In mathematics, a (B, N) pair is a structure on groups of Lie type that allows one to give uniform proofs of many results, instead of giving a large number of case by case proofs. Roughly speaking, it shows that all such groups are similar to the general linear group over a field. They were invented by the mathematician Jacques Tits, and are also sometimes known as Tits systems.

Definition

A (B, N) pair is a pair of subgroups B and N of a group G such that the following axioms hold:

• G is generated by B and N.
• The intersection H of B and N is a normal subgroup of N.
• The group W = N/H is generated by a set of elements w_i of order 2, for i in some non-empty set I.
• If w_i is one of the generators of W and w is any element of W, then w_iBw is contained in the union of Bw_iwB and BwB.
• No generator w_i normalizes B.

The idea of this definition is that B is an analogue of the upper triangular matrices of the general linear group GL_n(K), H is an analogue of the diagonal matrices, and N is an analogue of the normalizer of H.

The subgroup B is sometimes called the Borel subgroup, H is sometimes called the Cartan subgroup , and W is called the Weyl group.

The number of generators w_i is called the rank.

Examples

• Suppose that G is any doubly transitive permutation group on a set X with more than 2 elements. We let B be the subgroup of G fixing a point x, and we let N be the subgroup fixing or exchanging 2 points x and y. The subgroup H is then the set of elements fixing both x and y, and W has order 2 and its nontrivial element is represented by anything exchanging x and y.

• Conversely, if G has a BN pair of rank 1, then the action of G on the cosets of B is doubly transitive. So BN pairs of rank 1 are more or less the same as doubly transitive actions on sets with more than 2 elements.

• Suppose that G is the general linear group GL_n(K) over a field K. We take B to be the upper triangular matrices, H to be the diagonal matrices, and N to be the matrices with exactly one non-zero element in each row and column. There are n-1 generators w_i, represented by the matrices obtained by swapping two adjacent rows of a diagonal matrix.

• More generally, any group of Lie type has the structure of a BN-pair.

Properties of groups with a BN pair.

The map taking w to BwB is an isomorphism from the set of elements of W to the set of double cosets of B.

The subgroups of G containing B are called parabolic subgroups . There are exactly 2ⁿ of them, and they correspond to subsets of I.

Applications

BN-pairs can be used to prove that most groups of Lie type are simple. More precisely, if G has a BN-pair such that B is a solvable group, the intersection of all conjugates of B is trivial, and the set of generators of W cannot be decomposed into two non-empty commuting sets, then G is simple whenever it is perfect. In practice all of these conditions except for G being perfect are easy to check. Checking that G is perfect needs some slightly messy calculations (and in fact there are a few small groups of Lie type which are not perfect or simple). But showing that a group is perfect is usually far easier than showing it is simple.

Further reading

The standard reference for BN pairs is

• Lie Groups and Lie Algebras: Chapters 4-6 (Elements of Mathematics)

by Nicolas Bourbaki ISBN 3540426507