In mathematics, a Ree group is a group of Lie type over a finite field constructed by from an exceptional automorphism of a Dynkin diagram that reverses the direction of the multiple bonds, generalizing the Suzuki groups found by Suzuki using a different method. They were the last of the infinite families of finite simple groups to be discovered. Unlike the Steinberg groups, the Ree groups are not given by the points of a connected reductive algebraic group defined over a finite field; in other words, there is no "Ree algebraic group" related to the Ree groups in the same way that (say) unitary groups are related to Steinberg groups. However, there are some exotic pseudo-reductive algebraic groups over non-perfect fields whose construction is related to the construction of Ree groups, as they use the same exotic automorphisms of Dynkin diagrams that change root lengths. defined Ree groups over infinite fields of characteristics 2 and 3. and introduced Ree groups of infinite-dimensional Kac–Moody algebras. Construction If is a Dynkin diagram, Chevalley constructed split algebraic groups corresponding to , in particular giving groups with values in a field . These groups have the following automorphisms: Any endomorphism of the field induces an endomorphism of the group Any automorphism of the Dynkin diagram induces an automorphism of the group . The Steinberg and Chevalley groups can be constructed as fixed points of an endomorphism of X(F) for the algebraic closure of a field. For the Chevalley groups, the automorphism is the Frobenius endomorphism of , while for the Steinberg groups the automorphism is the Frobenius endomorphism times an automorphism of the Dynkin diagram. Over fields of characteristic 2 the groups and and over fields of characteristic 3 the groups have an endomorphism whose square is the endomorphism associated to the Frobenius endomorphism of the field . Roughly speaking, this endomorphism comes from the order 2 automorphism of the Dynkin diagram where one ignores the lengths of the roots. Suppose that the field has an endomorphism whose square is the Frobenius endomorphism: . Then the Ree group is defined to be the group of elements of such that . If the field is perfect then and are automorphisms, and the Ree group is the group of fixed points of the involution of . In the case when is a finite field of order (with p = 2 or 3) there is an endomorphism