Quasi-Optimal Elimination Trees for 2D Grids with Singularities

Figure 9

Elimination of fully assembled rows from matrix of size . In this example and , and the row subtraction is performed in three steps, denoted on panels , , and . The dark gray squares denote rows to be eliminated, the red squares denote the value checked by partial pivoting, and the white squares denote zeros generated during the row subtractions.