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1 ## Copyright (C) 1993, 1998, 1999 Auburn University. All rights reserved. |
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2 ## |
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3 ## This file is part of Octave. |
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4 ## |
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5 ## Octave is free software; you can redistribute it and/or modify it |
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6 ## under the terms of the GNU General Public License as published by |
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7 ## the Free Software Foundation; either version 3 of the License, or (at |
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8 ## your option) any later version. |
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9 ## |
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10 ## Octave is distributed in the hope that it will be useful, but |
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11 ## WITHOUT ANY WARRANTY; without even the implied warranty of |
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12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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13 ## General Public License for more details. |
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14 ## |
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15 ## You should have received a copy of the GNU General Public License |
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16 ## along with Octave; see the file COPYING. If not, see |
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17 ## <http://www.gnu.org/licenses/>. |
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18 |
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19 ## -*- texinfo -*- |
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20 ## @deftypefn {Function File} {[@var{u}, @var{h}, @var{nu}] =} krylov (@var{a}, @var{v}, @var{k}, @var{eps1}, @var{pflg}) |
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21 ## Construct an orthogonal basis @var{u} of block Krylov subspace |
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22 ## |
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23 ## @example |
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24 ## [v a*v a^2*v ... a^(k+1)*v] |
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25 ## @end example |
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26 ## |
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27 ## @noindent |
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28 ## Using Householder reflections to guard against loss of orthogonality. |
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29 ## |
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30 ## If @var{v} is a vector, then @var{h} contains the Hessenberg matrix |
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31 ## such that @code{a*u == u*h}. Otherwise, @var{h} is meaningless. |
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32 ## |
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33 ## The value of @var{nu} is the dimension of the span of the krylov |
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34 ## subspace (based on @var{eps1}). |
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35 ## |
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36 ## If @var{b} is a vector and @var{k} is greater than @var{m-1}, then |
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37 ## @var{h} contains the Hessenberg decomposition of @var{a}. |
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38 ## |
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39 ## The optional parameter @var{eps1} is the threshold for zero. The |
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40 ## default value is 1e-12. |
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41 ## |
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42 ## If the optional parameter @var{pflg} is nonzero, row pivoting is used |
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43 ## to improve numerical behavior. The default value is 0. |
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44 ## |
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45 ## Reference: Hodel and Misra, "Partial Pivoting in the Computation of |
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46 ## Krylov Subspaces", to be submitted to Linear Algebra and its |
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47 ## Applications |
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48 ## @end deftypefn |
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49 |
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50 ## Author: A. Scottedward Hodel <a.s.hodel@eng.auburn.edu> |
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51 |
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52 function [Uret, H, nu] = krylov (A, V, k, eps1, pflg); |
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53 |
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54 defeps = 1e-12; |
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55 |
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56 if (nargin < 3 || nargin > 5) |
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57 print_usage (); |
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58 elseif (nargin < 5) |
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59 pflg = 0; # default permutation flag |
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60 endif |
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61 |
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62 if(nargin < 4) |
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63 eps1 = defeps; # default tolerance parameter |
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64 endif |
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65 |
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66 if (isempty (eps1)) |
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67 eps1 = defeps; |
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68 endif |
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69 |
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70 na = issquare (A); |
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71 if (! na) |
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72 error ("A(%d x %d) must be square", rows (A), columns (A)); |
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73 endif |
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74 |
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75 [m, kb] = size(V); |
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76 if (m != na) |
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77 error("A(%d x %d), V(%d x %d): argument dimensions do not match", |
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78 na, na, m, kb) |
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79 endif |
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80 |
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81 if (! isscalar (k)) |
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82 error ("krylov: third argument must be a scalar integer"); |
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83 endif |
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84 |
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85 Vnrm = norm (V, Inf); |
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86 |
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87 ## check for trivial solution |
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88 if (Vnrm == 0) |
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89 Uret = []; |
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90 H = []; |
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91 nu = 0; |
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92 return; |
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93 endif |
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94 |
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95 # identify trivial null space |
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96 abm = max (abs ([A, V]')); |
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97 zidx = find (abm == 0); |
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98 |
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99 # set up vector of pivot points |
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100 pivot_vec = 1:na; |
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101 |
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102 iter = 0; |
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103 alpha = []; |
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104 nh = 0; |
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105 while (length(alpha) < na) && (columns(V) > 0) && (iter < k) |
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106 iter++; |
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107 |
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108 ## get orthogonal basis of V |
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109 jj = 1; |
