The number $12$ (or, probably we shall say Bernoulli numbers in general) appears in many places in Mathematics, sometimes leading to unexpected connections between different topics.
For instance, some time ago there was a very interesting explanation for
1) its occurrence in the Todd class
2) its occurrence in the Euler-Maclaurin formula
in terms of Riemann-Roch for toric varieties, as explained in:
My question is, will there be some relation between 1) and
3) its occurrence in the Baker-Campbell-Hausdorff formula.
I guess this might be related to some explicit local expressions in some method of proving the index theorem on Lie groups, or even the Duflo map (which I don't really understand).
Thank you very much.