Homological algebra 5: Ext(A,B)

This lecture is part of an online course on commutative algebra, following the book
"Commutative algebra with a view toward algebraic geometry" by David Eisenbud.

We define the groups Ext(A,B) for modules over a ring by analogy with Tor groups. We calculate these for cyclic abelian groups and explain how they are related to extensions of modules.

Reading: Section
Exercises:

[adk]

At 2:06 should the statement be that the morphism isn't necessarily monic rather than not necessarily onto?