  In this paper we present a new method that can be used for analysis of time
of arrival of a pulsar pulses (TOAs). It is designated especially to detect
quasi-periodic variations of TOAs. We apply our method to timing observations
of PSR B1257+12 and demonstrate that using it it is possible to detect not only
first harmonics of a periodic variations, but also the presence of a resonance
effect. The resonance effect detected, independently of its physical origin,
can appear only when there is a non-linear interaction between two periodic
modes. The explanation of TOAs variations as an effect of the existence of
planets is, till now, the only known and well justified. In this context, the
existence of the resonance frequency in TOAs is the most significant signature
of the gravitational interaction of planets.
