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I am looking for a characterization of the long exact sequences in the equivalence classe of the zero element (for the Baer sum) in $Ext^n(U,V)$ for $n>1$.

If $n=1$, then these are the split short exact sequences.

If $n>1$, then the trivial extension $$0 \to V \to V \to 0 \to ... \to 0 \to U \to U \to 0$$ is a representative of the zero class, but how to characterizes the others ?


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