Fig. 6

Convergence of PCG solver preconditioned with the two-level a posteriori preconditioner computed for the small-circles scan with K = 128 circles and fknee = 1.0. Blue lines correspond to the solutions obtained with the deflation subspace operator, ZD, with different ranks as defined by the number of approximated eigenvectors with eigenvalues smaller than ϵtol, as indicated in the legend. Red curve shows the solution with the standard preconditioner, MBD.
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