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Singular integrals on Ahlfors-David regular subsets of the Heisenberg group

We investigate certain singular integral operators with Riesz-type kernels on s-dimensional Ahlfors-David regular subsets of Heisenberg groups. We show that $L^2$-boundedness, and even a little less, implies that $s$ must be an integer and the set can be approximated at some arbitrary small scales by homogeneous subgroups. It follows that the operators cannot be bounded on many self similar fractal subsets of Heisenberg groups.