Proof.
Write M_ n = H^ p(X, I^ n\mathcal{F}) for n \geq 1 and M_0 = H^ p(X, \mathcal{F}) so that we have maps \ldots \to M_3 \to M_2 \to M_1 \to M_0. Setting B = \bigoplus _{n \geq 0} I^ n, then M = \bigoplus _{n \geq 0} M_ n is a finite graded B-module, see Lemma 30.20.1. Observe that the products B_ n \otimes M_ m \to M_{m + n}, a \otimes m \mapsto a \cdot m are compatible with the maps in our inverse system in the sense that the diagrams
\xymatrix{ B_ n \otimes _ A M_ m \ar[r] \ar[d] & M_{n + m} \ar[d] \\ B_ n \otimes _ A M_{m'} \ar[r] & M_{n + m'} }
commute for n, m' \geq 0 and m \geq m'.
Proof of (1). Choose d_1, \ldots , d_ t \geq 0 and x_ i \in M_{d_ i} such that M is generated by x_1, \ldots , x_ t over B. For any c \geq \max \{ d_ i\} we conclude that B_{n - c} \cdot M_ c = M_ n for n \geq c and we conclude (1) is true.
Proof of (2). Let c be as in the proof of (1). Let n + m \geq c. We have M_{n + m} = B_{n + m - c} \cdot M_ c. If c > n then we use M_ c \to M_ n and the compatibility of products with transition maps pointed out above to conclude that the image of M_{n + m} \to M_ n is contained in I^{n + m - c}M_ n. If c \leq n, then we write M_{n + m} = B_ m \cdot B_{n - c} \cdot M_ c = B_ m \cdot M_ n to see that the image is contained in I^ m M_ n. This proves (2).
Let K_ n \subset M_ n be the kernel of the map M_ n \to M_0. The compatibility of products with transition maps pointed out above shows that K = \bigoplus K_ n \subset M is a graded B-submodule. As B is Noetherian and M is a finitely generated graded B-module, this shows that K is a finitely generated graded B-module. Choose d'_1, \ldots , d'_{t'} \geq 0 and y_ i \in K_{d'_ i} such that K is generated by y_1, \ldots , y_{t'} over B. Set c = \max (d'_ i, d'_ j). Since y_ i \in \mathop{\mathrm{Ker}}(M_{d'_ i} \to M_0) we see that B_ n \cdot y_ i \subset \mathop{\mathrm{Ker}}(M_{n + d'_ i} \to M_ n). In this way we see that K_ n = \mathop{\mathrm{Ker}}(M_ n \to M_{n - c}) for n \geq c. This proves (3).
Consider the following commutative solid diagram
\xymatrix{ I^ n \otimes _ A M_0 \ar[r] \ar[d] & I^ nM_0 \ar[r] \ar@{..>}[d] & M_0 \ar[d] \\ M_ n \ar[r] & M_{n - c} \ar[r] & M_0 }
Since the kernel of the surjective arrow I^ n \otimes _ A M_0 \to I^ nM_0 maps into K_ n by the above we obtain the dotted arrow and the composition I^ nM_0 \to M_{n - c} \to M_0 is the canonical map. Then clearly the composition I^ nM_0 \to M_{n - c} \to I^{n - 2c}M_0 is the canonical map for n \geq 2c. Consider the composition M_ n \to I^{n - c}M_0 \to M_{n - 2c}. The first map sends an element of the form a \cdot m with a \in I^{n - c} and m \in M_ c to a m' where m' is the image of m in M_0. Then the second map sends this to a \cdot m' in M_{n - 2c} and we see (4) is true.
Part (5) is an immediate consequence of (4) and the definition of morphisms of pro-objects.
\square
Comments (2)
Comment #7348 by Yijin Wang on
Comment #7568 by Stacks Project on