Several conjectures and results in the theory of integrable Hamiltonian systems of hydrodynamic type, which do not possess Riemann invariants

We formulate several conjectures concerning the structure and general properties of the $n\times n$ integrable nondiagonalizable hamiltonian systems of hydrodynamic type. For $n=3$ our results are in fact complete: a $3\times 3$ nondiagonalizable hamiltonian system is integrable if and only if it is weakly nonlinear (linearly degenerate).