DeWitt notation

Physics often deals with classical models where the dynamical variables are a collection of functions

{φα}α over a d-dimensional space/spacetime manifold M where α is the "flavor" index. This involves functionals over the φ's, functional derivatives, functional integrals, etc. From a functional point of view this is equivalent to working with an infinite-dimensional smooth manifold where its points are an assignment of a function for each α, and the procedure is in analogy with differential geometry where the coordinates for a point x of the manifold M are φα(x).

In the DeWitt notation (named after theoretical physicist Bryce DeWitt), φα(x) is written as φi where i is now understood as an index covering both α and x.

So, given a smooth functional A, A,i stands for the functional derivative

A

,

i

[

φ

]

=

d

e

f

δ

δ

φ

α

(

x

)

A

[

φ

]

{\displaystyle A_{,i}[\varphi ]\ {\stackrel {\mathrm {def} }{=}}\ {\frac {\delta }{\delta \varphi ^{\alpha }(x)}}A[\varphi ]}

as a functional of φ. In other words, a "1-form" field over the infinite dimensional "functional manifold".

In integrals, the Einstein summation convention is used. Alternatively,

A

i

B

i

=

d

e

f

∫

M

∑

α

A

α

(

x

)

B

α

(

x

)

d

d

x

{\displaystyle A^{i}B_{i}\ {\stackrel {\mathrm {def} }{=}}\ \int _{M}\sum _{\alpha }A^{\alpha }(x)B_{\alpha }(x)d^{d}x}

References

Kiefer, Claus (April 2007). Quantum gravity (hardcover) (2nd ed.). Oxford University Press.