# Homework Help: QM: Infinitesimal Generator for Scale Transformation

1. Nov 17, 2008

### brooke1525

1. The problem statement, all variables and given/known data

The scale transformation is a continuous transformation which acts on a function f(x) according to

$$D_{s}$$f(x) = f(sx)

where s is a real number. There is a continuous family of such transformations, including the identity transformation corresponding to s = 1. Calculate the infinitesimal generator for scale transformation in terms of familiar quantum operators.

2. Relevant equations

3. The attempt at a solution

This question was in a foreign language to me. I don't recall ever hearing of such a thing as an 'infinitesimal generator' in my quantum mechanics course, so I have absolutely NO clue what this question means or how to do it. Any guidance is much appreciated.

2. Nov 18, 2008

### weejee

To demonstrate how we found the generator, lets consider the case of the translation operator

$$T(x)[f(x)] = f(x-a)$$ (translation by a)

For an infinitesimal translation $$\delta$$

$$T(\delta)[f(x)] = f(x-\delta) = f(x) - \delta \frac{df(x)}{dx} = \left( 1 - \delta \frac{d}{dx} \right) f(x) = \left( 1 - i\delta\frac{p}{\hbar} \right) f(x)$$

In this case, $$\frac{p}{\hbar}$$ (or just p) is called the generator of the infinitesimal translation.

Maybe you can proceed accordingly for the scale trasformation?

For further information you may consider any graduate-level textbooks for QM. (e.g. Sakurai)

Last edited: Nov 18, 2008
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