- #1
pellman
- 684
- 5
This is strictly a math question but I figured that since it is something which would show up in QM, the quantum folks might be already familiar with it.
Suppose we have an operator valued function A(x) of a real parameter x and another function f, both of which have well defined derivatives.
consider [tex]\frac{d}{dx}f(A(x))[/tex]
Does this equal
[tex]\frac{df}{dA}\frac{dA}{dx}[/tex]
or
[tex]\frac{dA}{dx}\frac{df}{dA} [/tex]
or something else? Of course, if A and dA/dx commute, then either expression is good. But it is not clear to me that A and dA/dx would necessarily commute.
Suppose we have an operator valued function A(x) of a real parameter x and another function f, both of which have well defined derivatives.
consider [tex]\frac{d}{dx}f(A(x))[/tex]
Does this equal
[tex]\frac{df}{dA}\frac{dA}{dx}[/tex]
or
[tex]\frac{dA}{dx}\frac{df}{dA} [/tex]
or something else? Of course, if A and dA/dx commute, then either expression is good. But it is not clear to me that A and dA/dx would necessarily commute.