Hard-disc cage approximation
= Hard-disc cage approximation
{title2=$\mathcal A=\pi(R-2r)^2$}
A circular free-area model for a disc surrounded by fixed neighbours a centre-to-centre distance $R$ away. The inscribed circle available to the moving centre has radius $R-2r$. It is an approximation to the actual curved cage boundary, not the area of a disc whose radius is $R-r$.