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            | Abstract |  
            | Given a map of vector bundles on a smooth variety, consider the deepest
 degeneracy locus where its rank is smallest. We show it carries a natural perfect
 obstruction theory whose virtual cycle can be calculated by the Thom–Porteous
 formula.
     We show nested Hilbert schemes of points on surfaces can be expressed as
 degeneracy loci. We show how to modify the resulting obstruction theories to recover
 the virtual cycles of Vafa–Witten and reduced local DT theories. The result computes
 some Vafa–Witten invariants in terms of Carlsson–Okounkov operators. This proves
 and extends a conjecture of Gholampour, Sheshmani, and Yau and generalises a
 vanishing result of Carlsson and Okounkov.
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            | Keywords
                Hilbert scheme, degeneracy locus, Thom–Porteous formula,
                local Donaldson–Thomas theory, Vafa–Witten invariants
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            | Mathematical Subject Classification 2010
                Primary: 14D20, 14J60, 14N35
               
                Secondary: 14C05, 57R57
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            | Milestones
                Received: 11 February 2019
               
                Accepted: 20 June 2019
               
                Published: 9 October 2019
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