## See also

Commutative Diagram,

Diagram Lemma,

Exact Sequence,

Five
Lemma
*This entry contributed by Margherita
Barile*

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## References

Mac Lane, S. *Homology.* Berlin: Springer-Verlag, p. 364, 1967.## Referenced on Wolfram|Alpha

Four Lemma
## Cite this as:

Barile, Margherita. "Four Lemma." From *MathWorld*--A Wolfram Web Resource, created by Eric
W. Weisstein. https://mathworld.wolfram.com/FourLemma.html

## Subject classifications