Classification of bi-Hamiltonian pairs extended by isometries

The aim of this article is to classify pairs of the first-order Hamiltonian operators of Dubrovin–Novikov type such that one of them has a non-local part defined by an isometry of its leading coefficient. An example of such a bi-Hamiltonian pair was recently found for the constant astigmatism equation. We obtain a classification in the case of two dependent variables, and a significant new example with three dependent variables that is an extension of a hydrodynamic-type system obtained from a particular solution of the Witten–Dijkgraaf–Verlinde–Verlinde equations.

Isospectral hermitian counterpart of complex nonhermitian Hamiltonian p2 – gx4 + a/x2

We show that the nonhermitian Hamiltonians H = p2 – gx4 + a/x2 and the conventional hermitian Hamiltonians h = p2 + 4gx4 + bx [Formula: see text] are isospectral if a = (b2 – 4gℏ2)/16g and a ≥ –ℏ2/4. This new class includes the equivalent nonhermitian–hermitian Hamiltonian pair, p2 – gx4 and [Formula: see text] found by Jones and Mateo six years ago as a special case. When a = (b2 – 4gℏ2)/16g and a < –ℏ2/4 although h and H are still isospectral, b is pure imaginary, and h is no longer the hermitian counterpart of H.