Kazhdan's Property (T)

In mathematics, a locally compact topological group G has property (T) if the trivial representation is an isolated point in its unitary dual under the Fell topology. Informally, this means that if G acts unitarily on a Hilbert space and has almost invariant vectors, then it has a nonzero invariant vector. The formal definition was introduced by David Kazhdan in 1967 to give this idea a precise, quantitative meaning. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/