For each pair (x, y), each object X is from the symbols of the first set and the Y is from the symbols of the second set. The relation \(U\) on the set \(\mathbb{Z}^*\) is defined as \[a\,U\,b \,\Leftrightarrow\, a\mid b. To check symmetry, we want to know whether \(a\,R\,b \Rightarrow b\,R\,a\) for all \(a,b\in A\). ) R, Here, (1, 2) R and (2, 3) R and (1, 3) R, Hence, R is reflexive and transitive but not symmetric, Here, (1, 2) R and (2, 2) R and (1, 2) R, Since (1, 1) R but (2, 2) R & (3, 3) R, Here, (1, 2) R and (2, 1) R and (1, 1) R, Hence, R is symmetric and transitive but not reflexive, Get live Maths 1-on-1 Classs - Class 6 to 12. Irreflexive Symmetric Antisymmetric Transitive #1 Reflexive Relation If R is a relation on A, then R is reflexiveif and only if (a, a) is an element in R for every element a in A. Additionally, every reflexive relation can be identified with a self-loop at every vertex of a directed graph and all "1s" along the incidence matrix's main diagonal. You will write four different functions in SageMath: isReflexive, isSymmetric, isAntisymmetric, and isTransitive. Not symmetric: s > t then t > s is not true Hence, these two properties are mutually exclusive. Write the definitions above using set notation instead of infix notation. y Example \(\PageIndex{5}\label{eg:proprelat-04}\), The relation \(T\) on \(\mathbb{R}^*\) is defined as \[a\,T\,b \,\Leftrightarrow\, \frac{a}{b}\in\mathbb{Q}.\]. `Divides' (as a relation on the integers) is reflexive and transitive, but none of: symmetric, asymmetric, antisymmetric. 4 0 obj Antisymmetric relation is a concept of set theory that builds upon both symmetric and asymmetric relation in discrete math. , c (c) Here's a sketch of some ofthe diagram should look: For most common relations in mathematics, special symbols are introduced, like "<" for "is less than", and "|" for "is a nontrivial divisor of", and, most popular "=" for "is equal to". More specifically, we want to know whether \((a,b)\in \emptyset \Rightarrow (b,a)\in \emptyset\). For example, "1<3", "1 is less than 3", and "(1,3) Rless" mean all the same; some authors also write "(1,3) (<)". Displaying ads are our only source of revenue. Yes. Functions Symmetry Calculator Find if the function is symmetric about x-axis, y-axis or origin step-by-step full pad Examples Functions A function basically relates an input to an output, there's an input, a relationship and an output. x 1 0 obj This page titled 7.2: Properties of Relations is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Harris Kwong (OpenSUNY) . x So, \(5 \mid (a-c)\) by definition of divides. Anti-reflexive: If the elements of a set do not relate to itself, then it is irreflexive or anti-reflexive. , This operation also generalizes to heterogeneous relations. , Give reasons for your answers and state whether or not they form order relations or equivalence relations. See also Relation Explore with Wolfram|Alpha. It is possible for a relation to be both symmetric and antisymmetric, and it is also possible for a relation to be both non-symmetric and non-antisymmetric. Varsity Tutors 2007 - 2023 All Rights Reserved, ANCC - American Nurses Credentialing Center Courses & Classes, Red Hat Certified System Administrator Courses & Classes, ANCC - American Nurses Credentialing Center Training, CISSP - Certified Information Systems Security Professional Training, NASM - National Academy of Sports Medicine Test Prep, GRE Subject Test in Mathematics Courses & Classes, Computer Science Tutors in Dallas Fort Worth. Since \((1,1),(2,2),(3,3),(4,4)\notin S\), the relation \(S\) is irreflexive, hence, it is not reflexive. and Then \(\frac{a}{c} = \frac{a}{b}\cdot\frac{b}{c} = \frac{mp}{nq} \in\mathbb{Q}\). Hence, \(T\) is transitive. Pierre Curie is not a sister of himself), symmetric nor asymmetric, while being irreflexive or not may be a matter of definition (is every woman a sister of herself? (14, 14) R R is not reflexive Check symmetric To check whether symmetric or not, If (a, b) R, then (b, a) R Here (1, 3) R , but (3, 1) R R is not symmetric Check transitive To check