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110 while (jj <= columns (V) && length (alpha) < na) |
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111 ## index of next Householder reflection |
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112 nu = length(alpha)+1; |
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113 |
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114 short_pv = pivot_vec(nu:na); |
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115 q = V(:,jj); |
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116 short_q = q(short_pv); |
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117 |
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118 if (norm (short_q) < eps1) |
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119 ## insignificant column; delete |
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120 nv = columns (V); |
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121 if (jj != nv) |
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122 [V(:,jj), V(:,nv)] = swap (V(:,jj), V(:,nv)); |
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123 ## FIXME -- H columns should be swapped too. Not done |
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124 ## since Block Hessenberg structure is lost anyway. |
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125 endif |
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126 V = V(:,1:(nv-1)); |
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127 ## one less reflection |
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128 nu--; |
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129 else |
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130 ## new householder reflection |
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131 if (pflg) |
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132 ## locate max magnitude element in short_q |
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133 asq = abs (short_q); |
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134 maxv = max (asq); |
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135 maxidx = find (asq == maxv, 1); |
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136 pivot_idx = short_pv(maxidx); |
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137 |
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138 ## see if need to change the pivot list |
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139 if (pivot_idx != pivot_vec(nu)) |
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140 swapidx = maxidx + (nu-1); |
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141 [pivot_vec(nu), pivot_vec(swapidx)] = ... |
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142 swap (pivot_vec(nu), pivot_vec(swapidx)); |
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143 endif |
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144 endif |
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145 |
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146 ## isolate portion of vector for reflection |
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147 idx = pivot_vec(nu:na); |
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148 jdx = pivot_vec(1:nu); |
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149 |
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150 [hv, av, z] = housh (q(idx), 1, 0); |
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151 alpha(nu) = av; |
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152 U(idx,nu) = hv; |
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153 |
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154 # reduce V per the reflection |
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155 V(idx,:) = V(idx,:) - av*hv*(hv' * V(idx,:)); |
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156 if(iter > 1) |
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157 ## FIXME -- not done correctly for block case |
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158 H(nu,nu-1) = V(pivot_vec(nu),jj); |
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159 endif |
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160 |
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161 ## advance to next column of V |
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162 jj++; |
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163 endif |
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164 endwhile |
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165 |
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166 ## check for oversize V (due to full rank) |
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167 if ((columns (V) > na) && (length (alpha) == na)) |
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168 ## trim to size |
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169 V = V(:,1:na); |
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170 elseif (columns(V) > na) |
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171 krylov_V = V |
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172 krylov_na = na |
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173 krylov_length_alpha = length (alpha) |
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174 error ("This case should never happen; submit a bug report"); |
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175 endif |
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176 |
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177 if (columns (V) > 0) |
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178 ## construct next Q and multiply |
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179 Q = zeros (size (V)); |
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180 for kk = 1:columns (Q) |
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181 Q(pivot_vec(nu-columns(Q)+kk),kk) = 1; |
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182 endfor |
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183 |
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184 ## apply Householder reflections |
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185 for ii = nu:-1:1 |
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186 idx = pivot_vec(ii:na); |
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187 hv = U(idx,ii); |
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188 av = alpha(ii); |
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189 Q(idx,:) = Q(idx,:) - av*hv*(hv'*Q(idx,:)); |
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190 endfor |
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191 endif |
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192 |
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193 ## multiply to get new vector; |
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194 V = A*Q; |
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195 ## project off of previous vectors |
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196 nu = length (alpha); |
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197 for i = 1:nu |
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198 hv = U(:,i); |
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199 av = alpha(i); |
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200 V = V - av*hv*(hv'*V); |
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201 H(i,nu-columns(V)+(1:columns(V))) = V(pivot_vec(i),:); |
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202 end |
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203 |
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204 endwhile |
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205 |
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206 ## Back out complete U matrix |
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207 ## back out U matrix ; |
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208 j1 = columns (U); |
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209 for i = j1:-1:1; |
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210 idx = pivot_vec(i:na); |
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211 hv = U(idx,i); |
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212 av = alpha(i); |
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213 U(:,i) = zeros (na, 1); |
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214 U(idx(1),i) = 1; |
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215 U(idx,i:j1) = U(idx,i:j1)-av*hv*(hv'*U(idx,i:j1)); |
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216 endfor |
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217 |
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218 nu = length (alpha); |
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219 Uret = U; |
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220 if (max (max (abs (Uret(zidx,:)))) > 0) |
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221 warning ("krylov: trivial null space corrupted; set pflg = 1 or eps1 > %e", |
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222 eps1); |
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223 endif |
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224 |
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225 endfunction |
