Four Lemma
المؤلف:
Mac Lane, S.
المصدر:
Homology. Berlin: Springer-Verlag
الجزء والصفحة:
p. 364
10-5-2021
2056
Four Lemma
A diagram lemma which states that, given the above commutative diagram with exact rows, the following holds:
1. If 
is surjective, and 
and 
are injective, then 
is injective;
2. If 
is injective, and 
and 
are surjective, then 
is surjective.
This lemma is closely related to the five lemma, which is based on a similar diagram obtained by adding a single column.
REFERENCES:
Mac Lane, S. Homology. Berlin: Springer-Verlag, p. 364, 1967.
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