An infinite filiform Lie algebra L is residually nilpotent and its graded associated with respect to the lower central series has smallest possible dimension in each degree but is still infinite. This means that gr(L) is of dimension two in degree one and of dimension one in all higher degrees. We prove that if L is solvable, then already [L,L] is abelian. The isomorphism classes in this case are given in a paper by Bratzlavsky, but the proof there is incomplete. We make the necessary additional computations and restate Bratzlavskys result when the ground field is the complex numbers.

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