% Matlab code LanczosDemo.m % For "Applied Numerical Linear Algebra", Chapter 7 % Written by James Demmel, Jun 6, 1997 % % This program sets up the data for the plots of the % Lanczos algorithm in Chapter 7 % % e contains the eigenvalues, in decreasing order clear e; clear A; e = [ 2.814172761275914e+00 2.702868092389970e+00 2.599089388576309e+00 2.590000000000000e+00 2.567541525093273e+00 2.464503386572409e+00 2.297364163401737e+00 2.264314579523848e+00 2.223670214225475e+00 2.221554601045395e+00 2.205565815386506e+00 2.191477513171730e+00 2.176997451239667e+00 2.141411405120702e+00 2.114364167621812e+00 2.082245457397133e+00 2.038797314674318e+00 1.983336237065439e+00 1.981158505826463e+00 1.976752525746760e+00 1.960011713185635e+00 1.946408217258373e+00 1.899539105865933e+00 1.896394134910090e+00 1.885266074552361e+00 1.875151167753231e+00 1.874919401677734e+00 1.872410928929237e+00 1.809780809862668e+00 1.804991115962216e+00 1.800713681981315e+00 1.794575633324331e+00 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-3.026665892769740e+00 ]; % % A is a 1000x1000 diagonal sparse matrix with e as eigenvalues % A = spdiags(e,0,1000,1000); % % q is the starting vector for Lanczos (unnormalized) % q = ones(1000,1); q(999) = 1e-5; % % m is the number of Lanczos steps to take % m = 99; % % The following paramters control the printing % % mm1,mm2 = indices of extreme eigenvalues for which to plot error % and error bounds mm1 = 1; mm2 = 4; % % mine, maxe = range in which to plot eigenvalues mine = -4; maxe = 4; % % steplabel = array of values at which to draw vertical lines in plots steplabel = (10:10:150); % % recomp = 1 to perform algorithm from scratch % = 0 just to redisplay data, with possibly different % mine, maxe, mm1, mm2, steplabel recomp = 1; % % whichbound = 2 to plot all errors and error bounds % = 1 to plot local error and error bounds % = 0 to plot error bounds only whichbound = 2; % % displayselection = 1 to display details of which Ritz vectors % are selected, as Lanczos runs % = 0 not to display details displayselection = 0;