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DTSTAMP:20221207T034428Z
UID:20221207T024428-01238384550@www.mn.uio.no
DTSTART:20221117T131500Z
DTEND:20221117T150000Z
LOCATION:NHA B1120
DESCRIPTION:Counterexamples to the integral Hodge conjecture can arise eit
her from torsion cohomology classes (as in Atiyah's and Hirzebruch's orig
inal counterexample from 1961) or from non-torsion classes (as first seen
in Kollár's counterexample from 1991). After Voisin proved the IHC for u
niruled threefolds, Schreieder found a unirational fourfold where the IHC
fails. His construction of a non-algebraic Hodge class relies on abstrac
t arguments with unramified cohomology. It was an open question whether t
his class is of torsion type. In this talk, I want to explain a new metho
d that gives an explicit geometric description of the unramified cohomolo
gy class appearing in his argument. In particular, this approach allows t
o prove that Schreieder's unirational counterexample is of torsion type.
SUMMARY:Matthias Paulsen (Hannover) - Unirational counterexamples to the I
HC of torsion type
URL:https://www.mn.uio.no/math/english/research/groups/algebra/events/semi
nars/2022/paulsen.html
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