The form factors may be written in a partial wave expansion in
the variable 
.
We consider a definite isospin 
state. Suppressing isospin
indices, one has [56,57]
where
The partial wave amplitudes 
and 
depend on
and 
. Their phase coincides with the phase shifts
in elastic 
scattering (angular momentum
l, isospin I).
More precisely, the quantities
are real in the physical region of 
decay. The form
factors 
and 
therefore have a simple expansion
On the other hand, the phase of the projected amplitudes
is not given by 
, e.g., 
is
not real in the isospin one case. A similar remark applies to
.
