The last step of the proof in [2] was omitted. To complete the argument, we proceed in the following way. We had shown that H = H(S) = H(S+S) = H(S-S), that |S-S| = 2|S| - |H| and hence that in the factor group G* = G/H of order 3m, the maximal sum-free set S* = S/H and its set of differences S* - S* are aperiodic, with

so that

By (1) and Theorem 2.1 of [1], S* - S* is either quasiperiodic or in arithmetic progression.

In my paper [1], part (b) of Lemma 3.4 and the remark following it are incorrect and should be omitted.

The isometry P in page 215, line 2, need not be a generator of T(E, q) as asserted, but can be shown to lie in T(E, q).

I am indebted to Professor Tits for pointing out that extended ideas of quadratic forms have already appeared in his paper [2] and in Wall's paper [3]. Professor Tits also discusses the corresponding groups and Clifford algebras in detail.

“Aspects of Topoi” [1] appeared in July 1972 through the Herculean efforts of Bernard Neumann with the able assistance of Tim Brook and Max Kelly. The original manuscript was written in haste during the last two weeks of the author's visit to Australia. Somehow just five months later it appeared in print. Among the difficulties surmounted was a set of page proofs which having been sent via slow steamer by the U. of Penn. administration arrived after the paper was in press.