# 8-Relations - EECS 210 Discrete Structures David O Johnson...

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Relations David O. Johnson EECS 210 (Fall 2016) 1 EECS 210 Discrete Structures David O. Johnson Fall 2016

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Reminders Homework 2 due: Today at the beginning of the lecture (Thursday, September 22) Connect 2 due: 11:59 PM, Thursday, September 29 Relations David O. Johnson EECS 210 (Fall 2016) 2
Any Questions? Relations 3 David O. Johnson EECS 210 (Fall 2016)

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Closure of Relations (Section 9.4) What Is a Closure? Reflexive Closure Symmetric Closure Transitive Closures Paths in Directed Graphs Connectivity Relation R * Warshall’s Algorithm Relations David O. Johnson EECS 210 (Fall 2016) 4
What Is a Closure? Example: A computer network has data centers in Boston, Chicago, Denver, Detroit, New York, and San Diego. There are direct, one-way telephone lines from Boston to Chicago, from Boston to Detroit, from Chicago to Detroit, from Detroit to Denver, and from New York to San Diego. We want to add telephone lines so that at least one link (possibly indirect) is between any two data centers. How do we determine the minimum number of direct links to add using the properties of a relation? Relations David O. Johnson EECS 210 (Fall 2016) 5 Chicago Boston San Diego Denver Detroit New York Solution: Let R be the relation containing (a, b) if there is a telephone line from the data center in a to that in b. The relation would be transitive, if all the data centers were interconnected. Transitivity : If ( x,y ) and ( y,z ) are edges, then so is ( x,z ) .

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What Is a Closure? Solution (continued): As we will show in this section, we can find all pairs of data centers that have a link by constructing a transitive relation S containing R such that S is a subset of every transitive relation containing R. Here, S is the smallest transitive relation that contains R. This relation is called the transitive closure of R. Thus S – R = the direct links we need to add. What is a Closure? In general, let R be a relation on a set A. R may or may not have some property P, such as reflexivity, symmetry, or transitivity. If there is a relation S with property P containing R such that S is a subset of every relation with property P containing R, then S is called the closure of R with respect to P. Note that the closure of a relation with respect to a property may not exist. Relations David O. Johnson EECS 210 (Fall 2016) 6
Reflexive Closure R is reflexive iff ( a,a ) R for every element a A . Written symbolically, R is reflexive if and only if x [ x U ⟶ ( x , x ) ∊ R ] The relation R = {(1, 1), (1, 2), (2, 1), (3, 2)} on the set A = {1, 2, 3} is not reflexive. How can we produce a reflexive relation containing R that is as small as possible? This can be done by adding (2, 2) and (3, 3) to R, because these are the only pairs of the form (a, a) that are not in R.

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