I have seen two definitions for the functional derivative. Why are there two definitions?
- In Goldstein's Classical mechanics 3rd edition page 574 eq. (13.63), and also in a Student's Guide to Lagrangians and Hamiltonans by Patrick Hamill on page 55 eq. (2.10), the functional derivative of a function Φ(y,y′,x), where y=y(x), is given by δΦδy=∂Φ∂y−ddx∂Φ∂y′.
- The second definition of a functional derivative is given by δF[y(x)]δy(x′)=limε→01ε(F[y(x)+εδ(x−x′)]−F[y(x)]).This definition is found on wikipedia and is used in QFT. This definition tells us that for the functional S[y(x)]=∫Φ(y,y′,x)dxwhere y=y(x), the functional derivative is given by δSδy=∂Φ∂y−ddx∂Φ∂y′
One definition is in terms of a function and the other in terms of a functional, but both give the same output?
No comments:
Post a Comment