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
\bigoplus \nolimits _{j = 1, \ldots , c} K \longrightarrow \bigoplus \nolimits _{i = 1, \ldots , n} K\text{d}x_ i
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 M/L/K be field extensions. Then the Jacobi-Zariski sequence
0 \to H_1(L_{L/K}) \otimes _ L M \to H_1(L_{M/K}) \to H_1(L_{M/L}) \to \Omega _{L/K} \otimes _ L M \to \Omega _{M/K} \to \Omega _{M/L} \to 0
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
\xymatrix{ K \ar[r] & K' \\ k \ar[u] \ar[r] & k' \ar[u] }
with k'/k and K'/K finitely generated field extensions the kernel and cokernel of the maps
\alpha : \Omega _{K/k} \otimes _ K K' \to \Omega _{K'/k'} \quad \text{and}\quad \beta : H_1(L_{K/k}) \otimes _ K K' \to H_1(L_{K'/k'})
are finite dimensional and
\dim \mathop{\mathrm{Ker}}(\alpha ) - \dim \mathop{\mathrm{Coker}}(\alpha ) -\dim \mathop{\mathrm{Ker}}(\beta ) + \dim \mathop{\mathrm{Coker}}(\beta ) = \text{trdeg}_ k(k') - \text{trdeg}_ K(K')
Proof. The Jacobi-Zariski sequences for k \subset k' \subset K' and k \subset K \subset K' are
0 \to H_1(L_{k'/k}) \otimes K' \to H_1(L_{K'/k}) \to H_1(L_{K'/k'}) \to \Omega _{k'/k} \otimes K' \to \Omega _{K'/k} \to \Omega _{K'/k'} \to 0
and
0 \to H_1(L_{K/k}) \otimes K' \to H_1(L_{K'/k}) \to H_1(L_{K'/K}) \to \Omega _{K/k} \otimes K' \to \Omega _{K'/k} \to \Omega _{K'/K} \to 0
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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