Anharmonic collective excitation in a solvable model

We apply the time-dependent variational principle, the nuclear field theory , and the boson expansion method to the Lipkin model to discuss anharmonici ties of collective vibrational excitations, It is shown that all of these a pproaches lead to the same anharmonicity to leading order in the number of particles, Comparison with the exact solution of the Lipkin model shows tha t these theories reproduce quite well. (C) 1999 Elsevier Science B.V. All r ights reserved.