Iwasawa decomposition

In mathematics, the Iwasawa decomposition (aka KAN from its expression) 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

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 analog 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]

References

↑ Iwasawa, Kenkichi (1949). "On Some Types of Topological Groups". Annals of Mathematics. 50 (3): 507–558. doi:10.2307/1969548. JSTOR 1969548.
↑ Bump, Daniel (1997), Automorphic forms and representations, Cambridge: Cambridge University Press, doi:10.1017/CBO9780511609572, ISBN 0-521-55098-X , Prop. 4.5.2

Fedenko, A.S.; Shtern, A.I. (2001) [1994], "Iwasawa decomposition", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4
Knapp, A. W. (2002). Lie groups beyond an introduction (2nd ed.). ISBN 9780817642594.

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[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, Daniel (1997), Automorphic forms and representations, Cambridge: Cambridge University Press, doi:10.1017/CBO9780511609572, ISBN 0-521-55098-X , Prop. 4.5.2

Iwasawa decomposition

Definition

Examples

Non-Archimedean Iwasawa decomposition

See also

References