Variability of blazars : proceedings of a conference in honour of the 100th anniversary of the birth of Academican Yrjö Väisälä, held at Tuorla Observatory, University of Turku, Finland, January 6-10, 1991 /
Valtaoja, Esko.
Valtonen, Mauri.
Cambridge : Cambridge University Press,
1992 [i.e. 1991]