Conjectured energy-critical Type II singularity formation

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Let X=({v≤0}∪{t≤−0.9})∩{−1≤u<0}\mathcal{X}=(\{v\leq 0\}\cup \{t\leq -0.9\})\cap\{-1\leq u<0\} and let C\mathcal{C} be the backward light cone. The symbols ϕ\phi, rr, ν\nu, dd, pp, and kk have the meanings in the corresponding wave-map or semilinear equation, and Cs−C^{s-} denotes regularity strictly below CsC^s; infinite conormal regularity is regularity to all orders with respect to the vector fields tangent to the relevant characteristic structure. Energy-critical Type II conjecture. The listed Type II solutions should exist in X\mathcal{X}: for any ν>1\nu>1, the three solutions for (d,k)=(2,1)(d,k)=(2,1) wave maps, (d,p)=(3,5)(d,p)=(3,5), and (d,p)=(4,3)(d,p)=(4,3) have respectively the stated bounds ϕ∣C=O(rν−1)\phi|_{\mathcal{C}}=\mathcal{O}(r^{\nu-1}), O(r(ν−2)/2)\mathcal{O}(r^{(\nu-2)/2}), and O(rν−2)\mathcal{O}(r^{\nu-2}), with the stated Cν−1/2−C^{\nu-1/2-} or Cν/2−C^{\nu/2-} regularity and infinite conormal regularity across C\mathcal{C}; for ν∈N≥2\nu\in\mathbb{N}_{\geq2} there is a smooth wave-map solution with (d,k)=(2,1)(d,k)=(2,1) and ϕ∣C=O(rν)\phi|_{\mathcal{C}}=\mathcal{O}(r^\nu); and there are smooth solutions with the three additional decay rates O(e−∣log⁡r∣)\mathcal{O}(e^{-\sqrt{|\log r|}}), O(∣log⁡r∣−1/(2k−2))\mathcal{O}(|\log r|^{-1/(2k-2)}) for d=2d=2, k≥2k\geq2, and O(r−1e−∣log⁡r∣+O(1))\mathcal{O}(r^{-1}e^{-\sqrt{|\log r|}+O(1)}) for (d,p)=(4,3)(d,p)=(4,3). In all cases the solutions are smooth for v<0v<0, have scale-critical norm bounded away from zero as τ→0\tau\to0, and the stated smoothness and conormality may be replaced by finite CkC^k versions. These conjectures organise known Type II constructions according to their exterior regularity and decay, while the cited results do not uniformly record the precise decay or high r∂x,r∂tr\partial_x,r\partial_t regularity across C\mathcal{C}; the claims therefore remain open in the supplied source context.

References

Primary source

Istvan Kadar and Lionor Kehrberger, “A note on exterior stability of isolated singularity formation for nonlinear wave equations”, arXiv:2602.03963 (2026).

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