Usage:
<j_invariant f j
computes the j-invariant of a quartic form f in 4 variables
(that is, the j invariant of its 4 roots.)
Parameters:
f = 1x1 matrix containing a cubic form in 3 variables
Output values:
j = 1x2 matrix representing the point of P1 which is
the j-invariant of f.
Uses the equations of the divisors of forms of j-invariant 0 and 1728