Suppose that
and
are two algebras and
is a unital
-bimodule. Then
with the usual
matrix-like addition and matrix-like multiplication is an algebra.
An algebra
is called a triangular algebra if there exist algebras
and
and an
-bimodule
such that
is (algebraically) isomorphic to
under matrix-like addition and matrix-like multiplication.
For example, the algebra of
upper triangular
matrices over the complex field
may be viewed as a triangular algebra when
.