The paper deals with the application of the restricted-denominator rational Krylov method, recently
discussed in (BIT 2004; 44(3):595–615; SIAM J. Sci. Comput. 2005; 27:1438–1457), to the computation
of the action of the so-called -functions, which play a fundamental role in several modern exponential
integrators. The analysis here presented is devoted in particular to the construction of error estimates of
easy practical use.
