John Christian Ottem: The Integral Hodge conjecture for 3-folds

The Hodge conjecture predicts which rational cohomology classes on a smooth complex projective variety can be represented by linear combinations of complex subvarieties. The integral Hodge conjecture, the analogous conjecture for integral homology classes, is known to be false in general (the first counterexamples were given in dimension 7 by Atiyah and Hirzebruch). I'll give a short survey talk on this conjecture, and present some new counterexamples in dimension three.