Iwasawa decomposition

In mathematics, the Iwasawa decomposition KAN of a semisimple Lie group generalises the way a square real matrix can be written as a product of an orthogonal matrix and an upper triangular matrix (a consequence of Gram–Schmidt orthogonalization). It is named after Kenkichi Iwasawa, the Japanese mathematician who developed this method.^[1]

Definition

• G is a connected semisimple real Lie group.
• ${\mathfrak {g}}_{0}$ is the Lie algebra of G
• ${\mathfrak {g}}$ is the complexification of ${\mathfrak {g}}_{0}$.
• θ is a Cartan involution of ${\mathfrak {g}}_{0}$
• ${\mathfrak {g}}_{0}={\mathfrak {k}}_{0}\oplus {\mathfrak {p}}_{0}$ is the corresponding Cartan decomposition
• ${\mathfrak {a}}_{0}$ is a maximal abelian subalgebra of ${\mathfrak {p}}_{0}$
• Σ is the set of restricted roots of ${\mathfrak {a}}_{0}$, corresponding to eigenvalues of ${\mathfrak {a}}_{0}$ acting on ${\mathfrak {g}}_{0}$.
• Σ⁺ is a choice of positive roots of Σ
• ${\mathfrak {n}}_{0}$ is a nilpotent Lie algebra given as the sum of the root spaces of Σ⁺
• K, A, N, are the Lie subgroups of G generated by ${\mathfrak {k}}_{0},{\mathfrak {a}}_{0}$ and ${\mathfrak {n}}_{0}$.

Then the Iwasawa decomposition of ${\mathfrak {g}}_{0}$ is

${\mathfrak {g}}_{0}={\mathfrak {k}}_{0}\oplus {\mathfrak {a}}_{0}\oplus {\mathfrak {n}}_{0}$

and the Iwasawa decomposition of G is

${\displaystyle G=KAN}$

The dimension of A (or equivalently of ${\mathfrak {a}}_{0}$) is equal to the real rank of G.

Iwasawa decompositions also hold for some disconnected semisimple groups G, where K becomes a (disconnected) maximal compact subgroup provided the center of G is finite.

The restricted root space decomposition is

${\mathfrak {g}}_{0}={\mathfrak {m}}_{0}\oplus {\mathfrak {a}}_{0}\oplus _{\lambda \in \Sigma }{\mathfrak {g}}_{\lambda }$

where ${\mathfrak {m}}_{0}$ is the centralizer of ${\mathfrak {a}}_{0}$ in ${\mathfrak {k}}_{0}$ and ${\mathfrak {g}}_{\lambda }=\{X\in {\mathfrak {g}}_{0}:[H,X]=\lambda (H)X\;\;\forall H\in {\mathfrak {a}}_{0}\}$ is the root space. The number $m_{\lambda }={\text{dim}}\,{\mathfrak {g}}_{\lambda }$ is called the multiplicity of $\lambda$.

Examples

If G=SL_n(R), then we can take K to be the orthogonal matrices, A to be the positive diagonal matrices, and N to be the unipotent group consisting of upper triangular matrices with 1s on the diagonal.

Non-Archimedean Iwasawa decomposition

There is an analogon to the above Iwasawa decomposition for a non-Archimedean field $F$: In this case, the group $GL_{n}(F)$ can be written as a product of the subgroup of upper-triangular matrices and the (maximal compact) subgroup $GL_{n}(O_{F})$, where $O_{F}$ is the ring of integers of $F$. ^[2]

See also

• Lie group decompositions

References

1. Iwasawa, Kenkichi (1949). "On Some Types of Topological Groups". Annals of Mathematics. 50 (3): 507–558. doi:10.2307/1969548. JSTOR 1969548.
2. Bump, Automorphic Forms and Representations, Prop. 4.5.2

• Fedenko, A.S.; Shtern, A.I. (2001), "I/i053060", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
• A. W. Knapp, Structure theory of semisimple Lie groups, in ISBN 0-8218-0609-2: Representation Theory and Automorphic Forms: Instructional Conference, International Centre for Mathematical Sciences, March 1996, Edinburgh, Scotland (Proceedings of Symposia in Pure Mathematics) by T. N. Bailey (Editor), Anthony W. Knapp (Editor)