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starting an entry on the integer Heisenberg group.
For the moment it remains telegraphic as far as the text is concerned (no Idea-section)
but it contains a slick (I find) computation of the modular transformation of Chern-Simons/WZW states from the manifest modular automorphy of certain integer Heisenberg groups.
Hope to beautify this entry a little more tomorrow (but won’t have much time, being on an intercontinental flight) or else the days after (where I am however at a conference, but we’ll see).
I don’t have time but there might be connections to Ganter’s categorical tori, in how she contracts the crossed module from a lattice that is the kernel of exp, and a central extension.
Not sure what specifically you are thinking of?
Maybe I can amplify that the mere existence of the integer/discrete Heisenberg group is immediate and classical. The point that I meant to record here is that its evident action by the modular group (which is still obvious) induces a modular action on its irrep which is the famous modular functor of abelian Chern-Simons theory that is no longer that obvious and traditionally derived only over pages of analysis of geometric quantization.
This is an observation which I have not seen discussed anywhere. But since its proof is so elementary (as recorded in the entry), I won’t be surprised if this has been noted somewhere before. A reference would be welcome.
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