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I'm currently reading Sarah Witherspoon's book on Hochschild Cohomology. At the beginning of the fourth chapter it is given the following definition:

Definition 1. If $k$ is a field and $A$ is a $k$-algebra, we say that $A$ is smooth if its Hochschild dimension is finite and it has a finite projective resolution as an $A^e$-module by finitely generated projective modules.

However, in Weibel's book, the definition for smoothness is different (and it is given only for commutative $k$-algebras):

Definition 2. A commutative $k$-algebra $A$ is called smooth if for every square-zero extenstion $0\rightarrow M \rightarrow E \xrightarrow{\varepsilon} T \rightarrow 0$ of commuatative $k$-algebras and every algebra map $\nu:A\rightarrow T$, there is a $k$-algebra map $u:A\rightarrow E$ such that $\varepsilon u = \nu$.

As Witherspoon indicate in her book at page 51, these definitions should match for finitely generated commutative $k$-algebra $A$. Why is this so?

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    $\begingroup$ They do not seem to be equivalent in general. The second is called formally smooth which is satisfied by, say, a polynomial algebra on infinitely many variables. $\endgroup$
    – Z. M
    Commented May 4, 2022 at 18:45
  • $\begingroup$ What made me think that these definitions are equivalent is that Whitherspoon invoke in the proof of Theorem 3.2.6 (HKR Theorem) a result from Weibel's book on smooth algebras, although she gave a different definition for this notion. $\endgroup$ Commented May 4, 2022 at 19:07
  • $\begingroup$ Also, it seems to me that formally smooth means quasi-free in Weibel's book (page 313), not smooth $\endgroup$ Commented May 4, 2022 at 19:36
  • $\begingroup$ quasi-free is noncommutative. Every ring on SP (as in texts in algebraic geometry) is commutative by default. By the way, formal smoothness coincides with classical smoothness under finiteness. $\endgroup$
    – Z. M
    Commented May 5, 2022 at 7:27

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