I attach some Pari code which computes various things, including
Nagell's algorithm, including forward and reverse transforms.
Accompanying it is some test code which attempts to verify that the
inverse transform does what it is supposed to, i.e. it composes with
the forward transform to give the
Andrey,
Check out the hg bundle attached to this ticket:
http://trac.sagemath.org/sage_trac/ticket/1136
Bobby Moretti was implementing code for doing this transformation, but
never finished due to a subtle bug in the transformation maps (he gets the
right cubic but not the right maps). It's
