Our notation is the following:
denotes a multi-index, made of
(spatial) indices. Similarly, we write for
instance
(in practice, we generally do not need to write explicitly the “carrier” letter
or
), or
. Always understood in expressions such as Eq. (34) are
summations over the indices
ranging from 1 to 3. The derivative operator
is a short-hand for
. The function
(for any space-time
indices
) is symmetric and trace-free (STF) with respect to the
indices composing
. This means that for any pair
of indices
, we have
and that
(see Ref. [403*] and
Appendices A and B in Ref. [57*] for reviews about the STF formalism). The STF projection is denoted with a hat,
so
, or sometimes with carets around the indices,
. In particular,
is the
STF projection of the product of unit vectors
, for instance
and
; an expansion into STF tensors
is equivalent to the usual
expansion in spherical harmonics
, see Eqs. (75) below. Similarly, we denote
where
, and
. The Levi-Civita antisymmetric symbol is denoted
(with
).
Parenthesis refer to symmetrization,
. Superscripts
indicate
successive time
derivations.