whether transitive or not, If (a,b) R & (b,c) R , then (a,c) R Here, (1, 3) R and (3, 9) R but (1, 9) R. R is not transitive Hence, R is neither reflexive, nor . = So we have shown an element which is not related to itself; thus \(S\) is not reflexive. By going through all the ordered pairs in \(R\), we verify that whether \((a,b)\in R\) and \((b,c)\in R\), we always have \((a,c)\in R\) as well. Again, the previous 3 alternatives are far from being exhaustive; as an example over the natural numbers, the relation xRy defined by x > 2 is neither symmetric nor antisymmetric, let alone asymmetric. example: consider \(D: \mathbb{Z} \to \mathbb{Z}\) by \(xDy\iffx|y\). For each of the following relations on \(\mathbb{N}\), determine which of the three properties are satisfied. Decide if the relation is symmetricasymmetricantisymmetric (Examples #14-15), Determine if the relation is an equivalence relation (Examples #1-6), Understanding Equivalence Classes Partitions Fundamental Theorem of Equivalence Relations, Turn the partition into an equivalence relation (Examples #7-8), Uncover the quotient set A/R (Example #9), Find the equivalence class, partition, or equivalence relation (Examples #10-12), Prove equivalence relation and find its equivalence classes (Example #13-14), Show ~ equivalence relation and find equivalence classes (Examples #15-16), Verify ~ equivalence relation, true/false, and equivalence classes (Example #17a-c), What is a partial ordering and verify the relation is a poset (Examples #1-3), Overview of comparable, incomparable, total ordering, and well ordering, How to create a Hasse Diagram for a partial order, Construct a Hasse diagram for each poset (Examples #4-8), Finding maximal and minimal elements of a poset (Examples #9-12), Identify the maximal and minimal elements of a poset (Example #1a-b), Classify the upper bound, lower bound, LUB, and GLB (Example #2a-b), Find the upper and lower bounds, LUB and GLB if possible (Example #3a-c), Draw a Hasse diagram and identify all extremal elements (Example #4), Definition of a Lattice join and meet (Examples #5-6), Show the partial order for divisibility is a lattice using three methods (Example #7), Determine if the poset is a lattice using Hasse diagrams (Example #8a-e), Special Lattices: complete, bounded, complemented, distributed, Boolean, isomorphic, Lattice Properties: idempotent, commutative, associative, absorption, distributive, Demonstrate the following properties hold for all elements x and y in lattice L (Example #9), Perform the indicated operation on the relations (Problem #1), Determine if an equivalence relation (Problem #2), Is the partially ordered set a total ordering (Problem #3), Which of the five properties are satisfied (Problem #4a), Which of the five properties are satisfied given incidence matrix (Problem #4b), Which of the five properties are satisfied given digraph (Problem #4c), Consider the poset and draw a Hasse Diagram (Problem #5a), Find maximal and minimal elements (Problem #5b), Find all upper and lower bounds (Problem #5c-d), Find lub and glb for the poset (Problem #5e-f), Determine the complement of each element of the partial order (Problem #5g), Is the lattice a Boolean algebra? Antisymmetric if \(i\neq j\) implies that at least one of \(m_{ij}\) and \(m_{ji}\) is zero, that is, \(m_{ij} m_{ji} = 0\). Co-reflexive: A relation ~ (similar to) is co-reflexive for all . For example, \(5\mid(2+3)\) and \(5\mid(3+2)\), yet \(2\neq3\). <> No edge has its "reverse edge" (going the other way) also in the graph. Note: If we say \(R\) is a relation "on set \(A\)"this means \(R\) is a relation from \(A\) to \(A\); in other words, \(R\subseteq A\times A\). More precisely, \(R\) is transitive if \(x\,R\,y\) and \(y\,R\,z\) implies that \(x\,R\,z\). CS202 Study Guide: Unit 1: Sets, Set Relations, and Set. hands-on exercise \(\PageIndex{1}\label{he:proprelat-01}\). Suppose divides and divides . Sind Sie auf der Suche nach dem ultimativen Eon praline? real number n m (mod 3), implying finally nRm. Nonetheless, it is possible for a relation to be neither