Sunday, July 8, 2018

classical mechanics - What are Hamilton's equations with respect to a nonstandard symplectic form?


Hamilton's equations for a Hamiltonian H(q,p) w.r.t. to a standard symplectic from ω=dqdp are ˙q=Hp,˙p=Hq


How do Hamilton's equations write w.r.t. a nonstandard symplectic form F(q,p)dqdp, where F(q,p) is some smooth function?



Answer






  1. More generally, let there be given a Poisson manifold (M,π), where π = 12πIJzIzJ

    is a Poisson bi-vector, and {f,g}PB = fzJπIJgzJ
    is the corresponding Poisson bracket. Let the Hamiltonian H be a globally defined function on M. Then Hamilton's equations read ˙zI = {zI,H}PB,
    i.e. time-evolution is given by (minus) the Hamiltonian vector field XH = {H,}PB.




  2. If the Poisson structure is invertible, then M is a symplectic manifold with symplectic 2-form ω = 12ωIJ dzIdzJ,

    where ωIJ is the inverse matrix: πIJωJK = δIK.




  3. In canonical/Darboux coordinates (z1,,z2n) = (q1,,qn,p1,,pn),

    the above construction reduces to the standard Poisson bi-vector π = qipi,
    and the standard symplectic 2-form ω = dpidqi.





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