  Largest Lyapunov exponent has been examined in the dynamical-mode locking
phenomena of the ac+dc driven dissipative Frenkel-Kontorova model with
deformable substrate potential. Due to deformation, large fractional and higher
order subharmonic steps appear in the response function of the system.
Computation of the largest Lyapunov exponent as a way to verify their presence
led to the observation of the Farey sequence. In the standard regime, between
the large harmonic steps, the appearance of halfinteger and subharmonic steps,
and their relative sizes follow the Farey construction. In the nonstandard
regime, though halfinteger steps are larger than harmonic ones, Farey
construction is still present in the appearance of higher order subharmonic
steps. The examination of Lyapunov exponents also shows that there is no chaos
in the system.