reflexive nor irreflexive. Note: (1) \(R\) is called Congruence Modulo 5. Hence, \(S\) is not antisymmetric. Finally, a relation is said to be transitive if we can pass along the relation and relate two elements if they are related via a third element. Relations that satisfy certain combinations of the above properties are particularly useful, and thus have received names by their own. Since \(\sqrt{2}\;T\sqrt{18}\) and \(\sqrt{18}\;T\sqrt{2}\), yet \(\sqrt{2}\neq\sqrt{18}\), we conclude that \(T\) is not antisymmetric. Antisymmetric: For al s,t in B, if sGt and tGs then S=t. s > t and t > s based on definition on B this not true so there s not equal to t. Therefore not antisymmetric?? Reflexive, Symmetric, Transitive Tutorial LearnYouSomeMath 94 Author by DatumPlane Updated on November 02, 2020 If $R$ is a reflexive relation on $A$, then $ R \circ R$ is a reflexive relation on A. hands-on exercise \(\PageIndex{2}\label{he:proprelat-02}\). character of Arthur Fonzarelli, Happy Days. The power set must include \(\{x\}\) and \(\{x\}\cap\{x\}=\{x\}\) and thus is not empty. The relation \(U\) is not reflexive, because \(5\nmid(1+1)\). We find that \(R\) is. Explain why none of these relations makes sense unless the source and target of are the same set. 3 David Joyce Reflexive: Each element is related to itself. The following figures show the digraph of relations with different properties. \(a-a=0\). Strange behavior of tikz-cd with remember picture. a function is a relation that is right-unique and left-total (see below). Let us define Relation R on Set A = {1, 2, 3} We will check reflexive, symmetric and transitive R = { (1, 1), (2, 2), (3, 3), (1, 2), (2, 3), (1, 3)} Check Reflexive If the relation is reflexive, then (a, a) R for every a {1,2,3} (Python), Chapter 1 Class 12 Relation and Functions. Let x A. But it also does not satisfy antisymmetricity. , c Transitive if for every unidirectional path joining three vertices \(a,b,c\), in that order, there is also a directed line joining \(a\) to \(c\). if Kilp, Knauer and Mikhalev: p.3. It is clearly reflexive, hence not irreflexive. A relation \(R\) on \(A\) is reflexiveif and only iffor all \(a\in A\), \(aRa\). , then The contrapositive of the original definition asserts that when \(a\neq b\), three things could happen: \(a\) and \(b\) are incomparable (\(\overline{a\,W\,b}\) and \(\overline{b\,W\,a}\)), that is, \(a\) and \(b\) are unrelated; \(a\,W\,b\) but \(\overline{b\,W\,a}\), or. Checking that a relation is refexive, symmetric, or transitive on a small finite set can be done by checking that the property holds for all the elements of R. R. But if A A is infinite we need to prove the properties more generally. Mathematical theorems are known about combinations of relation properties, such as "A transitive relation is irreflexive if, and only if, it is asymmetric". Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. In mathematics, a relation on a set may, or may not, hold between two given set members. Let \({\cal T}\) be the set of triangles that can be drawn on a plane. y If a relation \(R\) on \(A\) is both symmetric and antisymmetric, its off-diagonal entries are all zeros, so it is a subset of the identity relation. Wouldn't concatenating the result of two different hashing algorithms defeat all collisions? Part 1 (of 2) of a tutorial on the reflexive, symmetric and transitive properties (Here's part 2: https://www.youtube.com/watch?v=txNBx.) We conclude that \(S\) is irreflexive and symmetric. Because\(V\) consists of only two ordered pairs, both of them in the form of \((a,a)\), \(V\) is transitive. So Congruence Modulo is symmetric. We'll show reflexivity first. As another example, "is sister of" is a relation on the set of all people, it holds e.g. Share with Email, opens mail client Since \((a,b)\in\emptyset\) is always false, the implication is always true. Transcribed Image Text:: Give examples of relations with declared domain {1, 2, 3} that are a) Reflexive and transitive, but not symmetric b) Reflexive and symmetric, but not transitive c) Symmetric and transitive, but not