∞-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
Let $X$ be a set of symbols. The free associative k-algebra $k\langle X \rangle$ on the set $X$ where $k$ is a commutative unital ring, will be denoted $A(X)$. It is clearly graded (by the length of the word) as $A(X) = \oplus_n A^n(X)$. The product of $k$-modules $\hat{A}(X) = \prod_n A^n(X)$ has a natural multiplication
where $a = (a_n)_n$ and $b = (b_n)_n$. Furthermore, $\hat{A}(X)$ has the topology of the product of discrete topological spaces. This makes $\hat{A}(X)$ a Hausdorff topological algebra, where the ground field is considered discrete and $A(X)$ is dense in $\hat{A}(X)$. We say that $\hat{A}(X)$ is the Magnus algebra with coefficients in $k$. (Bourbaki-Lie gr. II.5).
An element in $\hat{A}(X)$ is invertible (under multiplication) iff it’s free term is invertible in $k$.
The Magnus group is the (multiplicative) subgroup of the Magnus algebra consisting of all elements in the Magnus algebra with free term $1$.
The free Lie algebra $L(X)$ naturally embeds in (the Lie algebra corresponding to the associative algebra) $A(X)\hookrightarrow \hat{A}(X)$; one defines $\hat{L}(X)$ as the closure of $L(X)$ in $\hat{A}(X)$. The exponential series and the makes sense in $\hat{A}(X)$; when restricted to $\hat{L}(X)$ it gives a bijection between $\hat{L}(X)$ and a closed subgroup of the Magnus group which is sometimes called the Hausdorff group $exp(\hat{L}(X))$.
Hausdorff series $H(U,V)$ is an element $log(exp(U)exp(V))$ in $\hat{L}(\{U,V\})$.
The formula $exp(X)exp(Y) = (exp(Y)exp(X))^{-1}$ implies the basic symmetry of the Hausdorff series: $H(-Y,-X) = -H(X,Y)$.
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 $exp(U)exp(V) = exp(H(U,V))$.
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 $H(X,Y) = \sum_{N=0}^\infty H_N(X,Y)$ where Dynkin’s Lie polynomials $H_N = H_N(X,Y)$ are defined recursively by $H_1 = X+Y$ and
where the sum over $s$ is the sum over all $2r$-tuples $s = (s_1,\ldots,s_{2r})$ of strictly positive integers whose sum $s_1 +\ldots+s_{2r} = N$.
Hausdorff series satisfies the symmetry $H(-Y,-X) = -H(X,Y)$.
First few terms of Hausdorff series are
Related $n$Lab entries include Lie theory, Malcev completion, exponential map
Here we list mostly references about the classical part of the subject. The references connecting Hausdorff series to Drinfeld associators, Grothendieck-Teichmueller group and Kashiwara-Vergne conjecture see in the corresponding entries.
N. Bourbaki, Lie groups and algebras, chapter II
M M Postnikov, Lectures on geometry, Semester V, Lie groups and algebras
E. B. Dynkin, Calculation of the coefficents in the Campbell-Hausdorff formula, Doklady Akad. Nauk SSSR (N.S.) 57, 323-326, (1947)
Terence Tao, 254A, Notes 1, Lie groups, Lie algebras, and the Baker-Campbell-Hausdorff formula, blog entry
V. Kurlin, Exponential Baker-Campbell-Hausdorff formula, http://arxiv.org/abs/math/0606330
Terry Tao’s blog: the-c11-baker-campbell-hausdorff-formula
Kuo-Tsai Chen, Integration of paths, geometric invariants and a generalized Baker-Hausdorff formula, Annals of Mathematics 65:1 (1957) 163–178 doi jstor
Wilhelm Magnus, A connection between the Baker-Hausdorff formula and a problem of Burnside, Ann. of Math. 52 (1950) 111-126
Federico Zadra et al. The flow method for the Baker-Campbell-Hausdorff formula: exact results, J. Phys. A: Math. Theor. 56 (2023) 385206 doi
Last revised on September 5, 2023 at 09:52:25. See the history of this page for a list of all contributions to it.