The discovery of high-z blazars ensures the census, free from obscuration effects, of supermassive black holes (SMBHs) hosted by radio-loud active galactic nuclei (AGNs) in the early Universe, and provides strong and critical constraints on the accretion mode, mass and spin of the seed of SMBHs.
Blazars are rare sources: only $8$ blazars have been discovered to date at $z>5$, with the most...
I will present the studies on variability detection of AGNs based on the VST surveys of the CDFS and COSMOS region, that are compatible in depth to the expectations from LSST. I will discuss the completeness and contamination of variability-selected samples in order to characterise the AGN population and understand the selection biases with respect to optical/infrared/X-ray selection methods.
