In mathematics, the Baker–Campbell–Hausdorff formula gives a value of that solves the equation
for possibly noncommutative and in the Lie algebra of a Lie group. There are various ways of writing the formula, but all ultimately yield an expression for in Lie algebraic terms, that is, as a formal series (not necessarily convergent) in and and iterated commutators thereof. The first few terms of this series are:
where "" indicates terms involving higher commutators of and . If and are sufficiently small elements of the Lie algebra of a Lie group , the series is convergent. Meanwhile, every element sufficiently close to the identity in can be expressed as for a small in . Thus, we can say that near the identity the group multiplication in —written as —can be expressed in purely Lie algebraic terms. The Baker–Campbell–Hausdorff formula can be used to give comparatively simple proofs of deep results in the Lie group–Lie algebra correspondence.
If and are sufficiently small matrices, then can be computed as the logarithm of , where the exponentials and the logarithm can be computed as power series. The point of the Baker–Campbell–Hausdorff formula is then the highly nonobvious claim that can be expressed as a series in repeated commutators of and .
Modern expositions of the formula can be found in, among other places, the books of Rossmann and Hall.
History The formula is named after Henry Frederick Baker, John Edward Campbell, and Felix Hausdorff who stated its qualitative form, i.e. that only commutators and commutators of commutators, ad infinitum, are needed to express the solution. An earlier statement of the form was adumbrated by Friedrich Schur in 1890F. Schur (1890), "Neue Begründung der Theorie der endlichen Transformationsgruppen," Mathematische Annalen, 35 (1890), 161–197. online copy where a convergent power series is given, with terms recursively defined.see, e.g., Shlomo Sternberg, Lie Algebras (2004) Harvard University. (cf p 10.) This qualitative form is what is used in the most important applications, such as the relatively accessible proofs of the Lie correspondence and in quantum field theory. Following Schur, it was noted in print by CampbellJohn Edward Campbell, Proceedings of the London Mathematical Society 28 (1897) 381–390; (cf pp386-7 for the eponymous lemma); J. Campbell, Proceedings of the London Mathematical Society 29 (1898) 14–32. (1897); elaborated by Henri PoincaréHenri Poincaré, Comptes rendus de l'Académie des Sciences 128 (1899) 1065–1069; Transactions of the Cambridge Philosophical Society 18 (1899) 220–255. online (1899) and Baker (1902);Henry Frederick Baker, Proceedings of the London Mathematical Society (1) 34 (1902) 347–360; H. Baker, Proceedings of the London Mathematical Society (1) 35 (1903) 333–374; H. Baker, Proceedings of the London Mathematical Society (Ser 2) 3 (1905) 24–47. and systematized geometrically, and linked to the Jacobi identity by Hausdorff (1906).Felix Hausdorff, "Die symbolische Exponentialformel in der Gruppentheorie", Ber Verh Saechs Akad Wiss Leipzig 58 (1906) 19–48. The first actual explicit formula, with all numerical coefficients, is due to Eugene Dynkin (1947). p. 23 The history of the formula is described in detail in the article of Achilles and Bonfiglioli and in the book of Bonfiglioli and Fulci.
Explicit forms For many purposes, it is only necessary to know that an expansion for in terms of iterated commutators of and exists; the exact coefficients are often irrelevant. (See, for example, the discussion of the relationship between Lie group and Lie algebra homomorphisms in Section 5.2 of Hall's book, where the precise coefficients play no role in the argument.) A remarkably direct existence proof was given by Martin Eichler, see also the "Existence results" section below.
In other cases, one may need detailed information about and it is therefore desirable to compute as explicitly as possible. Numerous formulas exist; we will describe two of the main ones (Dynkin's formula and the integral formula of Poincaré) in this section.
Dynkin formula Let G be a Lie group with Lie algebra . Let
be the exponential map. The following general combinatorial formula was introduced by Eugene Dynkin (1947),
where the sum is performed over all nonnegative values of and , and the following notation has been used:
with the understanding that .
The series is not convergent in general; it is convergent (and the stated formula is valid) for all sufficiently small and . Since , the term is zero if or if and .A.A. Sagle & R.E. Walde, "Introduction to Lie Groups and Lie Algebras", Academic Press, New York, 1973. .
The first few terms are well-known, with all higher-order terms involving and commutator nestings thereof (thus in the Lie algebra):
The above lists all summands of order 6 or lower (i.e. those containing 6 or fewer 's and 's). The (anti-)/symmetry in alternating orders of the expansion, follows from . A complete elementary proof of this formula can be found in the article on the derivative of the exponential map.
An integral formula There are numerous other expressions for , many of which are used in the physics literature.; Veltman, M, 't Hooft, G & de Wit, B (2007), Appendix D. A popular integral formula isW. Miller, Symmetry Groups and their Applications, Academic Press, New York, 1972, pp 159–161. Theorem 5.3
involving the generating function for the Bernoulli numbers,
utilized by Poincaré and Hausdorff.Recall
for the Bernoulli numbers, B0 = 1, B1 = 1/2, B2 = 1/6, B4 = −1/30, ...
