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