The BTZ black hole, named after Máximo Bañados, Claudio Teitelboim, and Jorge Zanelli, is a black hole solution for (2+1)-dimensional topological gravity with a negative cosmological constant[clarification needed].
This came as a surprise, because when the cosmological constant is zero, a vacuum solution of (2+1)-dimensional gravity is necessarily flat (the Weyl tensor vanishes in three dimensions, while the Ricci tensor vanishes due to the Einstein field equations, so the full Riemann tensor vanishes), and it can be shown that no black hole solutions with event horizons exist.
The similarities to the ordinary black holes in 3+1 dimensions: Since (2+1)-dimensional gravity has no Newtonian limit, one might fear[why?]
(Carlip 1995, section 3 Black Holes and Gravitational Collapse) The BTZ solution is often discussed in the realm on (2+1)-dimensional quantum gravity.
[2] A rotating BTZ black hole admits closed timelike curves.