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The linear shear: the second-kind generating function and the matrix test
Example
Let , let , and let , . Then:
- is a generating function of the second kind with , and the local map it generates is , defined on all of (Generating functions of the first and second kind for a local canonical map);
- is canonical, both by the generating-function theorem (The local map generated by a first- or second-kind generating function is a canonical local diffeomorphism) and by direct verification of the matrix identity (A map between phase domains preserves the standard two-form iff , and a canonical diffeomorphism carries to ).
Facts & Assumptions
Given: The real number and the function .
Derivatives (Directional derivatives and partial derivatives of a map ): and , so and the second-kind relations , read , ; solving, and , so the generated map is on all of , a diffeomorphism (Generating functions of the first and second kind for a local canonical map, Euclidean maps and diffeomorphisms).
Matrix test (A map between phase domains preserves the standard two-form iff , and a canonical diffeomorphism carries to , The standard two-form of a phase domain is closed and nondegenerate, with contraction , Rectangular matrix multiplication and the identity matrix , including zero-sized shapes): a map between phase domains is canonical iff ; here .
Verification
Clause 1 is [F1]: , and the relations , give , which is a linear isomorphism of and therefore a diffeomorphism.
Clause 2: the derivative is , and by direct block multiplication; hence is canonical by [F2], while the generating-function theorem gives the same conclusion because and force .
Depends on
Generating functions of the first and second kind for a local canonical map
The local map generated by a first- or second-kind generating function is a canonical local diffeomorphism
A map between phase domains preserves the standard two-form iff , and a canonical diffeomorphism carries to
The standard two-form of a phase domain is closed and nondegenerate, with contraction
A phase domain, its canonical one-form and its standard symplectic two-form
Euclidean maps and diffeomorphisms
Directional derivatives and partial derivatives of a map
The Jacobian matrix of partial derivatives and the gradient in the scalar-valued case
Rectangular matrix multiplication and the identity matrix , including zero-sized shapes
Linear map between vector spaces over the same field
Dependency tree · 2 levels
Sources
- D. Tong, Classical Dynamics (University of Cambridge Part II Mathematical Tripos), Section 4.4.3 (worked generating functions and the infinitesimal shear as a canonical transformation), printed pages 104-105
- D. Cline, Variational Principles in Classical Mechanics (2nd edition, University of Rochester River Campus Libraries, open textbook, CC BY-NC-SA 4.0), Section 15.3 (canonical transformations and simple generating functions)