nLab Hausdorff series

Contents

Context

\infty-Lie theory

∞-Lie theory (higher geometry)

Background

Smooth structure

Higher groupoids

Lie theory

∞-Lie groupoids

∞-Lie algebroids

Formal Lie groupoids

Cohomology

Homotopy

Related topics

Examples

\infty-Lie groupoids

\infty-Lie groups

\infty-Lie algebroids

\infty-Lie algebras

Contents

Magnus algebras and Magnus group

Let XX be a set of symbols and let kk denote the ground field.

The free associative k-algebra kXk\langle X \rangle on the set XX where kk is a commutative unital ring, will be denoted A(X)A(X). It is clearly graded (by the length of the word) as A(X)= nA n(X)A(X) = \oplus_n A^n(X). The product of kk-modules A^(X)= nA n(X)\hat{A}(X) = \prod_n A^n(X) has a natural multiplication

(ab) n= i=0 na ib ni (ab)_n = \sum_{i = 0}^n a_i b_{n-i}

where a=(a n) na = (a_n)_n and b=(b n) nb = (b_n)_n. Furthermore, A^(X)\hat{A}(X) has the topology of the product of discrete topological spaces. This makes A^(X)\hat{A}(X) a Hausdorff topological algebra, where the ground field is considered discrete and A(X)A(X) is dense in A^(X)\hat{A}(X). We say that A^(X)\hat{A}(X) is the Magnus algebra with coefficients in kk. (Bourbaki-Lie gr. II.5).

An element in A^(X)\hat{A}(X) is invertible (under multiplication) iff it’s free term is invertible in kk.

The Magnus group is the (multiplicative) subgroup of the Magnus algebra consisting of all elements in the Magnus algebra with free term 11.

Hausdorff group and Hausdorff series

The free Lie algebra L(X)L(X) naturally embeds in (the Lie algebra corresponding to the associative algebra) A(X)A^(X)A(X)\hookrightarrow \hat{A}(X); one defines L^(X)\hat{L}(X) as the closure of L(X)L(X) in A^(X)\hat{A}(X). The exponential series and the makes sense in A^(X)\hat{A}(X); when restricted to L^(X)\hat{L}(X) it gives a bijection between L^(X)\hat{L}(X) and a closed subgroup of the Magnus group which is sometimes called the Hausdorff group exp(L^(X))exp(\hat{L}(X)).

Hausdorff series H(U,V)H(U,V) is an element log(exp(U)exp(V))log(exp(U)exp(V)) in L^({U,V})\hat{L}(\{U,V\}).

The formula exp(X)exp(Y)=(exp(Y)exp(X)) 1exp(X)exp(Y) = (exp(Y)exp(X))^{-1} implies the basic symmetry of the Hausdorff series: H(Y,X)=H(X,Y)H(-Y,-X) = -H(X,Y).

The specializations of the Hausdorff series in Lie algebras which are not necessarily free are known as the Baker-Campbell-Hausdorff series and play the role in the corresponding BCH formula exp(U)exp(V)=exp(H(U,V))exp(U)exp(V) = exp(H(U,V)).

The BCH formula can be written in many ways, the most important which belong to Dynkin. The part which is linear in one of the variables involves Bernoulli numbers.

There is a decomposition H(X,Y)= N=0 H N(X,Y)H(X,Y) = \sum_{N=0}^\infty H_N(X,Y) where Dynkin’s Lie polynomials H N=H N(X,Y)H_N = H_N(X,Y) are defined recursively by H 1=X+YH_1 = X+Y and

(N+1)H N+1=12[XY,H N]+ r=0 N/21B 2r(2r)! s[H s 1,[H s 2,[,[H s 2r,X+Y]]]] (N+1)H_{N+1} = \frac{1}{2}[X-Y,H_N] + \sum_{r = 0}^{\lfloor N/2 -1\rfloor} \frac{B_{2r}}{(2r)!}\sum_s [H_{s_1},[H_{s_2},[ \ldots, [H_{s_{2r}},X+Y]\ldots]]]

where the sum over ss is the sum over all 2r2r-tuples s=(s 1,,s 2r)s = (s_1,\ldots,s_{2r}) of strictly positive integers whose sum s 1++s 2r=Ns_1 +\ldots+s_{2r} = N.

The Hausdorff series satisfies the symmetry H(Y,X)=H(X,Y)H(-Y,-X) = -H(X,Y).

First few terms of Hausdorff series are:

H(X,Y)=X+Y+12[X,Y]+112([X,[X,Y]]+[Y,[Y,X]])+124[Y,[X,[Y,X]]]+. H(X,Y) \;=\; X + Y + \tfrac{1}{2}[X,Y] + \tfrac{1}{12} \Big( \big[X,[X,Y]\big] + \big[Y,[Y,X]\big] \Big) + \tfrac{1}{24} \Big[ Y, \big [X,[Y,X] \big] \Big] + \ldots \,.

Literature

The following references concern the classical part of the subject. For the references connecting Hausdorff series to Drinfeld associators, the Grothendieck-Teichmueller group and the Kashiwara-Vergne conjecture see there.

Early discussion:

  • Rüdiger Achilles, Andrea Bonfiglioli: The early proofs of the theorem of Campbell, Baker, Hausdorff, and Dynkin, Arch. Hist. Exact Sci. 66 (2012) 295–358 [doi:10.1007/s00407-012-0095-8]

  • Eugene B. Dynkin, Calculation of the coefficents in the Campbell-Hausdorff formula, Doklady Akad. Nauk SSSR (N.S.) 57 (1947) 323-326

  • Wilhelm Magnus, A connection between the Baker-Hausdorff formula and a problem of Burnside, Ann. of Math. 52 (1950) 111-126

  • Kuo-Tsai Chen, Integration of paths, geometric invariants and a generalized Baker–Hausdorff formula, Annals of Mathematics 65:1 (1957) 163–178 doi jstor

Most monographs on Lie algebras discuss the Hausdorff series, for instance:

See also:

See also:

  • V. Kurlin, The Baker-Campbell-Hausdorff formula in the free metabelian Lie algebra [arXiv:math/0606330]

  • Federico Zadra et al. The flow method for the Baker-Campbell-Hausdorff formula: exact results, J. Phys. A: Math. Theor. 56 (2023) 385206 [doi:10.1088/1751-8121/acf102]

category: algebra

Last revised on September 3, 2024 at 10:15:27. See the history of this page for a list of all contributions to it.