We organise the constraints introduced in the previous section in the following four systems:
Each system is a linear homogeneous system subject to a quadratic constraint, that is,
where 
is a vector composed by the the first three components of
x, and k is a real number.
The four systems above are solved in sequence;
the solution of each can be computed using
the decomposition of 
suggested by [4], or casting
the problem as a
generalised eigenvector problem.