  We present a systematic analysis of the energy gap in underdoped Bi2212
superconductor as a function of temperature and hole doping level. Within the
framework of the theoretical model containing the electron-phonon and
electron-electron-phonon pairing mechanism, we reproduced the measurement
results of modern ARPES experiments with very high accuracy. We showed that the
energy-gap amplitude is very weakly dependent on the temperature but clearly
dependent on the level of doping. The evidence for a non-zero energy gap above
the critical temperature, referred to as a pseudogap, was also obtained.
