Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Reflexive = transistive relation?

  1. May 9, 2010 #1
    Given A = {1,2,3}
    R1 = {1,1 2,2 3,3}

    I know it is reflexive, and I know it is symmetric. But what about its transitivity?

    Def of transitive: a,b in R, b,c in R, then a,c is also in R

    let a = 1
    let b = 1
    let c = 1

    (1,1) and (1,1)
    So yes, the book says it is an equivalence relation, so its transitivity is also valid.

    therefore, are all reflexive relations transitive?

    but what if the questions asks: constructs a reflexive and sysmmetric but not transitive?

    I can do 11 22 33 and add 12 21 to make them both reflexive and symmetric. since i added 12 and 21, thus these 2 ordered pairs destroyed the transitivity?
  2. jcsd
  3. May 9, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    No. You need to construct a relation where (a, b) and (b, c) are in the relation, but (a, c) is not.
  4. May 9, 2010 #3
    Well can't I make a = 1 b = 1 and c =1?

    The book said 11 22 33 is an equivalence relations on A, so I am guessing that's how he did it.
  5. May 10, 2010 #4


    User Avatar
    Science Advisor

    This is wrong. You can't just "take" a,b,c=1. You need to check that FOR ALL a,b,c in R the implication "if (a,b) and (b,c) are in R, then (a,c) is in R" is valid.

    Btw, you need to be careful with dropping brackets.
    R1 = {(1,1), (2,2), (3,3)} is correct,
    R1 = {1,1 2,2 3,3} is nonsensical.
  6. May 10, 2010 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    No. See the second example here:
    (Bold emphasis added by me.)
  7. May 10, 2010 #6


    User Avatar
    Science Advisor

    This is not even a relation! A relation is a collection of order pairs from A, a subset of [itex]A\times A[/itex]. Did you mean {(1,1), (2,2), (3,3)}? If that is the case, then, yes it is transitive. Transitive means "if (a, b) and (b, c) are in the relation, then (a, c) is also in the relation. Here, in order that (a, b) and (b, c) be in the relation, we must have a= b= c. so that (a, b)= (a, c) is in the set.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook