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With block methods get. 1) basic triangular shape. 2) super long columns. 3) short columns which don't fit into rank k c
Variations on the retraction algorithm for symmetric indefinite banded matrices Linda Kaufman

Standard B-K produces a bulge when pivoting for stabilityretract or prevent it. x x x x x x

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Use planar transformations to eliminate With block methods get 1) basic triangular shape 2) super long columns 3) short columns which don’t fit into rank k correction or vanish

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