Abstract. These five lectures constitute a tutorial on the Euler elastica and the Kirchhoff elastic rod. We consider the classical variational problem in Euclidean space and its generalization to Riemannian manifolds. We describe both the Lagrangian and the Hamiltonian formulation of the rod, with the goal of examining the (Liouville-Arnol'd) integrability. We are particularly interested in determining closed (i.e., periodic) solutions.Keywords: elastica, Kirchhoff rod PACS: 02.30.Xx,02.30.Yy
CLASSICAL BERNOULLI-EULER ELASTICAThe classical curve known as the elastica is the solution to a variational problem proposed by Daniel Bernoulli to Leonhard Euler in 1744, that of minimizing the bending energy of a thin inextensible wire (See, e.g., [27], §263). The mathematical idealization of this problem is that of minimizing the integral of the squared curvature for curves of a fixed length satisfying given first order boundary data. In this lecture, we will use the classical techniques of the calculus of variations to derive the equations of the elastica.Consider regular curves (curves with nonvanishing velocity vector) in Euclidean space defined on a fixed interval [a 1 , a 2 ]:We will assume (for technical reasons) that the (geodesic) curvature k of γ is nonvanishing. This will allow us to define the Frenet Frame along the curve.The Frenet frame {T, N, B} is orthonormal and satisfies γ = vT dT ds = kN dN ds = −kT + τB dB ds = −τN The elastica minimizes the bending energywith fixed length and boundary conditions. Accordingly, let α 1 and α 2 be points in R 3 and α 1 and α 2 nonzero vectors. We will consider the space of smooth curves