Anharmonic O(N) oscillator model in next-to-leading order of the 1/N   expansion
  An extended Holstein-Primakoff mapping which incorporates both single- and double boson mappings is used in the context of O(N+1) anharmonic oscillator with the global symmetry broken down to O(N) to study the next-to-leading order of the 1/N expansion. It is shown that the Goldstone theorem is obeyed to that order and that the Ward identity is satisfied. 
Alan Dzhioev (BLTP, JINR, Dubna), J.Wambach (IKP TU Darmstadt and GSI,   Darmstadt), A. Vdovin (BLTP, JINR, Dubna), and Z. A...