reflexive Symmetric and antisymmetric Reflexive, transitive, and a total function d) e) f) Antisymmetric and a one-to-one correspondence What is reflexive, symmetric, transitive relation? E.g. Which of the above properties does the motherhood relation have? a) \(U_1=\{(x,y)\mid 3 \mbox{ divides } x+2y\}\), b) \(U_2=\{(x,y)\mid x - y \mbox{ is odd } \}\), (a) reflexive, symmetric and transitive (try proving this!) R is said to be transitive if "a is related to b and b is related to c" implies that a is related to c. dRa that is, d is not a sister of a. aRc that is, a is not a sister of c. But a is a sister of c, this is not in the relation. Example \(\PageIndex{6}\label{eg:proprelat-05}\), The relation \(U\) on \(\mathbb{Z}\) is defined as \[a\,U\,b \,\Leftrightarrow\, 5\mid(a+b).\], If \(5\mid(a+b)\), it is obvious that \(5\mid(b+a)\) because \(a+b=b+a\). The relation \(T\) is symmetric, because if \(\frac{a}{b}\) can be written as \(\frac{m}{n}\) for some integers \(m\) and \(n\), then so is its reciprocal \(\frac{b}{a}\), because \(\frac{b}{a}=\frac{n}{m}\). Our interest is to find properties of, e.g. The complete relation is the entire set A A. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. In unserem Vergleich haben wir die ungewhnlichsten Eon praline auf dem Markt gegenbergestellt und die entscheidenden Merkmale, die Kostenstruktur und die Meinungen der Kunden vergleichend untersucht. Relationship between two sets, defined by a set of ordered pairs, This article is about basic notions of relations in mathematics. 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A relation R in a set A is said to be in a symmetric relation only if every value of a,b A,(a,b) R a, b A, ( a, b) R then it should be (b,a) R. ( b, a) R. Example \(\PageIndex{2}\label{eg:proprelat-02}\), Consider the relation \(R\) on the set \(A=\{1,2,3,4\}\) defined by \[R = \{(1,1),(2,3),(2,4),(3,3),(3,4)\}. The statement (x, y) R reads "x is R-related to y" and is written in infix notation as xRy. Example \(\PageIndex{4}\label{eg:geomrelat}\). For the relation in Problem 9 in Exercises 1.1, determine which of the five properties are satisfied. Clash between mismath's \C and babel with russian. For the relation in Problem 7 in Exercises 1.1, determine which of the five properties are satisfied. Example \(\PageIndex{6}\label{eg:proprelat-05}\), The relation \(U\) on \(\mathbb{Z}\) is defined as \[a\,U\,b \,\Leftrightarrow\, 5\mid(a+b). A similar argument shows that \(V\) is transitive. 2 0 obj The relation \(R\) is said to be reflexive if every element is related to itself, that is, if \(x\,R\,x\) for every \(x\in A\). Determine whether the relation is reflexive, symmetric, and/or transitive? Write the relation in roster form (Examples #1-2), Write R in roster form and determine domain and range (Example #3), How do you Combine Relations? If it is irreflexive, then it cannot be reflexive. Let \({\cal L}\) be the set of all the (straight) lines on a plane. Proof. The reason is, if \(a\) is a child of \(b\), then \(b\) cannot be a child of \(a\). Hence, it is not irreflexive. Then , so divides . The relation \(R\) is said to be symmetric if the relation can go in both directions, that is, if \(x\,R\,y\) implies \(y\,R\,x\) for any \(x,y\in A\). i.e there is \(\{a,c\}\right arrow\{b}\}\) and also\(\{b\}\right arrow\{a,c}\}\). R = {(1,1) (2,2)}, set: A = {1,2,3} Apply it to Example 7.2.2 to see how it works. (2) We have proved \(a\mod 5= b\mod 5 \iff5 \mid (a-b)\). It is clearly reflexive, hence not irreflexive. The notations and techniques of set theory are commonly used when describing and implementing algorithms because the abstractions associated with sets often help to clarify and simplify algorithm design. What's wrong with my argument? \nonumber\]. A directed line connects vertex \(a\) to vertex \(b\) if and only if the element \(a\) is related to the element \(b\). Exercise \(\PageIndex{12}\label{ex:proprelat-12}\). Using this observation, it is easy to see why \(W\) is antisymmetric. x Hence the given relation A is reflexive, but not symmetric and transitive. Since \(\frac{a}{a}=1\in\mathbb{Q}\), the relation \(T\) is reflexive; it follows that \(T\) is not irreflexive. Suppose is an integer. This shows that \(R\) is transitive. Do It Faster, Learn It Better. The relation \(R\) is said to be antisymmetric if given any two. [3][4] The order of the elements is important; if x y then yRx can be true or false independently of xRy. [1] What are examples of software that may be seriously affected by a time jump? Let B be the set of all strings of 0s and 1s. (b) is neither reflexive nor irreflexive, and it is antisymmetric, symmetric and transitive. The other type of relations similar to transitive relations are the reflexive and symmetric relation. It is not irreflexive either, because \(5\mid(10+10)\). Transitive Property The Transitive Property states that for all real numbers x , y, and z, If R is contained in S and S is contained in R, then R and S are called equal written R = S. If R is contained in S but S is not contained in R, then R is said to be smaller than S, written R S. For example, on the rational numbers, the relation > is smaller than , and equal to the composition > >. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. 1. Again, it is obvious that P is reflexive, symmetric, and transitive. This means n-m=3 (-k), i.e. (a) Reflexive: for any n we have nRn because 3 divides n-n=0 . Hence, \(T\) is transitive. If Instead, it is irreflexive. It is clearly irreflexive, hence not reflexive. (a) Since set \(S\) is not empty, there exists at least one element in \(S\), call one of the elements\(x\). \nonumber\], hands-on exercise \(\PageIndex{5}\label{he:proprelat-05}\), Determine whether the following relation \(V\) on some universal set \(\cal U\) is reflexive, irreflexive, symmetric, antisymmetric, or transitive: \[(S,T)\in V \,\Leftrightarrow\, S\subseteq T. \nonumber\], Example \(\PageIndex{7}\label{eg:proprelat-06}\), Consider the relation \(V\) on the set \(A=\{0,1\}\) is defined according to \[V = \{(0,0),(1,1)\}. Consider the relation \(R\) on \(\mathbb{Z}\) defined by \(xRy\iff5 \mid (x-y)\). z We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. t A reflexive relation is a binary relation over a set in which every element is related to itself, whereas an irreflexive relation is a binary relation over a set in which no element is related to itself. Identity Relation: Identity relation I on set A is reflexive, transitive and symmetric. \nonumber\]. Therefore, the relation \(T\) is reflexive, symmetric, and transitive. Indeed, whenever \((a,b)\in V\), we must also have \(a=b\), because \(V\) consists of only two ordered pairs, both of them are in the form of \((a,a)\). For instance, the incidence matrix for the identity relation consists of 1s on the main diagonal, and 0s everywhere else. A binary relation R defined on a set A may have the following properties: Reflexivity Irreflexivity Symmetry Antisymmetry Asymmetry Transitivity Next we will discuss these properties in more detail. \nonumber\] Determine whether \(S\) is reflexive, irreflexive, symmetric, antisymmetric, or transitive. Please login :). The relation is irreflexive and antisymmetric. (Problem #5i), Show R is an equivalence relation (Problem #6a), Find the partition T/R that corresponds to the equivalence relation (Problem #6b). Symmetric Property The Symmetric Property states that for all real numbers x and y , if x = y , then y = x . Define a relation \(S\) on \({\cal T}\) such that \((T_1,T_2)\in S\) if and only if the two triangles are similar. y Some important properties that a relation R over a set X may have are: The previous 2 alternatives are not exhaustive; e.g., the red binary relation y = x2 given in the section Special types of binary relations is neither irreflexive, nor reflexive, since it contains the pair (0, 0), but not (2, 2), respectively. For transitivity the claim should read: If $s>t$ and $t>u$, becasue based on the definition the number of 0s in s is greater than the number of 0s in t.. so isn't it suppose to be the > greater than sign. and To prove Reflexive. \(B\) is a relation on all people on Earth defined by \(xBy\) if and only if \(x\) is a brother of \(y.