  In this paper, we discuss the Hyers-Ulam stability of closable (unbounded)
operators with several interesting examples. We also present results pertaining
to the Hyers-Ulam stability of the sum and product of closable operators to
have the Hyers-Ulam stability and the necessary and sufficient conditions of
the Schur complement and the quadratic complement of $2 \times 2$ block matrix
$\mathcal A$ in order to have the Hyers-Ulam stability.
