  This exposition paper is devoted to the theory of Abram Vilgelmovich Shtraus
and his disciples and followers. This theory studies the so-called generalized
resolvents of symmetric and isometric operators in a Hilbert space and provides
alternative formulas to the well-known Krein-type formulas. We think that it
would be convenient to have an exposition of the corresponding theory with full
proofs and necessary corrections.
