Historical question about the dimension subgroup conjecture Let $G$ be a group and let $I$ be the augmentation ideal in the group ring $\mathbb{Z}[G]$.  The k-th dimension subgroup of $G$ is
$$D_k(G) = \{\text{$g \in G$ $|$ $g-1 \in I^k$}\}.$$
It is not hard to see that this is a central series, so if $\gamma_k(G)$ denotes the k-th term of the lower central series of $G$ then $\gamma_k(G) \subset D_k(G)$.
The dimension subgroup conjecture asserts that these are always equal.  Unfortunately, it is false (a counterexample was found by Rips).  However, it is often true.  In particular, it is known that if the lower central series quotients $\gamma_k(G)/\gamma_{k+1}(G)$ are all torsion-free, then $\gamma_k(G) = D_k(G)$ for all $k$.
I first learned about this from Quillen's paper
Quillen, Daniel G.,
On the associated graded ring of a group ring.,
J. Algebra 10 (1968), 411-418.
where it is Corollary 4.2.  However, there is a vast literature on the dimension subgroup problem that never mentions Quillen's paper at all (despite it being heavily cited elsewhere!).
In this literature, this result seems to be attributed to Hall and Jennings.  This is done for instance in
Hurley, Ted,
Dimension and Fox subgroups,
Around group rings (Jasper, AB, 2001).
Resenhas 5 (2002), no. 4, 293–304.
which can be downloaded here.  However, I have been unable to located any paper of Hall or Jennings that proves it.
What's the story here?  Whom should I attribute this result to?
 A: I'm going to write an answer based on information that I figured out while consulting the references that Benjamin Steinberg suggested in the comments.  It will involve a certain amount of speculation, so I invite clarifications from anyone who actually knows the historical record.
In his paper
Jennings, S. A.,
The group ring of a class of infinite nilpotent groups,
Canadian J. Math. 7 (1955), 169-187.
Jennings claims the result in question at the end of Part I.  He says that the proof will appear in a forthcoming paper.  I gather that this paper never appeared.  However, Philip Hall lectured about it at the Summer Seminar of the Canadian Mathematical Congress held at the University of Alberta in 1957.  These lectures were later published in 1969 as part of the Queen Mary College Mathematics Notes.  The reference is here:
Hall, Philip, The Edmonton notes on nilpotent groups,
Queen Mary College Mathematics Notes. Mathematics Department, Queen Mary College, London 1969 iii+76 pp.
The theorem in question is Theorem 7.1, which Hall proves and attributes to Jennings.
So what's the deal with Quillen's paper?  I'm going to have to speculate here.  Quillen's paper appeared in 1968, which is between Jennings's paper and the formal publication of Hall's lecture notes.  My guess is that he wasn't aware of Jennings's claim since it is buried 2/3 of the way through a paper on something else and not proved there.  Hall's notes were apparently circulated privately among people working in group theory long before they were published, but since Quillen was a topologist it stands to reason that he would not know about them.
Quillen's proof is quite different from the ones given by the group-theorists, and depends in part on Milnor-Moore's work on Hopf algebras.  So this is really just a matter of the imperfect circulation of informal word-of-mouth mathematical knowledge, especially before the internet.
As for why Quillen's paper never seems to be mentioned by group theorists working on dimension subgroups, they certainly would not be obligated to cite it since he was not the first to prove the result.  Also, I suspect that the methods/language used were not very natural to hard-core group theorists, so I guess it didn't influence them (though Quillen's paper has 77 citations in mathscinet, so certainly other people found it useful!).
