The classification of real simple Lie algebras is one of the major achievements of Wilhelm Killing and Élie Cartan.
A direct sum of simple Lie algebras is called a semisimple Lie algebra.
A finite-dimensional simple complex Lie algebra is isomorphic to either of the following:
[1] To each finite-dimensional complex semisimple Lie algebra
, there exists a corresponding diagram (called the Dynkin diagram) where the nodes denote the simple roots, the nodes are jointed (or not jointed) by a number of lines depending on the angles between the simple roots and the arrows are put to indicate whether the roots are longer or shorter.
All possible connected Dynkin diagrams are the following:[3] where n is the number of the nodes (the simple roots).
The correspondence of the diagrams and complex simple Lie algebras is as follows:[2] If
is a finite-dimensional real simple Lie algebra, its complexification is either (1) simple or (2) a product of a simple complex Lie algebra and its conjugate.
thought of as a real Lie algebra is
Thus, a real simple Lie algebra can be classified by the classification of complex simple Lie algebras and some additional information.