Maximal degree variational principles and Liouville dynamics

Let M be smooth n-dimensional manifold, fibered over a k-dimensional submanifold B as (pi) :M(right arrow)B, and (zeta)(element of) (lambda)^k(M) one can consider the functional on sections (phi) of the bundle (pi) defined by (integral)D(phi)*((upsilon), with D a domain in B. We show that for k=n-2 the variational principle based on this functional identifies a unique (up to multiplication by a smooth function) nontrivial vector field in M, i.e., a system of ODEs. Conversely, any vector field X on M satisfying X(downward right)d (upsilon)=0 for some (upsilon)(elemnt of)(lambda)^n-2(M) admits such a variational characterization. We consider the general case, and also the particular case M=P×R where one of the variables (the time) has a distinguished role; in this case our results imply that any Liouville (volume-preserving) vector field on the phase space P admits a variational principle of the kind considered here.