- #1

Tsunoyukami

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*A relation ##R## is defined on ##N## by ##aRb## if ##\frac{a}{b} \in N##. For ##c, d \in N##, under what conditions is ##c R^{-1} d##?*(Exercise 8.6 from Chartrand, Polemni & Zhang's

*Mathematical Proofs: A Transition to Advanced Mathematics; 3rd edition*; page 210).

From page 192 of the same text, "Let ##A## and ##B## be two sets. By a relation ##R## from ##A## to ##B## we mean a subset of ##A x B##. That is, ##R## is a set of ordered pairs, where the first coordinate of the pair belongs to ##A## and the second coordinate belongs to ##B##. If ##(a,b) \in R##, then we say that ##a## is related to ##b## by ##R## and write ##a R b##. If ##(a,b) \notin R##, then ##a## is

*not*related to ##b## by ##R## and we write ##a ~R b##." (Here I have used ##~R## to denote "not ##R##" because I was unsure how to cross ##R## out as it appears in the text).

On the next page (193), "Let ##R## be a relation from ##A## to ##B##. By the inverse relation of ##R## is meant the relation ##R^{-1}## from ##B## to ##A## defined by

##R^{-1}=\left\{ (b,a) : (a,b) \in R \right\}##."

Given these definitions I am having difficulty understanding what the notation means. For example, I understand what ##a R b## means, but I do not understand what ##a R^{-1} b## means. Because of the way ##R^{-1}## is defined (that is, ##R^{-1}=\left\{ (b,a) : (a,b) \in R \right\}##) I am not sure whether this definition is precisely ##a R^{-1} b## or ##b R^{-1} a##.

Any help with regard to this would be much appreciated. Thank you in advance!