### Saddle point problem in Navier-Stokes equation

#### by Yi Zhang

It is well known that the the optimization problem

is equal to solving a linear operator equation

provided that is symmetric and positive definite. Here which is a Hilbert space, and while is a bilinear operator.

This is a very general problem considered widely in many areas. However, in the real world, some further condition are to be applied on the space where lies on. If this is described as constraint equation

then we are dealing with constraint optimization problem ,to which Lagrange multiplier is usually the first one can come up. We then try to solve the stationary point of Lagrangian

Now supposing we set , using stationary point condition on above equation, we arrive at

where is replaced by indicating the parallel unknown as in the problem. The stationary point nature of this solution gives the name of saddle point problem, whose general case is

So, putting a constraint to a minimization problem, one try to solve a saddle problem instead of a simple linear system with SPD matrix.

For a linear system, one can always decompose it in to the form of

and this is the most general form of saddle problem.

For incompressible Navier-Stokes equation, we have (without mentioning the boundary conditions)

Following -scheme by Glowinski, one has the time discretized plan as:

then

then

The first and last step in above scheme are to solve Stokes problem like

Multiply on both sides of the first equation, by using Green’s formula, we can see it is just the saddle point problem, with as and as :

Here pressure acts as Lagrange multiplier, and the Lagrangian is

where is the seminorm on and is in .