Hyperkähler quotient
In mathematics, the hyperkähler quotient of a hyperkähler manifold acted on by a Lie group G is the quotient of a fiber of a hyperkähler moment map  over a G-fixed point by the action of G. It was introduced by Nigel Hitchin, Anders Karlhede, Ulf Lindström, and Martin Roček in 1987. It is a hyperkähler analogue of the Kähler quotient. References

Differential geometry