Let $B$ be a commutative $A$-algebra, and let $M$, $N$ be two $B$-modules. We can talk about the set of $A$-linear module homomorphisms $M \to N$, i.e. the set $\text{Hom}\_A(M, N)$. Differential operators of order zero should be the $B$-linear maps from $M$ to $N$, i.e. $\text{Hom}\_B(M, N)$.


First, note that the commutator $[f, b]$ (where $b \in B$) is a well-defined morphism $M \to N$. Then we make our first definition, the "Weyl Algebra" one.



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> **Definition 1 (Weyl Algebra).** Let $\mathcal{D}\_A^0(M, N) = \text{Hom}\_B(M, N)$. Define $$\mathcal{D}\_A^n(M, N) = \{f \in \text{Hom}\_A(M, N) \text{ such that }[f, b] \in \mathcal{D}\_A^{n-1}(M,N)\}.$$We set $\mathcal{D}\_A(M, N) = \bigcup\_{n \ge 0}\mathcal{D}\_A^n(M, N)$.
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In order to formulate the crystalline definition, we introduce some notation. Let $D: M \to N$ be an $A$-linear map. Then, $D$ induces the map $\overline{D}: \delta\_{B/A} \otimes\_B M\to N$. We now have our "Crystalline" definition.



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> **Definition 2 (Crystalline).** Let $I$ be the kernel of the diagonal map (i.e., the map $B \otimes\_A B \to B, \ b \otimes b' \mapsto bb'$). Then $D: M \to N$ is said to be a differential operator of order $\le n$ if $\overline{D}$ annihilates $I^{n+1} \otimes\_B M$. Let $\mathcal{D}\_A^n(M, N)$ be the $B$-module of differential operators of order $\le n$. We define $\mathcal{D}\_A(M, N) = \bigcup\_{n \ge 0} \mathcal{D}\_A^n(M, N)$.
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My question is, what is the easiest way to see that/the intuition behind the definitions of rings of differential operators between modules given above are equivalent?




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**EDIT:** In the comments, Michael Bächtold is asking me to spell out the definition of $\delta\_{B/A}$ and $\overline{D}$.


So say we have $B$ a commutative $A$-algebra. We want to formalize the notion of an $A$-linear endomorphism of $B$ which is ``close" to being $B$-linear. Let $D: B \to B$ be an $A$-linear endomorphism of $B$. Using $D$, we obtain a map$$\tilde{D}: B \otimes\_A B \to B$$defined by $\tilde{D}: b \otimes b' \mapsto bD(b')$, which can also be viewed as a map$$\overline{D}: B \otimes\_A B \otimes\_B B \to B,$$where we have identified $B$ and $B \otimes\_B B$ and the map is defined by $\overline{D}: b \otimes b' \otimes b'' \mapsto bD(b'b'')$. Let us define $\delta\_{B/A} = B \otimes\_A B$. Then, we have a map:$$\overline{D}: \delta\_{B/A} \otimes\_B B \to B.$$



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> In order to formulate the crystalline definition, we introduce some notation. Let $D: M \to N$ be an $A$-linear map. Then, $D$ induces the map $\overline{D}: \delta\_{B/A} \otimes\_B M\to N$ defined by the same formula as above (that is, $\overline{D}: b \otimes b' \otimes b'' \mapsto bD(b'b'')$ for $b \in B$, $b' \in B$ and $b'' \in M$). We now have our "Crystalline" definition.
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In the quoted text, the inducing is in perfect analogy to what I wrote above.



Equivalence of "Weyl Algebra" and "Crystalline" definitions of rings of differential operators between modules?
It's straightforward to check that the definitions agree for order $0$ differential operators. 


To show that they agree for higher order DOs it suffices to show that $D$ is a crystalline DO of order $\leq n$ iff $[b,D]$ is a crystalline DO of order $\leq n-1$ for all $b\in B$. 


Keeping in mind that the kernel $I$ of $B\otimes\_A B \to B$ is [generated](https://mathoverflow.net/a/176653/745), as $B\otimes B$-module, by elements of the form $b\otimes 1 - 1 \otimes b$, it follows from the next computation, where $c\in I^n$ and $m\in M$ are arbitrary: 
$$ \overline{D}\left(( (b\otimes 1 - 1\otimes b)\cdot c) \otimes m\right )=\overline{D}((b\cdot c - c \cdot b)\otimes m) = b \overline{D}(c\otimes m) - \overline{D}(c\otimes bm)=\overline{[b, D]}(c\otimes m) .$$


Here $b\cdot c$ means multiplication on the first factor of $c\in B\otimes\_A B$ and similarly $c\cdot b$ is multiplication on the second factor. 


