  We consider the problem of expansion of Bose-Einstein condensate released
form a box. On the contrary to the standard situation of release from a
harmonic trap, in this case the dynamics is complicated by a process of
collision of two rarefaction waves propagating to the center of the initially
uniform distribution. Complete analytical solution of this problem is obtained
by Riemann method in hydrodynamic dispersionless approximation and the results
are compared with the exact numerical solution of the Gross-Pitaevskii
equation.
