1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Symmetric relations

  1. May 5, 2009 #1
    1. The problem statement, all variables and given/known data

    Is this relation symmetric?

    The relation in a set of people, "is brother of"

    2. Relevant equations

    aRb , bRa

    3. The attempt at a solution
    The answer is not symmetric. They gave example says that
    paul may be the bother of Anne but Anne is not the brother of paul
    I thought that if A is brother of B then automatically B is brother of A isnt it?
    I think the reason why i can not understand this is problem of English.
    In my country brother indicates both older and younger, is it different in other country?
  2. jcsd
  3. May 5, 2009 #2


    User Avatar
    Homework Helper

    It does seem like a linguistic problem. Paul is male Anne is female. Paul is the brother of Anne, Anne is the sister of Paul. It has nothing to do with age only with gender. I hope this clears things up.

    A few more clarifications. If the set would have been the set of all males then the relation would have been symmetric.
  4. May 5, 2009 #3
    Ohhhhh right why couldnt i think about that. Anne is name of girl indeed..
  5. May 5, 2009 #4
    an one more question
    {(x,y) l x^2 + y^2 = 9, x belongs to Z}
    I have to find the domain and range here and as x belongs to Z [interger]
    I thought the domain is all the integers. and if x = 5 then y^2 = -16 which gives answer
    y = 4i so range is all the complex numbers. But the answer is
    domain {-3,-2,-1,0,1,2,3}
    range {-3, -2(root2), -(root5), 0 , root5,2 root 2, 3
  6. May 5, 2009 #5


    User Avatar
    Homework Helper

    It is the equation of a circle with radius 3. You can be pretty sure that they want real values for y and not complex values. Secondly even if we were to allow complex values for y then the range would still not be all the complex numbers. Example: [itex]y=\frac{1}{2} \Rightarrow x=\frac{1}{2}\sqrt{35} \notin \mathbb{Z}[/itex]
  7. May 5, 2009 #6
    Right..ahh why couldnt i think about such things before register the replies..
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook