How to define using the GAP package, with the Lie algebra package included, some classical Lie algebras like sl(n,C), so(n,R), su(n) etc. without using commands referring to the type A_n?
Possibly one has first to define the corresponding associative algebras.
Or one has to define first the classical groups SL(n,C), SO(R,n), SU(n) and then apply a command which gives the Lie algebra of a Lie group? Which command?
Use the user-contributed Lie algebra package by Willem de Graaf and Thomas Breuer for GAP, see here.