Abstract:
We prove a homological result in additive exact categories, which we call the Square-Cross Lemma. Its
applications include a property related to the Green formula from the theory of Ringel-Hall algebras as well
as the Two-Square Lemma of Fay, Hardie and Hilton.
Key Words: Additive category, exact category, Two-Square Lemma.
2000 Mathematics Subject Classification: Primary: 18E10,
Secondary: 18E05, 18G50.
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