The Frobenius problem over real number fields
Given a number field $K$ that is a subfield of the real numbers, we generalize the notion of the classical Frobenius problem to the ring of integers $\mathfrak{O}_K$ of $K$ by describing certain Frobenius semigroups, $\mathrm{Frob}(\alpha_1,\dots,\alpha_n)$, for appropriate elements $\alpha_1,\dots,\alpha_n\in\mathfrak{O}_K$. We construct a partial ordering on $\mathrm{Frob}(\alpha_1,\dots,\alpha_n)$, and show that this set is completely described by the maximal elements with respect to this ordering. We also show that $\mathrm{Frob}(\alpha_1,\dots,\alpha_n)$ will always have finitely many such maximal elements, but in general, the number of maximal elements can grow without bound as $n$ is fixed and $\alpha_1,\dots,\alpha_n\in\mathfrak{O}_K$ vary. Explicit examples of the Frobenius semigroups are also calculated for certain cases in real quadratic number fields.