Eigenvalues of a function

I/O
1

How can I find the eigenvalues for sigma:

W = VectorFunctionSpace(mesh, 'CG', 1)
u, v = TrialFunction(W), TestFunction(W)
def epsilon(u):
    return sym(grad(u))
def sigma(u):
    return 2.0*mu*epsilon(u)+lmbda*tr(epsilon(u))*Identity(len(u))