  The conditions for an arbitrary jump occurrence in isentropic flow are
studied. It is shown that the jump in gas-dynamic parameters arises as a result
of the evolution of a self-similar flow. The concept of self-focusing Riemann
waves is introduced. It is shown that an arbitrary jump is formed only by these
waves and the conditions for its generation are found. It is shown that there
exists a critical velocity, below which a discontinuity cannot be formed
isentropically. Also found is the second critical value of velocity, exceeding
which a discontinuity is formed only in the presence of a vacuum region. It is
shown that there are only two classes of solitary shock waves: those that form
in a medium containing a vacuum region and those that form in a continuous
medium. It is shown that not every fall of the Riemann wave leads to the
appearance of a shock wave.
