartesian product \((X\times Y, \nabla )\) where \((x, y)\nabla (x^{\prime }, y^{\prime }):= (x*x^{\prime }, y\,\bullet \, y^{\prime })\) for all \((x, y), (x^{\prime }, y^{\prime })\in X\times Y\).
Proposition 21
If \((X,*)\) is \(\rho\)shrinking and \((Y,\bullet )\) is \(\sigma\)shrinking, then \((X\times Y, \nabla )\) is \([\rho , \sigma ]\)shrinking.
Proof
Since \((X,*)\) is \(\rho\)shrinking and \((Y,\bullet )\) is \(\sigma\)shrinking, we have \(\rho (x*y)\le \min \{\rho (x), \rho (y)\}\) and \(\sigma (x\,\bullet \, y) \le \min \{\sigma (x), \sigma (y)\}\) for all \(x, y\in X\). It follows that
which proves the proposition. \(\square\)
Conclusions
Above, we introduced four (fuzzy) rank functions in the semigroup of all binary systems (i.e., groupoids), and we investigated their roles related to selective groupoids and the notion of Bin(X). Using the notion of “right parallelism”, we showed that if \(\rho _i\) is a (fuzzy) ranksubalgebra (resp., (fuzzy) rankdfunction) of \((X,*)\) (\(i= 1,2\)) and if \(\rho _1 \rho _2\), then \(\rho _1 + \rho _2\) is also a (fuzzy) ranksubalgebra (resp., (fuzzy) rankdfunction) of \((X,*)\). By introducing the notion of \(\rho\)shrinking to groupoids, we found that if \((X,*)\) is both \(\rho _1\)shrinking and \(\rho _2\)shrinking and if \(\rho _1 \rho _2\), then it is both \((\rho _1 + \rho _2)\)shrinking and \(\rho _1\,\bullet \,\rho _2\)shrinking. This research may provide hyperfuzzy rank functions in the set of all binary systems naturally, and thus several welldeveloped theorems/propositions in the areas of soft fuzzy theory and intuitionistic fuzzy set theory can then possibly be applied in future research also.
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Kim, H.S., Neggers, J. & So, K.S. Fuzzy rank functions in the set of all binary systems. SpringerPlus 5, 1865 (2016). https://doi.org/10.1186/s400640163536z
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Keywords
 (Fuzzy) ranksubalgebra
 (Fuzzy) rankdfunction
 (Fuzzy) symmetricrankfunction
 Right parallel
 \(\rho\)Shrinking
 Selective
 Bin(X)
Mathematics Subject Classification
 Primary 08A72
 20N02