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Homework Help: Associative, Commutative, and Distributive Laws for Congruence Classes

  1. Mar 25, 2013 #1
    1. The problem statement, all variables and given/known data
    We are supposed to prove that the addition and multiplication of congruence classes for integers are associative, commutative, and distributive.

    2. Relevant equations

    3. The attempt at a solution
    To me, it seems like this would just be trivial as all these properties follow from the fact that we are working with integers, and integers are associative, commutative, and distributive (with both addition and multiplication). Am I on the right track here? Thanks.
  2. jcsd
  3. Mar 25, 2013 #2


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    It is fairly trivial, but you still need to show it. A congruence class of integers is not the same thing as an integer. What do you have so far?
  4. Mar 25, 2013 #3
    I think I have the idea of what to do, but just to be sure I'll try the commutative addition one.

    The addition is commutative if [itex]\overline{a}[/itex] + [itex]\overline{b}[/itex] = [itex]\overline{b}[/itex] + [itex]\overline{a}[/itex]

    This is the same condition as x + y = y + x for all x [itex]\in[/itex] [itex]\overline{a}[/itex] and all y [itex]\in[/itex] [itex]\overline{b}[/itex] where a,b,x,y are all integers.

    Thus, because for any integers x and y, x + y = y + x (commutativity of integers under addition) we would have [itex]\overline{a}[/itex] + [itex]\overline{b}[/itex] = [itex]\overline{b}[/itex] + [itex]\overline{a}[/itex]

    Is this more or less the correct approach to take here?
  5. Mar 25, 2013 #4


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    That's more or less right. If you want to be more formal, you might start from the definitions:
    $$\overline{a} + \overline{b} = \{x + y : x \in \overline{a}, y \in \overline{b}\}$$
    $$\overline{b} + \overline{a} = \{y + x : x \in \overline{a}, y \in \overline{b}\}$$
    To show equality of two sets, we show that each contains the other, i.e. ##\overline{a} + \overline{b} \subseteq \overline{b} + \overline{a}## and ##\overline{b} + \overline{a} \subseteq \overline{a} + \overline{b}##. To show the first inclusion, take an arbitrary element of ##\overline{a} + \overline{b}##. By definition, it is of the form ##x + y## with ##x \in \overline{a}## and ##y \in \overline{b}##. By commutativity of integer addition, ##x + y = y + x##, and the right hand side is clearly an element of ##\overline{b} + \overline{a}##. The reverse inclusion is similar.
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