A NOTE ON HOMOLOGY IN CATEGORIES
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exact category, although this was not the object (nor the result) of that paper.' Yoneda showed (using the existence of projectives) that Extn(A, C) is in 1-1 correspondence with the set of "n-fold extensions" of A by C:
[1] A. Heller. Homological Algebra in Abellian Categories , 1958 .
[2] David A. Buchsbaum,et al. Exact categories and duality , 1955 .