Baer's criterion, also known as Baer's test, states that a module 
over a unit
ring 
is injective iff every module
homomorphism from an ideal of 
to 
can be extended to a homomorphism
from 
to 
.
Baer's Criterion
See also
Commutative Diagram, Injective ModuleThis entry contributed by Margherita Barile
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References
Baer, R. "Abelian Groups that Are Direct Summands of Every Containing Abelian Group." Bull. Amer. Math. Soc. 46, 800-806, 1940.Faith, C. Algebra: Rings, Modules and Categories, I. Berlin, p. 157, 1973.Lam, T. Y. Lectures on Modules and Rings. New York: Springer-Verlag, p. 63, 1999.Referenced on Wolfram|Alpha
Baer's CriterionCite this as:
Barile, Margherita. "Baer's Criterion." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/BaersCriterion.html