[Math] the complement of empty set

elementary-set-theory

We have learned that for universal set $U$ and for set $A$ belonging to $U$,
$A^c=U-A$ but what about for the empty set?

Best Answer

The complement of the empty set would be $U$. Using your formula:

$\emptyset^C = U - \emptyset = U$

Also, the complement of $U$ is the empty set: $U^C = U - U = \emptyset$

Related Solutions

Elementary Set Theory – Is There an Empty Set in the Complement of an Empty Set?

The answer is yes. But there are several comments that need to be made:

There is only one empty set. So it is better to say that $A=B=\emptyset$ rather than saying that "both $A$ and $B$ are empty sets", as the latter erroneously suggests that there is more than one. This is because two sets are equal precisely if they have the same elements. So any two empty sets are equal, since they have precisely the same elements (namely, none).
I assume by "context" you mean that the complement of $B$ is computed with respect to the "universal set." In the standard system of set theory, there can be no "universal set", as assuming its existence leads to problems (Russell paradox). [Though, yes, it is usual to talk of a "universal set" as a way to delineate what objects we are interested in.]

The reason why $A$ is contained in the complement of $B$ is that $A$ (being the empty set) is a subset of any set. This is because we define "$A$ is contained in $C$" to mean that any element of $A$ is also an element of $C$. Now, since nothing is an element of $A$, this condition is satisfied in this case (one typically says that it is satisfied vacuously.)

[Math] the cartesian product of a non empty set and a set containing an empty set (or $X_1 \times \{ \emptyset \}$ )

The empty set, considered as an element of another set, is nothing special! It is just an element. You could do the problem by finding $$X_1\times\{\,a\,\}$$ and then replacing $a$ by $\varnothing$.

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Related Solutions

Elementary Set Theory – Is There an Empty Set in the Complement of an Empty Set?

[Math] the cartesian product of a non empty set and a set containing an empty set (or $X_1 \times \{ \emptyset \}$ )

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