A question on short exact sequences.

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The following is an excerpt from Atiyah-Macdonald on short exact sequences.

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I don't understand the part where the author says "Then $d(x'')$ is defined to be the image of $y'$ in Coker ($f'$)". Is $d$ a mapping from $N'$ to Coker ($f'$)?

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This is precisely the snake lemma http://en.wikipedia.org/wiki/Snake_lemma

the construction of the so called boundary operator $d$ is a little bit tidious but gets very clear after some time.

You have to do a little bit diagram chasing to see that it is defined and well-defined.

Have a look at this short video taken from the movie "It's my turn"

https://www.youtube.com/watch?v=etbcKWEKnvg

Sorry for only being informative and not constructive, but I think the best way for you is to get it straight yourself - maybe with different literature.