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Is $\Bbb R_{+}$ under multiplication cyclic? Would it suffice to show that this group is isomorphic to a cyclic group? For example, $\Bbb Z_+$ is cyclic under addition, $\Bbb R_{+}$ is isomorphic to it, then $\Bbb R_{+}$ is cyclic too?

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No, it is not cyclic; this would imply the reals were countable. If you haven't seen cardinality, you can argue explicitly as follows: if $x$ were a generator, then we can assume $x > 1$ (else take $1/x$ for the generator). But nothing between $x$ and $x^2$ can be in the group generated by $x$.