Matrix Lie group illustration For a matrix Lie group the Lie algebra is the tangent space of the identity I, and the commutator is simply ; the exponential map is the standard exponential map of matrices,
When one solves for Z in
using the series expansions for and one obtains a simpler formula: Equation (2) Section 1.3. For matrix Lie algebras over the fields and , the convergence criterion is that the log series converges for both sides of . This is guaranteed whenever in the Hilbert–Schmidt norm. Convergence may occur on a larger domain. See p. 24. The first, second, third, and fourth order terms are:
The formulas for the various 's is not the Baker–Campbell–Hausdorff formula. Rather, the Baker–Campbell–Hausdorff formula is one of various expressions for 's in terms of repeated commutators of and . The point is that it is far from obvious that it is possible to express each in terms of commutators. (The reader is invited, for example, to verify by direct computation that is expressible as a linear combination of the two nontrivial third-order commutators of and , namely and .) The general result that each is expressible as a combination of commutators was shown in an elegant, recursive way by Eichler.
A consequence of the Baker–Campbell–Hausdorff formula is the following result about the trace:
That is to say, since each with is expressible as a linear combination of commutators, the trace of each such term is zero.
Questions of convergence Suppose and are the following matrices in the Lie algebra (the space of matrices with trace zero):
Then
It is then not hard to show Example 3.41 that there does not exist a matrix in with . (Similar examples may be found in the article of Wei.)
This simple example illustrates that the various versions of the Baker–Campbell–Hausdorff formula, which give expressions for in terms of iterated Lie-brackets of and , describe formal power series whose convergence is not guaranteed. Thus, if one wants to be an actual element of the Lie algebra containing and (as opposed to a formal power series), one has to assume that and are small. Thus, the conclusion that the product operation on a Lie group is determined by the Lie algebra is only a local statement. Indeed, the result cannot be global, because globally one can have nonisomorphic Lie groups with isomorphic Lie algebras.
Concretely, if working with a matrix Lie algebra and is a given submultiplicative matrix norm, convergence is guaranteed if
Special cases If and commute, that is , the Baker–Campbell–Hausdorff formula reduces to .
Another case assumes that commutes with both and , as for the nilpotent Heisenberg group. Then the formula reduces to its first three terms.
This is the degenerate case used routinely in quantum mechanics, as illustrated below and is sometimes known as the disentangling theorem. In this case, there are no smallness restrictions on and . This result is behind the "exponentiated commutation relations" that enter into the Stone–von Neumann theorem. A simple proof of this identity is given below.
Another useful form of the general formula emphasizes expansion in terms of Y and uses the adjoint mapping notation :
which is evident from the integral formula above. (The coefficients of the nested commutators with a single are normalized Bernoulli numbers.)
Now assume that the commutator is a multiple of , so that . Then all iterated commutators will be multiples of , and no quadratic or higher terms in appear. Thus, the term above vanishes and we obtain:
Again, in this case there are no smallness restriction on and . The restriction on guarantees that the expression on the right side makes sense. (When we may interpret .) We also obtain a simple "braiding identity":
which may be written as an adjoint dilation:
Existence results
If and are matrices, one can compute using the power series for the exponential and logarithm, with convergence of the series if and are sufficiently small. It is natural to collect together all terms where the total degree in and equals a fixed number , giving an expression . (See the section "Matrix Lie group illustration" above for formulas for the first several 's.) A remarkably direct and concise, recursive proof that each is expressible in terms of repeated commutators of and was given by Martin Eichler.
Alternatively, we can give an existence argument as follows. The Baker–Campbell–Hausdorff formula implies that if and are in some Lie algebra defined over any field of characteristic 0 like or , then
can formally be written as an infinite sum of elements of . [This infinite series may or may not converge, so it need not define an actual element in .] For many applications, the mere assurance of the existence of this formal expression is sufficient, and an explicit expression for this infinite sum is not needed. This is for instance the case in the Lorentzian Section 5.7 construction of a Lie group representation from a Lie algebra representation. Existence can be seen as follows.
We consider the ring of all non-commuting formal power series with real coefficients in the non-commuting variables and . There is a ring homomorphism from to the tensor product of with over ,
called the coproduct, such that and (The definition of Δ is extended to the other elements of S by requiring R-linearity, multiplicativity and infinite additivity.)
One can then verify the following properties: The map , defined by its standard Taylor series, is a bijection between the set of elements of with constant term 0 and the set of elements of with constant term 1; the inverse of exp is log is grouplike (this means ) if and only if s is primitive (this means ). The grouplike elements form a group under multiplication. The primitive elements are exactly the formal infinite sums of elements of the Lie algebra generated by X and Y, where the Lie bracket is given by the commutator . (Friedrichs' theoremNathan Jacobson, Lie Algebras, John Wiley & Sons, 1966.)
The existence of the Campbell–Baker–Hausdorff formula can now be seen as follows: The elements X and Y are primitive, so and are grouplike; so their product is also grouplike; so its logarithm is primitive; and hence can be written as an infinite sum of elements of the Lie algebra generated by and .