  We consider the problem of obtaining the scattering length for a fermion
colliding with a dimer, formed from a fermion identical to the incident one and
another different fermion. This is done in the universal regime where the range
of interactions is short enough so that the scattering length $a$ for non
identical fermions is the only relevant quantity. This is the generalization to
fermions with different masses of the problem solved long ago by Skorniakov and
Ter-Martirosian for particles with equal masses. We solve this problem
analytically in the two limiting cases where the mass of the solitary fermion
is very large or very small compared to the mass of the two other identical
fermions. This is done both for the value of the scattering length and for the
function entering the Skorniakov-Ter-Martirosian integral equation, for which
simple explicit expressions are obtained.
