$U$-Monad Topologies of Hyperfinite Time Lines
Jin, Renling
J. Symbolic Logic, Tome 57 (1992) no. 1, p. 534-539 / Harvested from Project Euclid
In an $\omega_1$-saturated nonstandard universe a cut is an initial segment of the hyperintegers which is closed under addition. Keisler and Leth in [KL] introduced, for each given cut $U$, a corresponding $U$-topology on the hyperintegers by letting $O$ be $U$-open if for any $x \in O$ there is a $y$ greater than all the elements in $U$ such that the interval $\lbrack x - y, x + y\rbrack \subseteq O$. Let $U$ be a cut in a hyperfinite time line $\mathscr{H}$, which is a hyperfinite initial segment of the hyperintegers. The $U$-monad topology of $\mathscr{H}$ is the quotient topology of the $U$-topological space $\mathscr{H}$ modulo $U$. In this paper we answer a question of Keisler and Leth about the $U$-monad topologies by showing that when $\mathscr{H}$ is $\kappa$-saturated and has cardinality $\kappa$, (1) if the coinitiality of $U_1$ is uncountable, then the $U_1$-monad topology and the $U_2$-monad topology are homeomorphic iff both $U_1$ and $U_2$ have the same coinitiality; and (2) $\mathscr{H}$ can produce exactly three different $U$-monad topologies (up to homeomorphism) for those $U$'s with countable coinitiality. As a corollary $\mathscr{H}$ can produce exactly four different $U$-monad topologies if the cardinality of $\mathscr{H}$ is $\omega_1$.
Publié le : 1992-06-14
Classification: 
@article{1183743973,
     author = {Jin, Renling},
     title = {$U$-Monad Topologies of Hyperfinite Time Lines},
     journal = {J. Symbolic Logic},
     volume = {57},
     number = {1},
     year = {1992},
     pages = { 534-539},
     language = {en},
     url = {http://dml.mathdoc.fr/item/1183743973}
}
Jin, Renling. $U$-Monad Topologies of Hyperfinite Time Lines. J. Symbolic Logic, Tome 57 (1992) no. 1, pp.  534-539. http://gdmltest.u-ga.fr/item/1183743973/