 A root datum contains slightly more information than the Dynkin diagram, because it also determines the center of the group.For any root datum ("X"*, Δ,"X"*, Δv), we can define a dual root datum ("X"*, Δv,"X"*, Δ) by switching the characters with the 1-parameter subgroups, and switching the roots with the coroots.If "G" is a connected reductive algebraic group over the algebraically closed field "K", then its Langlands dual group "L""G" is the complex connected reductive group whose root datum is dual to that of "G".