Bi-Helly family

Bi-Helly family --- би-Хелли семейство.

A hypergraph [math]\displaystyle{ {\mathcal H} }[/math] is called a bi-Helly family if it satisfies the following property: if any two edges of a subhypergraph [math]\displaystyle{ {\mathcal H'} \subseteq {\mathcal H} }[/math] share at least two vertices, then

[math]\displaystyle{ \left | \bigcap_{H \in {mathcal H'}} H \right | \geq 2. }[/math]