Eq. (27) has been obtained by using a trigonometric identity: sinðw1 þ w2Þ ¼ sinðw1Þ cosðw2Þ þ cosðw1Þ sinðw2Þ
where w1 ¼ X0s and w2 ¼ X0s02. We left only the term cosðw2Þ (Eq. (28)) because of the odd parity of the velocity
function vðs s01Þ under the integral. Of course, such a simplification requires the parity of v* to be defined
(Eq. (20)) and a proper choice of the constant s01 (Eq. (19)).
Note, the above integrals may be, in principle, evaluated analytically [32] but here, because of their complicated
form is difficult to handle, we decided to combine analytic expressions of homoclinic orbits (Eq.
(17)) and numerical calculations of the Melnikov integrals (Eqs. (26) and (27)) using a procedure described
recently by Litak and Borowiec in Ref. [29]. Fig. 7 shows Ac as a function of X0 for a ffi 0:04 and
b ffi 2:69 (see Eqs. (6) and (7)) given by the curve labeled ‘1’ and b ffi 2:69=2 given by the curve labeled ‘2’.
One can see the characteristic double sack-like shape, similar to the structure observed by Lenci and Rega
[26]. This structure is governed by the oscillating term sinðX0sÞ in the denominator of the integral I2ðX0Þ
(Eq. (25)).