We discuss the induced star formation in dense walls of expanding shells. The fragmentation process is studied using the linear perturbation theory. The influence of the energy input, the ISM distribution and the ISM speed of sound is examined analytically and by numerical simulations. We formulate the universal condition for the gravitational fragmentation of expanding shells: if the total surface density of the disk is higher than a certain critical value, shells are unstable. The value of the critical density depends on the energy of the shell and the sound speed in the ISM.
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