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The Stacks project

Lemma 30.20.3. Let A be a Noetherian ring. Let I \subset A be an ideal. Let f : X \to \mathop{\mathrm{Spec}}(A) be a proper morphism. Let \mathcal{F} be a coherent sheaf on X. Then for every p \geq 0 there exists an integer c \geq 0 such that

  1. the multiplication map I^{n - c} \otimes H^ p(X, I^ c\mathcal{F}) \to H^ p(X, I^ n\mathcal{F}) is surjective for all n \geq c,

  2. the image of H^ p(X, I^{n + m}\mathcal{F}) \to H^ p(X, I^ n\mathcal{F}) is contained in the submodule I^{m - e} H^ p(X, I^ n\mathcal{F}) where e = \max (0, c - n) for n + m \geq c, n, m \geq 0,

  3. we have

    \mathop{\mathrm{Ker}}(H^ p(X, I^ n\mathcal{F}) \to H^ p(X, \mathcal{F})) = \mathop{\mathrm{Ker}}(H^ p(X, I^ n\mathcal{F}) \to H^ p(X, I^{n - c}\mathcal{F}))

    for n \geq c,

  4. there are maps I^ nH^ p(X, \mathcal{F}) \to H^ p(X, I^{n - c}\mathcal{F}) for n \geq c such that the compositions

    H^ p(X, I^ n\mathcal{F}) \to I^{n - c}H^ p(X, \mathcal{F}) \to H^ p(X, I^{n - 2c}\mathcal{F})

    and

    I^ nH^ p(X, \mathcal{F}) \to H^ p(X, I^{n - c}\mathcal{F}) \to I^{n - 2c}H^ p(X, \mathcal{F})

    for n \geq 2c are the canonical ones, and

  5. the inverse systems (H^ p(X, I^ n\mathcal{F})) and (I^ nH^ p(X, \mathcal{F})) are pro-isomorphic.

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

Maybe there is a typo in the proof of lemma 30.20.3: In paragraph four,the last but two lines,I think 'we see that B_n⋅y_i⊂Ker(M_{n+d_i}→M_n)' should be 'we see that B_n⋅y_i⊂Ker(M_{n+d'_i}→M_n)' .


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