Lagrange bracket explained

Lagrange brackets are certain expressions closely related to Poisson brackets that were introduced by Joseph Louis Lagrange in 1808–1810 for the purposes of mathematical formulation of classical mechanics, but unlike the Poisson brackets, have fallen out of use.

Definition

Suppose that (q₁, ..., q_n, p₁, ..., p_n) is a system of canonical coordinates on a phase space. If each of them is expressed as a function of two variables, u and v, then the Lagrange bracket of u and v is defined by the formula

[ u, v ] _{p,q} = \sum ^{n} _{i =1} \left( (\partial q _{i})/(\partial u) (\partial p _{i})/(\partial v) - (\partial p _{i})/(\partial u) (\partial q _{i})/(\partial v) \right).

Properties

• Lagrange brackets do not depend on the system of canonical coordinates (q, p). If (Q,P) = (Q₁, ..., Q_n, P₁, ..., P_n) is another system of canonical coordinates, so that

Q = Q(q,p), P = P(q,p)

is a canonical transformation, then the Lagrange bracket is an invariant of the transformation, in the sense that

[ u, v] _{q,p} = [u , v] _{Q,P}

Therefore, the subscripts indicating the canonical coordinates are often omitted.

• If Ω is the symplectic form on the 2n-dimensional phase space W and u₁,...,u_2n form a system of coordinates on W, the symplectic form can be written as

\Omega = (12)/(\Omega) _{ij} du ^{i} \wedge du ^{j}

where the matrix

\Omega _{ij} = [ u _{i,} u _{j} ] _{p,q} , 1 \leq i,j\leq 2 n ::

represents the components of, viewed as a tensor, in the coordinates u. This matrix is the inverse of the matrix formed by the Poisson brackets

\left(\Omega ^{-1} \right) _{ij} = \{u _{i,} u _{j\},} 1 \leq i,j \leq 2 n

of the coordinates u.

• As a corollary of the preceding properties, coordinates (Q₁, ..., Q_n, P₁, ..., P_n) on a phase space are canonical if and only if the Lagrange brackets between them have the form

[Q _{i,} Q _{j]} _{p,q} =0 , [P _{i,P} _{j]} _{p,q} =0 , [Q _{i,} P _{j]} _{p,q} =- [P _{j,} Q _{i]} _{p,q} = \delta _{ij} .

Lagrange matrix in canonical transformations

See main article: Canonical transformation. The concept of Lagrange brackets can be expanded to that of matrices by defining the Lagrange matrix.

Consider the following canonical transformation:$$\backslash eta\; =\; \backslash begin\; q\_1\backslash \backslash \; \backslash vdots\; \backslash \backslash \; q\_N\backslash \backslash \; p\_1\backslash \backslash \; \backslash vdots\backslash \backslash \; p\_N\backslash \backslash \; \backslash end\; \backslash quad\; \backslash rightarrow\; \backslash quad\; \backslash varepsilon\; =\; \backslash begin\; Q\_1\backslash \backslash \; \backslash vdots\; \backslash \backslash \; Q\_N\backslash \backslash \; P\_1\backslash \backslash \; \backslash vdots\backslash \backslash \; P\_N\backslash \backslash \; \backslash end$$

Defining $M\; :=\; \backslash frac$, the Lagrange matrix is defined as $\backslash mathcal\; L(\backslash eta)\; =\; M^TJM$, where

J is the symplectic matrix under the same conventions used to order the set of coordinates. It follows from the definition that:

$$\backslash mathcal\; L\_(\backslash eta)\; =\; [M^TJM]\_\; =\; \backslash sum\_^\; \backslash left(\backslash frac\; \backslash frac\; -\; \backslash frac\; \backslash frac\backslash right)\; =\; \backslash sum\_^\; \backslash left(\backslash frac\; \backslash frac\; -\; \backslash frac\; \backslash frac\backslash right)=\; [\backslash eta\_i,\backslash eta\_j]\_\backslash varepsilon$$

The Lagrange matrix satisfies the following known properties:

^2

\\\mathcal L^(\eta)&= -M^ J (M^)^T = - \mathcal P(\eta)\\\endwhere the $\backslash mathcal\; P(\backslash eta)$ is known as a Poisson matrix and whose elements correspond to Poisson brackets. The last identity can also be stated as the following:$$\backslash sum\_^\; \backslash [\backslash eta\_k,\backslash eta\_j]\; =\; -\backslash delta\_$$Note that the summation here involves generalized coordinates as well as generalized momentum.

The invariance of Lagrange bracket can be expressed as: $[\backslash eta\_i,\backslash eta\_j]\_\backslash varepsilon=[\backslash eta\_i,\backslash eta\_j]\_\backslash eta\; =\; J\_$, which directly leads to the symplectic condition: $M^TJM\; =\; J$.^[1]

See also

• Lagrangian mechanics
• Hamiltonian mechanics

References

• Cornelius Lanczos, The Variational Principles of Mechanics, Dover (1986), .
• Iglesias, Patrick, Les origines du calcul symplectique chez Lagrange [The origins of symplectic calculus in Lagrange's work], L'Enseign. Math. (2) 44 (1998), no. 3-4, 257–277.

Notes and References

1. Book: Giacaglia, Giorgio E. O. . Perturbation methods in non-linear systems . 1972 . Springer . 978-3-540-90054-2 . Applied mathematical sciences . New York Heidelberg . 8–9.