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Homework Help: Discrete math, defining an operator

  1. Sep 29, 2009 #1
    I have some troubles with this question.

    Define an operator * on [tex]R[/tex] by

    [tex]x*y = 2xy -x -y +1[/tex]

    a) is * commutative?
    b) is * associative?

    I can easily see that * is commutative, but how do i test for associativity?

    The rule states that (x*y)*z = x*(y*z)

    But what is z ?
    Last edited: Sep 30, 2009
  2. jcsd
  3. Sep 29, 2009 #2
    For associativity, you should look at both sides of the equation you listed ((x*y)*z = x*(y*z)) and use your rule to expand. For your associativity condition, z is any third element of R, as both x and y must be. So if you're asked to do y*z, then you fill in y and z in the place of x and y in your rule. Then check to see if you get the same thing on both sides.
  4. Sep 29, 2009 #3


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    [tex] (x \ast y) \ast z [/tex] refers to the expression you get by first computing [tex] x \ast y = 2xy -x -y - 1 [/tex], then calculating [tex] (2xy - x -y -1) \ast z [/tex] - z is another real number.
  5. Sep 30, 2009 #4


    [tex](x \ast y) \ast z = (2xy -x -y +1)z \astz \longrightarrow (2xyz -xz -yz +z)[/tex]

    Now i have to test for the expression [tex]x \ast (y \ast z)[/tex]

    But how do i do that when the operator is defined for x and y ?

    More precisely, i don't know what [tex](y \ast z) is[/tex]
    Last edited: Sep 30, 2009
  6. Sep 30, 2009 #5


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    The rule for * has been given, so the question is simply
    and see if it is zero
    your operator is defined for all x and y in particular
  7. Oct 1, 2009 #6
    Thanks, i think i understand. Is this correct?:

    [tex](x \ast y) = 2xy -x -y +1 [/tex]

    [tex](x \ast y)\ast z = 2(2xy -x -y +1)z -2xy -x -y -z +1[/tex]

    [tex]= 4xyz -2xy -2xz +x -2yz -y -z +2[/tex]

    [tex](y \ast z) = 2yz - y -z +1[/tex]

    [tex]x \ast(y \ast z) = 2x(2yz -y -z +1) -x -2yz -y -z +2 = 4xyz -2xy -2xz +x -2yz -y -z +2[/tex]

    They are equal
  8. Oct 1, 2009 #7


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    Yes, that is correct.

    Referring to your earlier question, "But how do i do that when the operator is defined for x and y ?" or basically, "how do I combine three things with a binary operator?" The answer was "do them two at a time". To find (x*y)*z, first you find x*y then you treat that result as a single "object" with z. To find x*(y*z) you first find y*z and treat that as a single object with z. The whole point of the "associative law", (x*y)*z= x*(y*z), is that we can do it either way and so don't really need the parentheses: we could write x*y*z to mean either (x*y)*z or x*(y*z) and get the same answer. And, we can extend that to any number of objects, writing u*v*w*x*y*z, just like we do with sums or products of numbers!
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