[2.5] What is a degree of freedom? The notion of "degrees of freedom" as it is used for Hamiltonian systems mea ns one canonical conjugate pair, a configuration, q, and its conjugate momen tum p. Hamiltonian systems (sometimes mistakenly identified with the notion of conservative systems) always have such pairs of variables, and so the pha se space is even dimensional. In the study of dissipative systems the term "degree of freedom" is often us ed differently, to mean a single coordinate dimension of the phase space. Th is can lead to confusion, and it is advisable to check which meaning of the term is intended in a particular context. Those with a physics background generally prefer to stick with the Hamiltoni an definition of the term "degree of freedom." For a more general system the proper term is "order" which is equal to the dimension of the phase space. Note that a dynamical system with N d.o.f. Hamiltonian nominally moves in a 2N dimensional phase space. However, if H(q,p) is time independent, then ene rgy is conserved, and therefore the motion is really on a 2N-1 dimensional e nergy surface, H(q,p) = E. Thus e.g. the planar, circular restricted 3 body problem is 2 d.o.f., and motion is on the 3D energy surface of constant "Jac obi constant." It can be reduced to a 2D area preserving map by Poincare sec tion (see [2.6]). If the Hamiltonian is time dependent, then we generally say it has an additi onal 1/2 degree of freedom, since this adds one dimension to the phase space . (i.e. 1 1/2 d.o.f. means three variables, q, p and t, and energy is no lon ger conserved). -- 在细雨的午後 书页里悉哩哩地传来 " 周期3 = ? " 然而我知道 当我正在日耳曼深处的黑森林 继续发掘海森堡未曾做过的梦时 康德的诺言早已远离......... 远来的传教士静静地看着山涧不断反覆叠代自己的 过去 现在 和 未来 於是仅以 一颗量子浑沌 一本符号动力学 祝那发生在周一下午的新生 --

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