Any lemma on commutative diagrams. It can give relations between maps (as in the five lemma) or tell how to construct new diagrams from old ones (as in the snake lemma).
Diagram Lemma
See also
Commutative Diagram, Diagram Chasing, Four Lemma, Five Lemma, Nine Lemma, Snake Lemma, Zig-Zag LemmaThis entry contributed by Margherita Barile
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References
Mac Lane, S. "Diagram Lemmas." Ch. 7.4 in Categories for the Working Mathematician. New York: Springer-Verlag, pp. 198-205, 1971.Referenced on Wolfram|Alpha
Diagram LemmaCite this as:
Barile, Margherita. "Diagram Lemma." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/DiagramLemma.html