DEGREE THEORY DIFFERENTIAL EQUATIONS ECUACIONES DIFERENCIALES KNOT THEORY MORSE THEORY SARD-SMALE THEOREM TEOREMA DE SARD-SMALE TEORIA DE GRADO TEORIA DE MORSE TEORIA DE NUDOS Averaging method 51M16 Central force Contractible domains Forced 2-body problem Forced oscillation Hartman-Nagumo condition Multiplicity Periodic solutions Primary 46E35 Second fundamental form Second order systems Secondary 46E39 Topological degree Winding number