Geometrical features of a nonlinear wavefront

We use the equations of weakly nonlinear ray theory (WNLRT), developed by u s over a number of years, to study all possible shapes which a nonlinear wa vefront in a polytropic gas can have. As seen in experiments, a converging nonlinear wavefront avoids folding itself in a caustic region of a linear t heory and emerges unfolded with a pair of kinks. We review the work of Bask ar, Potadar and Szeftel showing the way in which the solution of a Riemann problem of the conservation form of the equations of WNLRT can be used to s tudy the formation of new shapes of a nonlinear wavefront from a single sin gularity on it. We also study the ultimate result of interactions of elemen tary shapes on the front.