  In this paper, we report the multiwavelength observations of a very long
filament in active region (AR) 11112 on 2010 October 18. The filament was
composed of two parts, the eastern part (EP) and western part (WP). We focus on
longitudinal oscillations of the EP, which were enhanced by two homologous
C-class flares in the same AR. The C1.3 flare was confined without a CME. Both
EP and WP of the filament were slightly disturbed and survived the flare. After
5 hrs, eruption of the WP generated a C2.6 flare and a narrow, jet-like CME.
Three oscillating threads (thd$_a$, thd$_b$, thd$_c$) are obviously identified
in the EP and their oscillations are naturally divided into three phases by the
two flares. The initial amplitude ranges from 1.6 to 30 Mm with a mean value of
$\sim$14 Mm. The period ranges from 34 to 73 minutes with a mean value of
$\sim$53 minutes. The curvature radii of the magnetic dips are estimated to be
29 to 133 Mm with a mean value of $\sim$74 Mm. The damping times ranges from
$\sim$62 to $\sim$96 minutes with a mean value of $\sim$82 minutes. The value
of $\tau/P$ is between 1.2 and 1.8. For thd$_a$ in the EP, the amplitudes were
enhanced by the two flares from 6.1 Mm to 6.8 Mm after the C1.3 flare and
further to 21.4 Mm after the C2.6 flare. The period variation as a result of
perturbation from the flares was within 20\%. The attenuation became faster
after the C2.6 flare. To the best of our knowledge, this is the first report of
large-amplitude, longitudinal filament oscillations enhanced by flares.
Numerical simulations reproduce the oscillations of thd$_a$ very well. The
simulated amplitudes and periods are close to the observed values, while the
damping time in the last phase is longer, implying additional mechanisms should
be taken into account apart from radiative loss.
