  In high frequency financial data not only returns but also waiting times
between trades are random variables. In this work, we analyze the spectra of
the waiting-time processes for tick-by-tick trades. The numerical problem,
strictly related with the real inversion of Laplace transforms, is analyzed by
using Tikhonov's regularization method. We also analyze these spectra by a
rough method using a comb of Dirac's delta functions.
