Fig. 3.
(left) PCG residuals with M2l and MBD preconditioners for several choices of the size of the deflation subspace. Ritz eigenpairs are computed up to a fixed Arnoldi tolerance ϵArn. (right) The M2l is built by selecting a fixed number of Ritz eigenvectors, i.e. dim 𝒵 = 25, computed by running the Arnoldi algorithm with several choices for ϵArn.
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