Lagrangians for dissipative nonlinear oscillators: the method of Jacobi   Last Multiplier
  We present a method devised by Jacobi to derive Lagrangians of any second-order differential equation: it consists in finding a Jacobi Last Multiplier. We illustrate the easiness and the power of Jacobi's method by applying it to several equations and also a class of equations studied by Musielak with his own method [Musielak ZE, Standard and non-standard Lagrangians for dissipative dynamical systems with variable coefficients. J. Phys. A: Math. Theor. 41 (2008) 055205 (17pp)], and in particular to a Li\`enard type nonlinear oscillator, and a second-order Riccati equation. 
M.C. Nucci and K.M. Tamizhmani