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As has been previously shown by several authors (e.g., Ferland and Netzer 1983; Stasinska 1984a; Binette 1985), as well as by ourselves in a previous work (Storchi-Bergmann and Pastoriza 1989), the emission-line ratios of Seyfert 2's and LINERs can be successfully reproduced by model nebulae photoionized by a power-law continuum. So we have calculated the theoretical emission-line ratios using the photoionization code CLOUDY (Ferland 1990) for a constant density nebula photoionized by a power-law continuum of the form Fnu propto nualpha. In order to reproduce the observations we have varied the slope "alpha" of the ionizing continuum, the gas density, and the abundance of the elements, as described below. The typical total hydrogen column densities are in the range 1020-1021 cm-2.

As shock ionization also produces an enhancement of the [N II] / Halpha ratio (Dopita 1977), we have considered, for comparison, the integrated shock models of Viegas-Aldrovandi (1988), which take into account the contribution of clouds of different velocities (100 < v < 600 km s-1) according to the distribution law f(v) propto v-b.