I am reading Sakurai's Modern quantum mechanics and at some point it's trying to draw a parallel between classical and quantum mechanics.

It says

An infinitesimal translation in classical mechanics can be regarded as a canonical transformation, $$ \mathbf{x}_{\mathrm{new}} \equiv \mathbf{X} = \mathbf{x} + d\mathbf{x}, \quad \mathbf{p}_{\mathrm{new}} \equiv \mathbf{P} = \mathbf{p}, \tag{1.6.28} $$ obtainable from the generating function $$ F_2(\mathbf{x}, \mathbf{P}) = \mathbf{x}\cdot \mathbf{P} + \mathbf{p}\cdot d\mathbf{x}. \tag{1.6.29} $$

From the wikipedia page it seems that a generating function is something that one can differentiate to obtain the equation of motion of the system.

I mean I assume I need to differentiate $F$ with respect to $\mathbf{X}$ and $\mathbf{P}$? What equation of motion am I supposed to get from here?

  • $\begingroup$ You just read the instructions to differentiate w.r.t. the arguments, so x and P to get p and X, the canonical tfmation displayed. $\endgroup$ Commented Feb 12, 2018 at 23:22
  • 1
    $\begingroup$ Where do I even get $F$ from? I can see that somehow I need to have $\mathbf{X} \cdot$ something ? $\endgroup$
    – SuperCiocia
    Commented Feb 12, 2018 at 23:24
  • $\begingroup$ If it were not trivial to guess, a basic analytical mechanics book describes the methods.... Did you read up on cts? $\endgroup$ Commented Feb 12, 2018 at 23:27

1 Answer 1



  1. The infinitesimal change in position $d{\bf x}=\varepsilon~ {\bf f}({\bf x})$ can be thought of as an infinitesimal parameter $\varepsilon$ times a vector-valued function of the old position ${\bf x}$.

  2. A type-2 generating function $F_2({\bf x},{\bf P},t)$ for a canonical transformation depends on the old position ${\bf x}$, the new momentum ${\bf P}$, and possibly time $t$.

  3. If the above does not make sense, then you should study Hamiltonian mechanics and canonical transformations, as Cosmas Zachos suggests in above comment, e.g. Ref. 1.


  1. H. Goldstein, Classical Mechanics; Section 9.
  • $\begingroup$ Is there a reason you changed $\mathrm{d}{\bf x}$ to $d{\bf x}$? $\endgroup$
    – SuperCiocia
    Commented Feb 14, 2018 at 10:43
  • $\begingroup$ Yes. $d{\bf x}$ is the notation used by Sakurai. Personally I would prefer $\delta{\bf x}$ for an infinitesimal change. $\mathrm{d}{\bf x}$ looks like a one-form. $\endgroup$
    – Qmechanic
    Commented Feb 14, 2018 at 11:33

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