Alternatively, we could introduce different kinds of effective masses and , so that the momentum and the velocity of one fluid are aligned in the momentum rest frame of the other fluid,all greater or all smaller than the nucleon mass . From a stability analysis, Carter, Chamel and Haensel  proved that these effective masses obey the following inequalities () [90, 91], using for the first time the band theory of solids (see Section 3.2.4), show that these effective masses can be much larger than the bare nucleon mass . This is in sharp contrast to the situation encountered in the liquid core, where effective masses are slightly smaller than (see, for instance, Chamel & Haensel  and references therein). Note, however, that the relativistic effective neutron mass can be slightly larger than the bare mass in the liquid core, as recently shown by Chamel .
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