  We prove that if $f$ is a Lipschitz function on $\R$, $A$ and $B$ are
self-adjoint operators such that ${\rm rank} (A-B)=1$, then $f(A)-f(B)$ belongs
to the weak space $\boldsymbol{S}_{1,\be}$, i.e., $s_j(A-B)\le{\rm const}
(1+j)^{-1}$. We deduce from this result that if $A-B$ belongs to the trace
class $\boldsymbol{S}_1$ and $f$ is Lipschitz, then
$f(A)-f(B)\in\boldsymbol{S}_\Omega$, i.e.,
$\sum_{j=0}^ns_j(f(A)-f(B))\le\const\log(2+n)$. We also obtain more general
results about the behavior of double operator integrals of the form
$Q=\iint(f(x)-f(y))(x-y)^{-1}dE_1(x)TdE_2(y)$, where $E_1$ and $E_2$ are
spectral measures. We show that if $T\in\boldsymbol{S}_1$, then
$Q\in\boldsymbol{S}_\Omega$ and if $\rank T=1$, then
$Q\in\boldsymbol{S}_{1,\be}$. Finally, if $T$ belongs to the Matsaev ideal
$\boldsymbol{S}_\omega$, then $Q$ is a compact operator.
