“…[1,Corollary 11.24,Lemma 12.8]); -cases where k = 2 and γ 1 , γ 2 are diagonal are found in the thesis of Michel and his subsequent papers (e.g. [2]); -for k = 2, γ 1 = 1 and h = 0, we obtain the general 'correlation sums' (for the Fourier transform of K) defined in [3]; these are crucial to our works [3][4][5]; -special cases of this situation of correlation sums can be found (sometimes implicitly) in earlier works of Iwaniec [6], Pitt [7] and Munshi [8]; -the case k = 2, γ 1 and γ 2 diagonal, h arbitrary and K a Kloosterman sum in two variables (or a variant with K a Kloosterman sum in one variable and γ 1 , γ 2 not upper triangular) occurs in the work of Friedlander & Iwaniec [9], and it is also used in the work of Zhang [10] on gaps between primes; -cases where k is arbitrary, the γ i are upper triangular and distinct, and h may be non-zero appear in the work of Fouvry et al [11,Lemma 2.1], indeed in a form involving different trace functions K i (γ i • x) related to symmetric powers of Kloosterman sums; -the sums for k arbitrary and h = 0, with K a hyper-Kloosterman sum appear in the works of Fouvry et al [12] and Kowalski & Ricotta [13] (with γ i diagonal); -this last case, but with arbitrary h and the γ i being translations also appears in the work of Irving [14], and (for very different reasons) in work of Kowalski & Sawin [15]; and -another instance, with k = 4, h arbitrary and γ i upper triangular, occurs in the work of Blomer & Milićević [16,§11].…”