Ground-state subspace
= Ground-state subspace
{title2=$\ker(H-E_0I)$}
= Ground space
{synonym}
The <ground-state subspace> is the eigenspace of a <Hamiltonian operator> at its lowest <eigenvalue> $E_0$. A <computational history state> construction can have many valid <quantum witnesses>, hence a degenerate <ground space>. Perturbation bounds must optimize on this entire subspace, rather than on a chosen basis of ground states.