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CHAPTER 4. THEORY 109<br />

Table 4.1 shows the performance of GMRES when applied to the finite-dimensional<br />

problem where the preconditioner ˜ M =(K − 1<br />

τ I)−1 . Here, we show the average num-<br />

ber of Newton iterations per continuation step and the average number of GMRES<br />

iterations per Newton iteration. As the table shows, these values remain relatively<br />

flat as the grid is refined.<br />

Table 4.1: Krylov and Newton Iterations as Mesh Is Refined<br />

Nx Nk Avg. Newton Its. Per Cont. Step Avg. Krylov Its. Per Newton<br />

86 72 2.6 176<br />

172 144 2.5 174<br />

344 288 2.5 173<br />

688 576 2.5 186<br />

This is a reflection of the rapid convergence we expect in the infinite-dimensional<br />

setting. If we were able to prove that Z : L 2 → X was a compact operator, we could<br />

then show Z ′ : L 2 → X is a compact linear operator, and we would be able to apply<br />

the theory given in [21]. In [21], the authors look at a separable real Hilbert space ˜ H<br />

with a subspace X that has a compact linear operator Z ′ that maps ˜ H to X where<br />

they want to solve the equation (I − Z ′ )u = f. In this paper, by using a modified<br />

version of GMRES, the authors prove the error (not the residual) of the GMRES<br />

iterates converge in the X to 0.

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