On Lagrangians with Reduced-Order Euler–Lagrange Equations

If a Lagrangian defining a variational problem has order $k$ then its Euler–Lagrange equations generically have order $2k$. This paper considers the case where the Euler–Lagrange equations have order strictly less than $2k$, and shows that in such a case the Lagrangian must be a polynomial in the highest-order derivative variables, with a specific upper bound on the degree of the polynomial. The paper also provides an explicit formulation, derived from a geometrical construction, of a family of such $k$-th order Lagrangians, and it is conjectured that all such Lagrangians arise in this way.