Hi, i'm trying to find a relation between natural frequencies (or adimentional damping) and the modal parameters (mass, stiffness, damping) in 3 dof system. i wrote this:

qu = {{m1, 0, 0, 0, 0, 0}, {0, m2, 0, 0, 0, 0}, {0, 0, m3, 0, 0, 
0}, {0, 0, 0, 1, 0, 0}, {0, 0, 0, 0, 1, 0}, {0, 0, 0, 0, 0, 1}};
esse = {{d1 + d2, -d2, -d3, k1 + k2, -k2, -k3}, {-d2, d2, 
d1 + d3, -k2, k2, k3 + k1}, {-d3, d3 + d1, d3, -k3, k3 + k1, 
k3}, {-1, 0, 0, 0, 0, 0}, {0, -1, 0, 0, 0, 0}, {0, 0, -1, 0, 0, 
0}};
a = -Inverse[qu].esse;
Eigenvalues[a]

The program calculates 6 solutions in Root form so i can't find the right relation. How can i set conditions (like m,k,d>0) to have solutions in a closed-form?

thaks