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Two distinct oscillator solutions with the same zero endpoints at , and their common action value
Example
Take , , hence , and in The undamped oscillator: Dirichlet non-uniqueness at despite unique initial-value solutions, and consider the two curves
Both are solutions of on with , and they are distinct; they have different initial velocities and . On the chart over (A time-dependent Lagrangian chart, its action functional and its fixed-endpoint first variations) both are stationary curves of the action, and their action values agree:
This is the concrete case of the non-uniqueness in The undamped oscillator: Dirichlet non-uniqueness at despite unique initial-value solutions and of the signed-action checks of An explicit saddle of the oscillator action at : the two variations and ; all numbers are exact.
Facts & Assumptions
Given: The model on , its curves and , and the chart with its action .
The non-uniqueness proposition (The undamped oscillator: Dirichlet non-uniqueness at despite unique initial-value solutions): for and with , the set of solutions of on with is exactly , and each such curve is a stationary curve of the action of the chart (Stationarity of the time-dependent action is equivalent to the vanishing of the Euler-Lagrange expression, A time-dependent Lagrangian chart, its action functional and its fixed-endpoint first variations).
Trigonometric calculus and integrals (The derivatives of sine and cosine are cosine and minus sine, Sine and cosine defined by their real power series, The addition formulas for sine and cosine, Parity and the Pythagorean identity for sine and cosine, The zero sets of sine and cosine and the least positive common period 2 pi, The chain rule for total derivatives: , The second fundamental theorem: if is differentiable on with and is integrable, then , Integrable functions on form a set closed under sums and scalar multiples, and , The lower and upper Darboux integrals of a bounded on as and , Darboux integrability as their equality, and the notation , A continuous function on is Riemann integrable, by Heine-Cantor and Riemann's criterion, Intervals of : the nine order-convex forms, nondegeneracy, and length): and are differentiable with , , the zeros of are the integer multiples of and in particular ; the double-angle formula gives ; the fundamental theorem evaluates because ; and continuous integrands on are integrable.
Verification
Proof technique: direct; verify the two solutions and their endpoint data, note their distinctness, and evaluate the action along the family.
The two solutions: by [L2], and , so both curves solve on ; and , by the zero set of .
Distinctness with equal boundary data: (at the value is ), so the solutions are distinct, while both take the value at and at ; their initial velocities are and , so they are different initial-value solutions, in accordance with the initial-value uniqueness clause of [L1].
Action along the family: for , using and linearity of the integral, by [L2]; in particular .
Reading: by [L1] the curves are stationary for the action of the chart and share the endpoint data ; they are distinct and have the same action value , exhibiting in one explicit computation the failure of uniqueness of the fixed-endpoint problem at , with no claim about any other duration.
Depends on
The undamped oscillator: Dirichlet non-uniqueness at despite unique initial-value solutions
A time-dependent Lagrangian chart, its action functional and its fixed-endpoint first variations
Stationarity of the time-dependent action is equivalent to the vanishing of the Euler-Lagrange expression
The derivatives of sine and cosine are cosine and minus sine
Sine and cosine defined by their real power series
The addition formulas for sine and cosine
Parity and the Pythagorean identity for sine and cosine
The zero sets of sine and cosine and the least positive common period 2 pi
The chain rule for total derivatives:
The second fundamental theorem: if is differentiable on with and is integrable, then
Integrable functions on form a set closed under sums and scalar multiples, and
The lower and upper Darboux integrals of a bounded on as and , Darboux integrability as their equality, and the notation
A continuous function on is Riemann integrable, by Heine-Cantor and Riemann's criterion
Intervals of : the nine order-convex forms, nondegeneracy, and length
Dependency tree · 2 levels
Sources
- K. Likharev, Essential Graduate Physics: Classical Mechanics, Chapter 5 Section 5.1, Eqs. (5.1)-(5.3), printed pages 1-2 of Chapter 5 (PDF pages 87-88): the harmonic oscillator equation and its general two-constant solution
- D. Tong, Classical Dynamics (Cambridge lecture notes, 2015), Section 2.1, printed pages 10-12: the action functional with fixed endpoints and Lagrange's equations for its stationary paths