Re: strict static pivoting vs pliable static pivoting

On Wednesday 13 September 2006 10:47, you wrote:
> And Cleve Ashcraft writes:
>  - Pivoting within a supernode could be called "restricted pivoting".
>
> It could also be called "probably useless."

Not in my experience. 

Consider a matrix where the degrees of freedom are mixed, e.g., 
state and control variables, or translations and rotations. The 
scaling may be such that it is crucial to eliminate one type of 
variable before another within the supernode to avoid element growth.

Also think of a saddle point problem, where there may be some zeros
on the diagonals of the supernodes. It is better to pivot within
the supernode than to modify the zero diagonal elements and carry on.

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