  A systematic search for cyclical sources of gamma-ray emission on the period
range from days to years in the Fermi-LAT sky is performed. Looking for
cyclical emission, the sky is binned into equal-area pixels and the generalised
Lomb-Scargle periodogram is computed for each of these pixels. The search on
the period range between 2.5 and 30 days in the Galactic plane confirms
periodicities of three binaries, LSI +61 303, LS 5039, and 1FGL J1018.6-5856.
The all-sky search on the period range between 30 days and 2.5 years confirms
periodicities of three blazars, PG 1553+113, PKS 2155-304, and BL Lacertae.
Evidence for periodic behaviours of four blazars, 4C +01.28, S5 0716+71, PKS
0805-07, and PKS 2052-47, are presented. Three of these blazars, 4C +01.28, PKS
0805-07, and PKS 2052-47, are located at high redshifts. These three sources
are potential candidates to binary systems of supermassive black holes provided
that major galaxy mergers are more frequent and that galaxies are more gas-rich
at high redshifts.
