  We provide sufficient and necessary conditions guaranteeing equations
$(A+B)^*=A^*+B^*$ and $(AB)^*=B^*A^*$ concerning densely defined unbounded
operators $A,B$ between Hilbert spaces. We also improve the perturbation theory
of selfadjoint and essentially selfadjoint operators due to Nelson, Kato,
Rellich, and W\"ust. Our method involves the range of two-by-two matrices of
the form $M_{S,T}=\left(\begin{array}{cc}\!\! I & -T\!\!\\ \!\! S&
I\!\!\end{array}\right)$ that makes it possible to treat real and complex
Hilbert spaces jointly.
