We prove finite-time singularity formation for De Gregorio’s model of the three-dimensional vorticity equation in the class of Lp∩Cα(R)\documentclass[12pt]{minimal}
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\begin{document}$$L^p\cap C^\alpha (\mathbb {R})$$\end{document} vorticities for some α>0\documentclass[12pt]{minimal}
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\begin{document}$$\alpha >0$$\end{document} and p<∞\documentclass[12pt]{minimal}
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\begin{document}$$p<\infty $$\end{document}. We also prove finite-time singularity formation from smooth initial data for the Okamoto–Sakajo–Wunsch models in a new range of parameter values. As a consequence, we have finite-time singularity for certain infinite-energy solutions of the surface quasi-geostrophic equation which are Cα\documentclass[12pt]{minimal}
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\begin{document}$$C^\alpha $$\end{document}-regular. One of the difficulties in the models we consider is that there are competing nonlocal stabilizing effects (advection) and destabilizing effects (vortex stretching) which are of the same size in terms of scaling. Hence, it is difficult to establish the domination of one effect over the other without having strong control of the solution. We conjecture that strong solutions to the De Gregorio model exhibit the following behavior: for each 0<α<1\documentclass[12pt]{minimal}
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\begin{document}$$0<\alpha <1$$\end{document} there exists an initial ω0∈Cα(R)\documentclass[12pt]{minimal}
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\begin{document}$$\omega _0\in C^\alpha (\mathbb {R})$$\end{document} which is compactly supported for which the solution becomes singular in finite-time; on the other hand, solutions to De Gregorio’s equation are global whenever ω0∈Lp∩C1(R)\documentclass[12pt]{minimal}
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\begin{document}$$\omega _0\in L^p\cap C^{1}(\mathbb {R})$$\end{document} for some p<∞\documentclass[12pt]{minimal}
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\begin{document}$$p<\infty $$\end{document}. Such a dichotomy seems to be a genuinely non-linear effect which cannot be explained merely by scaling considerations since Cα\documentclass[12pt]{minimal}
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\begin{document}$$C^\alpha $$\end{document} spaces are scaling subcritical for each α>0\documentclass[12pt]{minimal}
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\begin{document}$$\alpha >0$$\end{document}.
