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Inversionless laser from a closed multilevel system.

We show that a closed four-level system can exhibit lasing without population inversion in any state basis. The same system can be also made to exhibit lasing without population inversion in the bare-state basis but with population inversion in the dressed-state basis. We derive the conditions of lasing without inversion and examine the dependence of the gain on various system parameters. The lasing threshold and the steady-state photon numbers are calculated. Comparison with a V-type, three-level model is presented