Let π€ be a Lie algebra over π.
A triangular decomposition of π€ is a triple of subalgebrasπ«Β±,π₯β€π€
such that
π€=π«ββπ₯βπ«+
where π₯ is abelian and [π₯,π«Β±]βπ«Β±.1lie
A Lie algebra with such a decomposition is called triangular.
This may be viewed as a generalization of a Heisenberg algebra.