Definition 87.5.1. Let \varphi : A \to B be a continuous map of linearly topologized rings. We say \varphi is taut1 if for every open ideal I \subset A the closure of the ideal \varphi (I)B is open and these closures form a fundamental system of open ideals.
87.5 Taut ring maps
It turns out to be convenient to have a name for the following property of continuous maps between linearly topologized rings.
If \varphi : A \to B is a continuous map of linearly topologized rings and I_\lambda a fundamental system of open ideals of A, then \varphi is taut if and only if the closures of I_\lambda B are open and form a fundamental system of open ideals in A.
Lemma 87.5.2. Let \varphi : A \to B be a continuous map of weakly admissible topological rings. The following are equivalent
\varphi is taut,
for every weak ideal of definition I \subset A the closure of \varphi (I)B is a weak ideal of definition of B and these form a fundamental system of weak ideals of definition of B.
Proof. The remarks following Definition 87.5.1 show that (2) implies (1). Conversely, assume \varphi is taut. If I \subset A is a weak ideal of definition, then the closure of \varphi (I)B is open by definition of tautness and consists of topologically nilpotent elements by Lemma 87.4.10. Hence the closure of \varphi (I)B is a weak ideal of definition. Furthermore, by definition of tautness these ideals form a fundamental system of open ideals and we see that (2) is true. \square
Lemma 87.5.3. Let A be a linearly topologized ring. The map A \to A^\wedge from A to its completion is taut.
Proof. Let I_\lambda be a fundamental system of open ideals of A. Recall that A^\wedge = \mathop{\mathrm{lim}}\nolimits A/I_\lambda with the limit topology, which means that the kernels J_\lambda = \mathop{\mathrm{Ker}}(A^\wedge \to A/I_\lambda ) form a fundamental system of open ideals of A^\wedge . Since J_\lambda is the closure of I_\lambda A^\wedge (compare with Lemma 87.4.11) we conclude. \square
Lemma 87.5.4. Let A \to B and B \to C be continuous homomorphisms of linearly topologized rings. If A \to B and B \to C are taut, then A \to C is taut.
Proof. Omitted. Hint: if I \subset A is an ideal and J is the closure of IB, then the closure of JC is equal to the closure of IC. \square
Lemma 87.5.5. Let A \to B and B \to C be continuous homomorphisms of linearly topologized rings. If A \to C is taut, then B \to C is taut.
Proof. Let J \subset B be an open ideal with inverse image I \subset A. Then the closure of JC contains the closure of IC. Hence this closure is open as A \to C is taut. Let I_\lambda be a fundamental system of open ideals of A. Let K_\lambda be the closure of I_\lambda C. Since A \to C is taut, these form a fundamental system of open ideals of C. Denote J_\lambda \subset B the inverse image of K_\lambda . Then the closure of J_\lambda C is K_\lambda . Hence we see that the closures of the ideals JC, where J runs over the open ideals of B form a fundamental system of open ideals of C. \square
Lemma 87.5.6. Let A \to B and A \to C be continuous homomorphisms of linearly topologized rings. If A \to B is taut, then C \to B \widehat{\otimes }_ A C is taut.
Proof. Let K \subset C be an open ideal. Choose any open ideal I \subset A whose image in C is contained in J. By assumption the closure J of IB is open. Since A \to B is taut we see that B \widehat{\otimes }_ A C is the limit of the rings B/J \otimes _{A/I} C/K over all choices of K and I, i.e, the ideals J(B \widehat{\otimes }_ A C) + K(B \widehat{\otimes }_ A C) form a fundamental system of open ideals. Now, since B \to B \widehat{\otimes }_ A C is continuous we see that J maps into the closure of K(B \widehat{\otimes }_ A C) (as I maps into K). Hence this closure is equal to J(B \widehat{\otimes }_ A C) + K(B \widehat{\otimes }_ A C) and the proof is complete. \square
Lemma 87.5.7. Let \varphi : A \to B be a continuous homomorphism of linearly topologized rings. If \varphi is taut and A has a countable fundamental system of open ideals, then B has a countable fundamental system of open ideals.
Proof. Immediate from the definitions. \square
Lemma 87.5.8. Let \varphi : A \to B be a continuous homomorphism of linearly topologized rings. If \varphi is taut and A is weakly pre-admissible, then B is weakly pre-admissible.
Proof. Let I \subset A be a weak ideal of definition. Then the closure J of IB is open and consists of topologically nilpotent elements by Lemma 87.4.10. Hence J is a weak ideal of definition of B. \square
Lemma 87.5.9. Let \varphi : A \to B be a continuous homomorphism of linearly topologized rings. If \varphi is taut and A is pre-admissible, then B is pre-admissible.
Proof. Let I \subset A be an ideal of definition. Let I_\lambda \subset A be a fundamental system of open ideals. Then the closure J of IB is open and the closures J_\lambda of I_\lambda B are open and form a fundamental system of open ideals of B. For every \lambda there is an n such that I^ n \subset I_\lambda . Observe that J^ n is contained in the closure of I^ nB. Thus J^ n \subset J_\lambda and we conclude J is an ideal of definition. \square
Lemma 87.5.10. Let \varphi : A \to B be a continuous homomorphism of linearly topologized rings. Assume
\varphi is taut and has dense image,
A is complete and has a countable fundamental system of open ideals, and
B is separated.
Then \varphi is surjective and open, B is complete, and B = A/K for some closed ideal K \subset A.
Proof. By the open mapping lemma (More on Algebra, Lemma 15.36.5) combined with tautness of \varphi , we see the map \varphi is open. Since the image of \varphi is dense, we see that \varphi is surjective. The kernel K of \varphi is closed as \varphi is continuous. It follows that B = A/K is complete, see for example Lemma 87.4.4. \square
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