Published online by Cambridge University Press: 18 June 2019
Let $\mathcal{H}$ and
$\mathcal{K}$ be infinite-dimensional separable Hilbert spaces and
$\text{Lat}\,\mathcal{H}$ the lattice of all
closed subspaces oh
$\mathcal{H}$. We describe the general form of pairs of bijective maps
$\phi ,\,\psi :\,\text{Lat}\,\mathcal{H}\,\to \,\text{Lat}\,\mathcal{K}$ having the property that for every pair
$U,\,V\,\in \,\text{Lat}\,\mathcal{H}$ we have
$\mathcal{H}\,=\,U\,\oplus \,V\,\Leftrightarrow \,\mathcal{K}\,=\,\phi \left( U \right)\,\oplus \,\psi \,\left( V \right)$. Then we reformulate this theorem as a description of bijective image equality and kernel equality preserving maps acting on bounded linear idempotent operators. Several known structural results for maps on idempotents are easy consequences.