The matrix element for 
is
where
and 
.
The matrix elements for the other channels
(5.2,5.3) may be obtained from (5.11,5.12) by isospin
symmetry, see below.
The form factors F,G,R and H are real analytic functions of the three
variables 
, 
and 
. Below,
we will use instead the variables 
or
.
Remark: In order to agree with the notation used by Pais and Treiman [52] and by Rosselet et al. [53], we have changed our previous convention [54,55] in the definition of the anomaly form factor H. See also the comments after Eq. (5.21).
