We use set-theoretic tools to make a model-theoretic contribution. In particular, we construct a singleLω1,ω\documentclass[12pt]{minimal}
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\begin{document}$${\mathcal {L}}_{\omega _1,\omega }$$\end{document}-sentence ψ\documentclass[12pt]{minimal}
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\begin{document}$$\psi $$\end{document} that codes Kurepa trees to prove the following statements: The spectrum of ψ\documentclass[12pt]{minimal}
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\begin{document}$$\psi $$\end{document} is consistently equal to [ℵ0,ℵω1]\documentclass[12pt]{minimal}
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\begin{document}$$[\aleph _0,\aleph _{\omega _1}]$$\end{document} and also consistently equal to [ℵ0,2ℵ1)\documentclass[12pt]{minimal}
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\begin{document}$$[\aleph _0,2^{\aleph _1})$$\end{document}, where 2ℵ1\documentclass[12pt]{minimal}
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\begin{document}$$2^{\aleph _1}$$\end{document} is weakly inaccessible.The amalgamation spectrum of ψ\documentclass[12pt]{minimal}
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\begin{document}$$\psi $$\end{document} is consistently equal to [ℵ1,ℵω1]\documentclass[12pt]{minimal}
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\begin{document}$$[\aleph _1,\aleph _{\omega _1}]$$\end{document} and [ℵ1,2ℵ1)\documentclass[12pt]{minimal}
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\begin{document}$$[\aleph _1,2^{\aleph _1})$$\end{document}, where again 2ℵ1\documentclass[12pt]{minimal}
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\begin{document}$$2^{\aleph _1}$$\end{document} is weakly inaccessible. This is the first example of an Lω1,ω\documentclass[12pt]{minimal}
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\begin{document}$${\mathcal {L}}_{\omega _1,\omega }$$\end{document}-sentence whose spectrum and amalgamation spectrum are consistently both right-open and right-closed. It also provides a positive answer to a question in Souldatos (Notre Dame J Form Log 55(4):533–551, 2014).Consistently, ψ\documentclass[12pt]{minimal}
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\begin{document}$$\psi $$\end{document} has maximal models in finite, countable, and uncountable many cardinalities. This complements the examples given in Baldwin et al. (Arch Math Log 55(3):545–565, 2016) and Baldwin and Souldatos (Math Log Q 65(4):444–452, 2019) of sentences with maximal models in countably many cardinalities.Consistently, 2ℵ0<ℵω1<2ℵ1\documentclass[12pt]{minimal}
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\begin{document}$$2^{\aleph _0}<\aleph _{\omega _1}<2^{\aleph _1}$$\end{document} and there exists an Lω1,ω\documentclass[12pt]{minimal}
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\begin{document}$${\mathcal {L}}_{\omega _1,\omega }$$\end{document}-sentence with models in ℵω1\documentclass[12pt]{minimal}
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\begin{document}$$\aleph _{\omega _1}$$\end{document}, but no models in 2ℵ1\documentclass[12pt]{minimal}
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\begin{document}$$2^{\aleph _1}$$\end{document}. This relates to a conjecture by Shelah that if ℵω1<2ℵ0\documentclass[12pt]{minimal}
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\begin{document}$$\aleph _{\omega _1}<2^{\aleph _0}$$\end{document}, then any Lω1,ω\documentclass[12pt]{minimal}
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\begin{document}$${\mathcal {L}}_{\omega _1,\omega }$$\end{document}-sentence with a model of size ℵω1\documentclass[12pt]{minimal}
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\begin{document}$$\aleph _{\omega _1}$$\end{document} also has a model of size 2ℵ0\documentclass[12pt]{minimal}
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\begin{document}$$2^{\aleph _0}$$\end{document}. Our result proves that 2ℵ0\documentclass[12pt]{minimal}
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\begin{document}$$2^{\aleph _0}$$\end{document} can not be replaced by 2ℵ1\documentclass[12pt]{minimal}
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\begin{document}$$2^{\aleph _1}$$\end{document}, even if 2ℵ0<ℵω1\documentclass[12pt]{minimal}
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\begin{document}$$2^{\aleph _0}<\aleph _{\omega _1}$$\end{document}.
