Quote by jcsd
But for me at least the whole concept of inertial mass is directly rooted to F = ma, so I'm not sure it's particualrly good idea to call any quanitty in relativty the inertial mass (except the rest mass of an object in it's rest frmae which is unambigiously the inertial mass in that frame). Anyway I think the issue has been discussed enough

I don't know that Symbreak has discussed it enough. He/she may wish to know more.
F = ma is incorrect even in nonrelativistic mechanics. F = ma is a relationship for when m is not a constant of motion. The correct relationship is F = dp/dt (as Feynman pointed out in his text). F = dp/dt was due to Newton and F = ma was due to Euler.
Pete