The negative-curve characterization of instability for line bundles

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Let XX be a smooth complex projective surface, let B,ω\biginNS⁡(X)⊗RB,\omega\bigin\operatorname{NS}(X)\otimes\mathbb{R} with ω\omega ample, and let σB,ω=(ZB,ω,AB,ω)\sigma_{B,\omega}=(Z_{B,\omega},\mathscr{A}_{B,\omega}) be the Bridgeland stability condition defined by the tilted heart and central charge above. For a line bundle LL on XX, write L(C)∣CL(C)|_C for the restriction to CC of L(C)L(C). Negative-curve instability conjecture. (a) If L∈AB,ωL\in\mathscr{A}_{B,\omega}, then LL is not σB,ω\sigma_{B,\omega}-stable if and only if there exists a curve CC of negative self-intersection such that L(−C)L(-C) is a subobject of LL in AB,ω\mathscr{A}_{B,\omega} and

0<Arg⁡ZB,ω(L)≤Arg⁡ZB,ω(L(−C))≤π.0<\operatorname{Arg} Z_{B,\omega}(L)\leq\operatorname{Arg} Z_{B,\omega}(L(-C))\leq\pi.

(b) If L[1]∈AB,ωL[1]\in\mathscr{A}_{B,\omega}, then LL is not σB,ω\sigma_{B,\omega}-stable if and only if there exists a curve CC of negative self-intersection such that L(C)∣CL(C)|_C is a subobject of L[1]L[1] in AB,ω\mathscr{A}_{B,\omega} and

0<Arg⁡ZB,ω(L[1])≤Arg⁡ZB,ω(L(C)∣C)≤π.0<\operatorname{Arg} Z_{B,\omega}(L[1])\leq\operatorname{Arg} Z_{B,\omega}(L(C)|_C)\leq\pi.

This conjecture aims to characterize precisely the mechanisms by which a line bundle can fail to be Bridgeland stable: instability should arise exactly from subobjects associated with curves of negative self-intersection. Its status is unresolved in the supplied source.

References

Primary source

Yu-Wei Fan, “Notes on the deformed Hermitian-Yang-Mills equations and the large scaling limits of stability conditions”, arXiv:2604.22246 (2026).

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