# De Morgan's Laws (Set Theory)/Set Complement/Complement of Union

## Theorem

Let $T_1, T_2$ be subsets of a universe $\mathbb U$.

Then:

$\overline {T_1 \cup T_2} = \overline T_1 \cap \overline T_2$

where $\overline T_1$ is the set complement of $T_1$.

It is arguable that this notation may be easier to follow:

$\map \complement {T_1 \cup T_2} = \map \complement {T_1} \cap \map \complement {T_2}$

## Proof

 $\displaystyle \map \complement {T_1 \cup T_2}$ $=$ $\displaystyle \mathbb U \setminus \paren {T_1 \cup T_2}$ Definition of Set Complement $\displaystyle$ $=$ $\displaystyle \paren {\mathbb U \setminus T_1} \cap \paren {\mathbb U \setminus T_2}$ De Morgan's Laws: Difference with Union $\displaystyle$ $=$ $\displaystyle \map \complement {T_1} \cap \map \complement {T_2}$ Definition of Set Complement

$\blacksquare$

## Demonstration by Venn Diagram $\overline T_1$ is depicted in yellow and $\overline T_2$ is depicted in red.

Their intersection, $\overline T_1 \cap \overline T_2$, is depicted in orange.

As can be seen by inspection, this also equals the complement of the union of $T_1$ and $T_2$.

## Source of Name

This entry was named for Augustus De Morgan.