# Cardinality of union of two families of finite sets

Let $F=\{S_1,\dots,S_n\}$ be a family of finite sets and $G=\{T_1,\dots,T_m\}$ also be a family of finite sets. Define the union of $F$ and $G$ naturally as $F \cup G=\{S_1 \cup T_1, S_2 \cup T_1, \dots , S_n \cup T_1, \dots, S_1 \cup T_m, \dots, S_n \cup T_m\}$.

The maximum number of distinct elements in $F \cup G$ is $mn$, which occurs when all of the sets are disjoint. What is the minimum number of distinct elements in $F \cup G$?

• Have you tried anything so far? – TomGrubb Nov 14 '17 at 4:17
• ThomasGrubb, No, I have not thought too deeply about this. My real motive for asking the question is to see if there is any good material out there which addresses questions like this. – Craig Feinstein Nov 14 '17 at 4:20

The minimum is $1,$ which occurs when $S_i=\{1,2,3,\dots,n+m\}\setminus\{i\}$ for $i=1,2,3,\dots,n$ and $T_j=\{1,2,3,\dots,n+m\}\setminus\{n+j\}$ for $j=1,2,3,\dots,m;$ more generally, whenever the complements of all the sets are disjoint.