Shift invert iteration

[kiso]

Hi

I'm having problems with the shift invert iteration. I can't understand how the matrix A-sI is computed. If "s" is a coifficient (computed with Rayleighs quotient for etc.) then the product sI for a 3x3 matrix would be

s 0 0
0 s 0
0 0 s right????

Is it this simple, because it doesn't work?
Without the shift "s" my program works.

Thanks
Kim