In this paper we revisit the construction by which the SL(2, R) symmetry of the Euler equations allows to obtain the simple pendulum from the rigid body. We begin reviewing the original relation found by Holm and Marsden in which, starting from the two Casimir functions of the extended rigid body with Lie algebra ISO(2) and introducing a proper momentum map, it is possible to obtain both the Hamiltonian and equations of motion of the pendulum. Important in this construction is the fact that both Casimirs have the geometry of an elliptic cylinder. By considering the whole SL(2, R) symmetry group, in this contribution we give all possible combinations of the Casimir functions and the corresponding momentum maps that produce the simple pendulum, showing that this system can also appear when the geometry of one of the Casimirs is given by a hyperbolic cylinder and the another one by an elliptic cylinder. As a result we show that from the extended rigid body with Lie algebra ISO(1, 1), it is possible to obtain the pendulum but only in circulating movement. Finally, as a by product of our analysis we provide the momentum maps that give origin to the pendulum with an imaginary time. Our discussion covers both the algebraic and the geometric point of view.
I. INTRODUCTIONThe simple pendulum and the torque free rigid body are two well understood physical systems in both classical and quantum mechanics. The first systematic study of the pendulum is attributed to Galileo Galilei around 1602 and its dynamical description culminated with the development of the elliptic functions by Abel [1] and Jacobi [2,3], which turn out to be the analytical solutions to the equation of motion of the pendulum (for a review of elliptic functions see for instance [4][5][6][7][8][9] and [10][11][12] for the solutions of the pendulum). The quantization of the pendulum is based on the equivalence between the Schrödinger equation and the Mathieu differential equation, result developed originally by Condon in 1928 [13] and source of subsequent analysis of different aspects of the quantum system [14,15]. On the other hand in 1758 Euler showed that the equations of motion that describe the rotation of a rigid body form a vectorial quasilinear first-order ordinary differential equations set [16]. A geometric construction of the solution was given later on by Poinsot [17] and analytically these solutions are given, as for the simple pendulum, by elliptic functions (see for instance [18][19][20][21] and references therein). The quantization of the problem was attacked first by Kramers and Ittmann [22] and since then many authors have contributed to understand deeper many aspects of the problem [23][24][25][26][27][28][29][30].Despite the old age of these problems, from time to time there are some new physical aspects uncovered about these systems that contribute to our knowledge and understanding of physics in general. The list is long and here we point out just four examples: i) In 1973 Y. Nambu, taking the Liouville theorem as a guiding principle, ...