Physics often deals with classical models where the dynamical variables are a collection of functions {φ^α}_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 φ^α(x).

In the DeWitt notation (named after theoretical physicist Bryce DeWitt), φ^α(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

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

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,

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

References

• Kiefer, Claus (April 2007). Quantum gravity (hardcover) (2nd ed.). Oxford University Press. p. 361. ISBN 978-0-19-921252-1.