Lemma 15.34.1 (Cartier equality). Let $K/k$ be a finitely generated field extension. Then $\Omega _{K/k}$ and $H_1(L_{K/k})$ are finite dimensional and $\text{trdeg}_ k(K) = \dim _ K \Omega _{K/k} - \dim _ K H_1(L_{K/k})$.

## 15.34 Cartier's equality and geometric regularity

A reference for this section and the next is [Section 39, MatCA]. In order to comfortably read this section the reader should be familiar with the naive cotangent complex and its properties, see Algebra, Section 10.134.

**Proof.**
We can find a global complete intersection $A = k[x_1, \ldots , x_ n]/(f_1, \ldots , f_ c)$ over $k$ such that $K$ is isomorphic to the fraction field of $A$, see Algebra, Lemma 10.158.11 and its proof. In this case we see that $\mathop{N\! L}\nolimits _{K/k}$ is homotopy equivalent to the complex

by Algebra, Lemmas 10.134.2 and 10.134.13. The transcendence degree of $K$ over $k$ is the dimension of $A$ (by Algebra, Lemma 10.116.1) which is $n - c$ and we win. $\square$

Lemma 15.34.2. Let $K \subset L \subset M$ be field extensions. Then the Jacobi-Zariski sequence

is exact.

**Proof.**
Combine Lemma 15.33.7 with Algebra, Lemma 10.158.11.
$\square$

Lemma 15.34.3. Given a commutative diagram of fields

with $k \subset k'$ and $K \subset K'$ finitely generated field extensions the kernel and cokernel of the maps

are finite dimensional and

**Proof.**
The Jacobi-Zariski sequences for $k \subset k' \subset K'$ and $k \subset K \subset K'$ are

and

By Lemma 15.34.1 the vector spaces $\Omega _{k'/k}$, $\Omega _{K'/K}$, $H_1(L_{K'/K})$, and $H_1(L_{k'/k})$ are finite dimensional and the alternating sum of their dimensions is $\text{trdeg}_ k(k') - \text{trdeg}_ K(K')$. The lemma follows. $\square$

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