Chain rule for fractional laplacian Does anyone know a formula of chain rule for fractional laplacian?
say we take the fractional laplacian of order a on function $g(U(x))$  $x\in \mathbb{R}^2$, $U \in \mathbb{R}$, $g \colon \mathbb{R} \to \mathbb{R}$ functional.
Thanks
 A: In the fractional case, it turns out that approximate  chain rules are more useful than exact formulae (at least for applications to the analysis of PDE).  See 
http://wiki.math.toronto.edu/TorontoMathWiki/index.php/Fractional_Derivative
In the case $0 \leq a \leq 1/2$, the rule roughly takes the form
$$ (-\Delta)^a g(U) \approx ((-\Delta^a) U) \cdot \nabla g(U) + \ldots$$
where the $\ldots$ error is a paraproduct which is "lower order" than the main term in some sense.  One popular way to make this formula precise is the Bony linearisation formula, originally developed in http://www.ams.org/mathscinet-getitem?mr=631751 .   This is part of a more general theory known as paradifferential calculus, discussed for instance in Taylor's book http://www.ams.org/mathscinet-getitem?mr=1766415
A: I showed that the Riesz potential is an integral formulation of general fractional two-side derivative, that is much more general in the sense that is valid for any order greater than -1 for a broad class of funtions. 
I can give copies by sending a mail to mdo@fct.unl.pt 
