An Algebraic Characterization of singular quasi-bi-Hamiltonian systems

In this paper we prove an algebraic criterion which characterizes singular quasi-bi-hamiltonian structures constructed on the lines of a general, simple, new formal procedure proposed by the authors. This procedure shows that for the definition of a quasi-bi-hamiltonian system the requirement of non-singular Poisson tensors, contained in the original definition by Brouzet et al., is not essential. Besides, it is incidentally shown that one method of constructing Poisson tensors available in the literature is a particular case of ours. We present 2 examples.