We proved sharp estimates for the weak type $(1,1)$
norm of such operators. The obtained result is optimal i.e. the lower bounds
are proportional to the upper bounds. Applying our result to
the standard Laplace
operator on ${\bf R}^N$ we get
$\|\Delta_N^{is}\|\sim (1+|s|)^{N/2}$.
This estimate is more precise than the estimate which we obtain
in virtue of the classical Hörmander multiplier theorem.