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Let $X$ be a set, $(x_\alpha)_{\alpha \in A}$ be a net in $X$ and $(y_\lambda)_{\lambda \in \Lambda}$ be a subnet of $(x_\alpha)_{\alpha \in A}$. If $A$ is a complete lattice (i.e. not only a directed set), then is this true that $\Lambda$ is also a complete lattice ? If not, can we get sufficient conditions on the subnet $(y_\lambda)_{\lambda \in \Lambda}$ ?

Here is my attempt: Let $E \subset \Lambda$. Because $(y_\lambda)_{\lambda \in \Lambda}$ is a subnet of $(x_\alpha)_{\alpha \in A}$, there exists $f: \Lambda \to A$ such that:

  1. For every $\lambda \in \Lambda$, $y_\lambda = x_{f(\lambda)}$.
  2. For every $\lambda, \lambda' \in \Lambda$, $\lambda \leq_\lambda \lambda'$ implies $f(\lambda) \leq_A f(\lambda')$.
  3. For every $\alpha \in A$, there exists $\lambda \in \Lambda$ such that $\alpha \leq_A f(\lambda)$.

Consider $f(E) \subset A$. Since $A$ is a complete lattice, there exists $\bar \alpha \in A$ such that for every $\alpha \in f(E)$, $\alpha \leq_A \bar \alpha$. By the third property, consider $\bar \lambda \in \Lambda$ such that $\bar \alpha \leq_A f(\bar \lambda)$, ... (and here, I do not really know what to do; I have the feeling that it is not going to work).

Thank you for your help !

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  • $\begingroup$ I'm not sure I understood the concept, but it seems to me this is a very simple counter-example: consider $A=\wp(X)$, where $X$ is an infinite set, and $\Lambda=FC(X)$, the family of finite and cofinite subsets of $X$, and $f:FC(X)\to\wp(X)$ the identity function. It seems to satisfy your conditions but $FC(X)$ is not complete, while $\wp(X)$ is. $\endgroup$
    – amrsa
    Commented Jun 27 at 18:06
  • $\begingroup$ The assumptions guarantee only that $\Lambda$ is a cofinal directed subset of the complete lattice $A$. This is not enough to force $\Lambda$ to be lattice. For example, let $A = {\mathcal P}(\{0,1\})$ and let $\Lambda = {\mathcal P}(\{0,1\})\setminus \{\emptyset\}$. Let $f\colon \Lambda\to A$ be the inclusion function. All conditins are met, but $\Lambda$ is not lattice-ordered. $\endgroup$ Commented Jun 30 at 17:10
  • $\begingroup$ Thank to both of you for your answers ! $\endgroup$
    – Julien Pw
    Commented Jul 1 at 14:13

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