Remark 2. Note that the coeffients a, b and c as in Eqs. (22), (28) and (29) assume such particular forms
to assure an energy integral for Eq. (1). These particular forms are due to our choice of φ(ξ) = 12 ω2ξ2. This
is a result of the theorem. Also as a result of the theorem, one has that: if the coeffients are particularly
stated by Eqs. (22), (28) and (29), then x = x(t) as in Eq. (26) is an exact solution of Eq. (1).