Algebraic treatment of the Pais-Uhlenbeck oscillator and its PT-variant
  The algebraic method enables one to study the properties of the spectrum of a quadratic Hamiltonian through the mathematical properties of a matrix representation called regular or adjoint. This matrix exhibits exceptional points where it becomes defective and can be written in canonical Jordan form. It is shown that any quadratic function of $K$ coordinates and $K$ momenta leads to a $2K$ differential equation for those dynamical variables. We illustrate all these features of the algebraic method by means of the Pais-Uhlenbeck oscillator and its PT-variant. We also consider a trivial quantization of the fourth-order differential equation for the quantum-mechanical dynamical variables. 
Francisco M. Fern\'andez
quant-ph