 Conversely for any root datum there is a reductive algebraic group.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"*, Φ,"X"*, Φv), we can define a dual root datum ("X"*, Φv,"X"*, Φ) by switching the characters with the 1-parameter subgroups, and switching the roots with the coroots.