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A new theory of higher gerbes Kottke, Chris


Complex line bundles are classified naturally up to isomorphism by degree two integer cohomology $H^2$, and it is of interest to find geometric objects which are similarly associated to higher degree cohomology. Gerbes (of which there are various versions, due respectively to Giraud, Brylinski, Hitchin and Chattergee, and Murray) provide a such theory associated to $H^3$. Various notions of"higher gerbes" have also been defined, though these tend to run into technicalities and complicted bookkeeping associated with higher categories. We propose a new geometric version of higher gerbes in the form of "multi simplicial line bundles", a pleasantly concrete theory which avoids many of the higher categorical difficulties, yet still captures key examples including the string (aka loop spin) obstruction associated to $\frac{1}{2}\ p_1$ in $H^4$. In fact, every integral cohomology class is represented by one of these objects in the guise of a line bundle on the iterated free loop space equipped with a "fusion product" (as defined by Stolz and Teichner and further developed by Waldorf) for each loop factor. This is joint work in progress with Richard Melrose.

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