Negativity : entanglement measure

What transformations on a bipartite state, rho preserve the
entanglement? SU(2)XSU(2) does - and this would have 6 parameters.
Also, if the eigenvalues are w,x,y,z subject to the condition that
their sum is 1 and another condition from the entanglement(some value
k), there remain 2 free parameters.
THis gives a total of 8 parameters. However, the set of all states
having a fixed entanglement, k have 14 parameters. Any clues about the
remaining 8?
-KM

any introductory reference whihc deals with the question would be very
useful.

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