Stability of multi-field cosmological solutions in the presence of a
  fluid
  We explore the stability properties of multi-field solutions in the presence
of a perfect fluid, as appropriate to assisted quintessence scenarios. We show
that the stability condition for multiple fields $\phi_i$ in identical
potentials $V_i$ is simply $d^2V_i/d \phi_i^2 > 0$, exactly as in the absence
of a fluid. A possible new instability associated with the fluid is shown not
to arise in situations of cosmological interest.
