Square lamina
= Square lamina
= Hollow square lamina
{synonym}
A square frame formed by removing a concentric square hole from a larger <square>. If outer and inner sides are <positive integers> $a>b$ with the same <integer parity>, the border has <integer> thickness $t=(a-b)/2$ and contains $a^2-b^2=4t(b+t)$ unit tiles.