  Within the framework of pure BCS(i.e. when the critical temperature is
proportional to the Debye temperature), we show that the isotope coefficient is
always less than 1/2 and could be extremely small in polyatomic superconductors
depending on the chemical formula unit. This finding leads to quantitative
explanation of observed values(correct orders of magnitude and sometimes exact
numerical values) in optimally doped cuprates, magnesium diboride and
alkali-doped fullerenes. Consequently, the smallness of the isotope coefficient
is not only compatible with pure electron-phonon interaction, but this is
perhaps the rule in polyatomic systems.
