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The isotropic oscillator: the explicit flow, YTJY=J and det⁡Y=1

Example

Let n=1, U=R2 with coordinates (q,p), and H(q,p)=(p2+q2)/2. Then:

  1. XH=(p,−q) and its flow is Φt(q,p)=(qcos⁡t+psin⁡t, −qsin⁡t+pcos⁡t), defined for all t∈R;
  2. the derivative Y(t)=DΦt(q,p)=(cos⁡tsin⁡t−sin⁡tcos⁡t) satisfies Y′(t)=J Hess H Y(t), Y(0)=I2, Y(t)TJ Y(t)=J and det⁡Y(t)=1 for all t, in accordance with The derivative of the Hamiltonian flow satisfies YTJY=J and det⁡Y=1.

Facts & Assumptions

Given: The Hamiltonian H(q,p)=(p2+q2)/2 on R2 and the matrix J=(01−10).

[F1]

Hamiltonian field (The Hamiltonian field of a C2 function on a phase domain is J∇H, and the Poisson bracket is ∑i(∂qif ∂pig−∂pif ∂qig)): Hq=q, Hp=p, so XH=(Hp,−Hq)=(p,−q); hence the flow solves q′=p, p′=−q.

[F2]

Trigonometric calculus (The derivatives of sine and cosine are cosine and minus sine, The addition formulas for sine and cosine, Sine and cosine defined by their real power series): sin⁡′=cos⁡, cos⁡′=−sin⁡, and Φt as displayed is the solution of the linear system with Φ0=id, since differentiating the displayed expressions gives q′=−qsin⁡t+pcos⁡t=p(t) and p′=−qcos⁡t−psin⁡t=−q(t).

[F3]

The matrix identity (The derivative of the Hamiltonian flow satisfies YTJY=J and det⁡Y=1, The standard two-form of a phase domain is closed and nondegenerate, with contraction ιvω=∑iai dpi−∑ibi dqi, The Hessian matrix and critical points of a scalar field): for a C2 Hamiltonian the flow derivative satisfies Y′=J Hess H Y, YTJY=J and det⁡Y=1, the Hessian being the matrix of second partial derivatives; here Hess H=I2.

Verification

technique · direct; verify the displayed flow by differentiation and multiply the two-by-two matrices
1.1F1F2algebra

Clause 1: by [F1] the field is XH=(p,−q) and the flow solves the linear system; the displayed Φt satisfies Φ0=id and, by [F2], its derivatives are q′=p(t) and p′=−q(t), so by uniqueness it is the flow, defined for all real t.

1.2F3algebra

Clause 2: differentiating Y(t) entrywise gives Y′(t)=(−sin⁡tcos⁡t−cos⁡t−sin⁡t), while J Hess H Y=JY=(−sin⁡tcos⁡t−cos⁡t−sin⁡t); thus Y′=JY with Y(0)=I2.

2.1F3step 1.2algebra∎

The orthogonality of Y(t) gives YTJY=J by direct multiplication, and det⁡Y=cos⁡2t+sin⁡2t=1; hence both conclusions of [F3] hold explicitly for this flow.

Depends on

Dependency tree · 2 levels

Sources