In the area of modern algebra known as group theory, the Janko groups are the four sporadic simple groups J1, J2, J3 and J4 introduced by Zvonimir Janko.
Janko constructed the first of these groups, J1, in 1965 and predicted the existence of J2 and J3.
The discovery of J1 caused a great "sensation"[1] and "surprise"[2] among group theory specialists.
This began the modern theory of sporadic groups.
It would be the last sporadic group (and, since the non-sporadic families had already been found, the last finite simple group) predicted and discovered, though this could only be said in hindsight when the Classification theorem was completed.