\). Why did the Soviets not shoot down US spy satellites during the Cold War? For each pair (x, y), each object X is from the symbols of the first set and the Y is from the symbols of the second set. Made with lots of love Symmetric if \(M\) is symmetric, that is, \(m_{ij}=m_{ji}\) whenever \(i\neq j\). This counterexample shows that `divides' is not asymmetric. Each square represents a combination based on symbols of the set. Example \(\PageIndex{5}\label{eg:proprelat-04}\), The relation \(T\) on \(\mathbb{R}^*\) is defined as \[a\,T\,b \,\Leftrightarrow\, \frac{a}{b}\in\mathbb{Q}. (a) is reflexive, antisymmetric, symmetric and transitive, but not irreflexive. \nonumber\]\[5k=b-c. \nonumber\] Adding the equations together and using algebra: \[5j+5k=a-c \nonumber\]\[5(j+k)=a-c. \nonumber\] \(j+k \in \mathbb{Z}\)since the set of integers is closed under addition. S Note that 4 divides 4. Relation is a collection of ordered pairs. (b) Symmetric: for any m,n if mRn, i.e. Let that is . Similarly and = on any set of numbers are transitive. Transitive: Let \(a,b,c \in \mathbb{Z}\) such that \(aRb\) and \(bRc.\) We must show that \(aRc.\) It is not antisymmetric unless | A | = 1. How do I fit an e-hub motor axle that is too big? Determine whether the following relation \(W\) on a nonempty set of individuals in a community is an equivalence relation: \[a\,W\,b \,\Leftrightarrow\, \mbox{$a$ and $b$ have the same last name}.\]. y The best-known examples are functions[note 5] with distinct domains and ranges, such as [vj8&}4Y1gZ] +6F9w?V[;Q wRG}}Soc);q}mL}Pfex&hVv){2ks_2g2,7o?hgF{ek+ nRr]n 3g[Cv_^]+jwkGa]-2-D^s6k)|@n%GXJs P[:Jey^+r@3 4@yt;\gIw4['2Twv%ppmsac =3. \nonumber\] Symmetric and transitive don't necessarily imply reflexive because some elements of the set might not be related to anything. is irreflexive, asymmetric, transitive, and antisymmetric, but neither reflexive nor symmetric. 12_mathematics_sp01 - Read online for free. Example \(\PageIndex{3}\label{eg:proprelat-03}\), Define the relation \(S\) on the set \(A=\{1,2,3,4\}\) according to \[S = \{(2,3),(3,2)\}. So, \(5 \mid (b-a)\) by definition of divides. The Transitive Property states that for all real numbers y Teachoo answers all your questions if you are a Black user! Given sets X and Y, a heterogeneous relation R over X and Y is a subset of { (x,y): xX, yY}. , It may sound weird from the definition that \(W\) is antisymmetric: \[(a \mbox{ is a child of } b) \wedge (b\mbox{ is a child of } a) \Rightarrow a=b, \label{eqn:child}\] but it is true! \(-k \in \mathbb{Z}\) since the set of integers is closed under multiplication. is divisible by , then is also divisible by . \(\therefore R \) is transitive. R = {(1,2) (2,1) (2,3) (3,2)}, set: A = {1,2,3} a b c If there is a path from one vertex to another, there is an edge from the vertex to another. Duress at instant speed in response to Counterspell, Dealing with hard questions during a software developer interview, Partner is not responding when their writing is needed in European project application. Exercise. Checking whether a given relation has the properties above looks like: E.g. = Define the relation \(R\) on the set \(\mathbb{R}\) as \[a\,R\,b \,\Leftrightarrow\, a\leq b.\] Determine whether \(R\) is reflexive, symmetric,or transitive. Hence, \(S\) is symmetric. Has 90% of ice around Antarctica disappeared in less than a decade? A relation \(R\) on \(A\) is symmetricif and only iffor all \(a,b \in A\), if \(aRb\), then \(bRa\). Show (x,x)R. How to prove a relation is antisymmetric Let \(S\) be a nonempty set and define the relation \(A\) on \(\scr{P}\)\((S)\) by \[(X,Y)\in A \Leftrightarrow X\cap Y=\emptyset.\] It is clear that \(A\) is symmetric. Thus, by definition of equivalence relation,\(R\) is an equivalence relation. Example \(\PageIndex{4}\label{eg:geomrelat}\). The reflexive property and the irreflexive property are mutually exclusive, and it is possible for a relation to be neither reflexive nor irreflexive. 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