# Trace class norms of special integral operators

Let $\mu$ be a finite compactly supported Borel measure on the real line. On the space $L^2(\mu)$ consider the integral operators $$(K_a f)(x)=\int k_a(x, y)f(y)d\mu(y)$$ with $$k_a(x, y)=\frac{a\cdot(x-y)}{|x-y|^2+a^2}, \qquad a>0.$$ From the representation $$k_a(x, y)=\int_{-\infty}^\infty e^{\frac{itx}a}e^{-\frac{ity}a}\ te^{-|t|}\ dt$$ it follows that the trace class norms of the operators $K_a$ are uniformly bounded. This is a consequence of the fact that the integral operators with kernels $e^{\frac{itx}a}e^{-\frac{ity}a}$ are rank-one operators whose norm is equal to the full mass of $\mu$.

Question. Describe the class of measures $\mu$ for which the trace class norms of the operators $K_a$ tend to 0 as $a\searrow 0$.

Of special interest is the question if this is possible for measures containing a continuous component.

• Erm... Looks like in the representation $a$ should be $a^{-1}$, the absolute value on $t$ should be absent, etc. (just compute the value and the derivative at $x=y$), but it doesn't really change anything. – fedja Jun 23 '14 at 17:19
• @fedja, I have corrected the formula. Please feel free to edit it if necessary. – limanac Jun 24 '14 at 14:41