# Operation table for A+B where + denotes the operation of symmetric difference

If someone could please verify if my operation table in the picture below is correct it'd be much appreciated.

$P_D=\{A: A \subset D\}$ and $D$ is a $3$-element set $D=\{a, b, c\}$ with the operation of symmetric difference defined as $A+B=(A-B) \cup (B-A)$. Write all the elements of $P_D$ and then write the operation table for $(P_D, +)$.

I got a little befuddled in doing this because I'm relatively new to dealing formally with sets, and I had trouble thinking of a, b, and as sets because in the previous problem it used A, B instead. Also, it was a little trickier with 3 sets and some simplified nicely while others did not...so I wasn't sure if that's okay or if I was perhaps doing something wrong or if you're only supposed to simplify to a certain point when doing an operation table (I didn't show my work here though...its on other pages and much messier).

I apologize for the small print and messy handwriting, but I think it's readable enough and I'd already re-written it several times so I thought that was good enough (I have trouble "organizing" my math).

In general, in operation tables you're supposed to simplify until each entry looks like one of the elements in the group, so you're probably expected to simplify more in most of them.

You're missing the curly brackets around each element of $P_D$; the power set of $D$ contains subsets of $D$, and $a$ is not a set, but $\{a\}$ is. This might be why you're having trouble simplifying them.

I'll write three of the simplifications to illustrate:

\begin{align} \{a\} &\triangle \{b\} = (\{a\}\setminus \{b\}) \cup (\{b\} \setminus \{a\}) = \{a\}\cup \{b\}=\{a,b\} \\ \{a,b\} &\triangle \{a,c\} = (\{a,b\}\setminus \{a,c\}) \cup (\{a,c\} \setminus \{a,b\}) = \{b\}\cup \{c\}=\{b,c\} \\ \{a,b\} &\triangle \{c\} = (\{a,b\}\setminus \{c\}) \cup (\{c\} \setminus \{a,b\}) = \{a,b\}\cup \{c\}=\{a,b,c\} \end{align}

You could compute all the others like this, or you could realize that the symmetric difference of two sets $X$ and $Y$ is the set containing all the elements that are in $X$ or in $Y$ but not in both: $$X \triangle Y = (X \setminus Y) \cup (Y \setminus X) = (X \cup Y) \setminus (Y \cap X) ,$$ so, since the sets are small, you could find the symmetric differences by inspection, by removing any elements that $X$ and $Y$ share and collecting the rest.
• You're right, $A-B\neq A$ in general. If $A$ and $B$ are sets, $A-B$ means $A \cap B^c$, or the intersection between $A$ and the complement of $B$, so it consists of the elements that are in $A$ and are not in $B$. Could you rephrase your second question? I don't really understand what you mean. – coldnumber Aug 16 '15 at 19:33
• Yes, that's clearer. Writing the difference as an intersection makes it easier to see: $\{a\}-\{b\}=\{a\}\cap \{b\}^c = \{a\} \cap \{a,c\} = \{a\}$ ( this is how set difference was actually defined in my class). I think it's better to think of $X-Y$ as removing anything that the sets $X$ and $Y$ have in common from $X$. If they don't have anything in common, there is nothing to remove. Does that make sense? – coldnumber Aug 17 '15 at 2:05