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Period-one to radian action-angle rescaling: θrad=2πθ and Irad=I/(2π) preserve ω

Statement

Let (I,θ) be period-one action-angle coordinates with ω=∑i=1ndθi∧dIi as supplied by Action-angle coordinates on a saturated proper submersion neighbourhood of a compact connected regular fibre, with the two-form primitive supplied locally and Action and angle coordinates. Use the quotient lift charts and global angle differentials specified in Action-angle coordinates on a saturated proper submersion neighbourhood of a compact connected regular fibre, with the two-form primitive supplied locally. Put Trad:=R/(2πZ), with quotient lift charts from intervals of length less than 2π; their transitions are translations by 2πk, k∈Z. The induced diffeomorphism s:R/Z→Trad is s([x])=[2πx], with inverse [y]↦[y/(2π)]. Thus the notation 2πθi below means s∘θi, with values in Trad, and its differential is defined by local real lifts. The action domain rescales from A to A/(2π). Define the rescaled coordinates

θirad:=2π θi,Iirad:=Ii2π(i=1,…,n).

Then (θrad,Irad) are C∞ coordinates on the same chart,

ω=∑i=1ndθirad∧dIirad,

the angles θrad are 2π-periodic (radian-valued), and if a Hamiltonian is H=h(I) with h of class C2 in the period-one coordinates then in the radian coordinates it is H=h~(Irad) with h~(Irad)=h(2πIrad) and θ˙irad=∂h~/∂Iirad.

Facts & Assumptions

Given: Period-one action-angle coordinates (I,θ) on a chart U, with ω=∑idθi∧dIi, and the rescaled functions θrad=2πθ, Irad=I/(2π).

[F1]

Coordinate calculus (Ck Euclidean maps and diffeomorphisms, Directional derivatives and partial derivatives of a map U⊆Rm→Rn): the rescaling is a linear invertible change of coordinates with constant differential dθirad=2π dθi and dIirad=(2π)−1dIi.

[F3]

Hamilton's equations in canonical coordinates (The Hamiltonian field of a C2 function on a phase domain is J∇H, and the Poisson bracket is ∑i(∂qif ∂pig−∂pif ∂qig)): on a phase domain with coordinates (q,p) and standard two-form ∑idqi∧dpi, a C2 function K has Hamiltonian field XK=J∇K, so its integral curves satisfy q˙i=Kpi and p˙i=−Kqi.

Proof

technique · direct; substitute the constant rescalings into the coordinate expression of $\omega$
1.1F1F2algebra

By [F1], dθirad∧dIirad=(2π dθi)∧((2π)−1dIi)=dθi∧dIi for each i; summing over i and using [F2] gives ω=∑idθirad∧dIirad. The angle map is well-defined because changing a lift by an integer changes its scaled lift by an element of 2πZ, and its inverse is the stated quotient map. On each lift chart its differential is the constant scalar 2π. Since this angle diffeomorphism and the action rescaling are bijective and smooth with smooth inverses, (θrad,Irad) are coordinates on the same chart.

2.1F1F3step 1.1algebra∎

The relation H=h(I) becomes H=h(2πIrad)=h~(Irad); in the radian chart the two-form is ∑idθirad∧dIirad by step 1.1, so [F3] applies there with (q,p)=(θrad,Irad): I˙irad=0 and θ˙irad=∂h~/∂Iirad. The chain rule gives ∂h~/∂Iirad=2π (∂ih)(2πIrad)=2πθ˙i, consistent with θirad=2πθi.

Remarks

  • The rescaling is the standard conversion between the period-one convention of Action and angle coordinates and the radian convention used, for instance, in the adiabatic-invariant estimates of the approximation layer; the symplectic form, and with it the canonical structure, is unchanged.

Depends on

Dependency tree · 2 levels

Sources