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Multi-Lagrangians, Hereditary Operators and Lax Pairs for the Korteweg-de Vries Positive and Negative Hierarchies

We present an approach to the construction of action principles for differential equations, and apply it to field theory in order to construct systematically, for integrable equations which are based on a Nijenhuis (or hereditary) operator, a ladder of action principles which is complementary to the well-known multi-Hamiltonian formulation. We work out results for the Korteweg-de Vries (KdV) equation, which is a member of the positive hierarchy related to a hereditary operator. Three negative hierarchies of (negative) evolution equations are defined naturally from the hereditary operator as well, in the context of field theory. The Euler-Lagrange equations arising from the action principles are equivalent to the original evolution equation + deformations, which are obtained in terms of the positive and negative evolution vectors. We recognize the Liouville, Sinh-Gordon, Hunter-Zheng and Camassa-Holm equations as negative equations. The ladder for KdV is directly mappable to a ladder for any of these negative equations and other positive equations (e.g., the Harry-Dym and a special case of the Krichever-Novikov equations): a new nonlocal action principle for the deformed system Sinh-Gordon + spatial translation vector is presented. Several nonequivalent, nonlocal time-reparametrization invariant action principles for KdV are constructed. Hamiltonian and Symplectic operators are obtained in factorized form. Alternative Lax pairs for all negative flows are constructed, using the flows and the hereditary operator as only input. From this result we prove that all positive and negative equations in the hierarchies share the same sets of local and nonlocal constants of the motion for KdV, which are explicitly obtained using the local and nonlocal action principles for KdV.