How to solve the equations:
- Problem: coupled system i.e v depends on T
                                           and T depends on v
- Analytic:
             -linearize equation
              -see if infinitesimal disturbance gets amplified
=> critical value for Ra ~ 600, independent of Pr
      - first instablities have a roll pattern
- other patterns also exist like: square patter, hexagon pattern, cross-roll pattern ...
- no extrema principal