5 typos

A lot of people will tell you that flatness means "continuously varying fibres" in some sense, and that flatness was invented to have correspondingly nice consequences, which is true. But there is a way to expect this (vague) interpretation a priori from an alternative, equivalent definition:

An $A$-module $M$ is flat $\iff$ $I \otimes_A M \to IM$ is an isomorphism for every ideal $I$.

I would prefer to present this as the definition of flatness, and present the fact that tensoring with $M$ preserves exact sequences as a theorem. Why?

Thinking "geometrically", $I$ just corresponds (uniquely) to a closed subscheme $Z=Z(I)\subseteq X=Spec(A)$Z=Z(I)==Spec(A/I)\subseteq Spec(A)$. If we think of$M$in the usual geometric way as a module of generalized functions on$X$(like sections of a bundle), and$M/I M/IM \simeq M\otimes_A A/I$as its restriction to$Z$, then the above definition of flatness can be interepreted directly to mean that$M$restricts nicely to closed subschemes$Z$. More precisely, it says that what we lose in this restriction, the submodule$IM$of elements which "vanish on$Z$", is easy to understand: it's just formal linear combinations of elements$i\otimes m$, with no surprise relations among them, i.e. the tensor product$I \otimes_A M$. In topology, continuous functions "restrict nicely" to points and closed sets (by taking limits), so you can see, without much experience at all, how this definition corresponds in an intuitive way to continuity. Having this motivation in place, the best thing to do is to check out examples along the lines of Dan Erman's answer to see the analogy with continuity and limits at work. 4 typos A lot of people will tell you that flatness means "continuously varying fibres" in some sense, and that flatness was invented to have correspondingly nice consequences, which is true. But there is a way to expect this (vague) interpretation a priori from an alternative, equivalent definition: An$A$-module$M$is flat$\iffI \otimes_A M \to IM$is an isomorphism for every ideal$I$. I would prefer to present this as the definition of flatness, and present the fact that tensoring with$M$preserves exact sequences as a theorem. Why? Thinking "geometrically",$I$just corresponds (uniquely) to a closed subscheme$Z=Z(I)\subseteq X=Spec(A)$. If we think of$M$in the usual geometric way as a module of generalized functions on$X$(like sections of a bundle), and$M/I \simeq M\otimes_A A/I$as its restriction to$V$, Z$, then the above definition of flatness can be interepreted directly to mean that $M$ restricts nicely to closed subschemes $Z$.

More precisely, it says that what we lose in this restriction, the submodule $IM$ of elements which "vanish on $Z$", is easy to understand: it's just formal linear combinations of elements $i\otimes m$, with no surprise relations among them, i.e. the tensor product $I \otimes_A M$.

In topology, continuous functions "restrict nicely" to points and closed sets (by taking limits), so you can see, without much experience at all, how this definition corresponds in an intuitive way to continuity.

Having this motivation in place, the best thing to do is to check out examples along the lines of Dan Erman's answer to see the analogy with continuity and limits at work.

3 rephrase

A lot of people will tell you that flatness means "continuously varying fibres" in some sense, and that flatness was invented to have correspondingly nice consequences, which is true. But there is a way to expect this (vague) interpretation a priori from an alternative, equivalent definition:

An $A$-module $M$ is flat $\iff$ $I \otimes_A M \to IM$ is an isomorphism for every ideal $I$.

I would prefer to present this as the definition of flatness, and present the fact that tensoring with $M$ preserves exact sequences as a theorem. Why?

Thinking "geometrically", $I$ just corresponds (uniquely) to a closed subscheme
$Z=Z(I)\subseteq X=Spec(A)$. If we think of $M$ in the usual geometric way as a module of generalized functions on $X$ (like sections of a bundle), and $M/I \simeq M\otimes_A A/I$ as its restriction to $V$, then the above definition of flatness can be interepreted directly to mean that $M$ restricts nicely to closed subschemes $Z$.

More precisely, it says that what we lose in this restriction, the submodule $IM$ of elements which "vanish on $Z$", is easy to understand: it's just formal linear combinations of elements $i\otimes m$, with no surprise relations among them, i.e. the tensor product $I \otimes_A M$.

In topology, continuous functions "restrict nicely" to points and closed sets (by taking limits), so you can see, without much experience at all, how this definition corresponds in an intuitive way to continuity.

Of course

Having this motivation in place, the best thing to fully appreciate flatness, experience and lots of do is to check out examples are indispensible, but I think when possible, motivation depending on minimal experience can be just what along the doctor ordered lines of Dan Erman's answer to get one startedsee the analogy with continuity and limits at work.

2 typos; deleted 3 characters in body
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