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Nullspace - Revision history
2024-03-29T08:36:46Z
Revision history for this page on the wiki
MediaWiki 1.25.3
http://mlwiki.org/index.php?title=Nullspace&diff=782&oldid=prev
Alexey at 09:51, 25 June 2017
2017-06-25T09:51:36Z
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<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 09:51, 25 June 2017</td>
</tr><tr><td colspan="2" class="diff-lineno" id="L24" >Line 24:</td>
<td colspan="2" class="diff-lineno">Line 24:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* i.e. all such $\mathbf x$ that solve $A \mathbf x = \mathbf 0$  </div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* i.e. all such $\mathbf x$ that solve $A \mathbf x = \mathbf 0$  </div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* $\mathbf 0 \in N(A)$ always</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* $\mathbf 0 \in N(A)$ always</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* <del class="diffchange diffchange-inline">$</del>\begin{bmatrix}</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* <ins class="diffchange diffchange-inline"><math></ins>\begin{bmatrix}</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 \\ 1 \\ -1</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 \\ 1 \\ -1</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix}<del class="diffchange diffchange-inline">$ </del>or any multiple of this vector <del class="diffchange diffchange-inline">$</del>c \cdot \begin{bmatrix}</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix}<ins class="diffchange diffchange-inline"></math> </ins>or any multiple of this vector <ins class="diffchange diffchange-inline"><math></ins>c \cdot \begin{bmatrix}</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 \\ 1 \\ -1</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 \\ 1 \\ -1</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix}<del class="diffchange diffchange-inline">$</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix}<ins class="diffchange diffchange-inline"></math></ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* so it's a subspace - a line in $\mathbb R^3$ through the origin</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* so it's a subspace - a line in $\mathbb R^3$ through the origin</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td colspan="2" class="diff-lineno" id="L66" >Line 66:</td>
<td colspan="2" class="diff-lineno">Line 66:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Let $A$ be some rectangular matrix and we find it's rref $R$</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Let $A$ be some rectangular matrix and we find it's rref $R$</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* <del class="diffchange diffchange-inline">$</del>A = \begin{bmatrix}</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* <ins class="diffchange diffchange-inline"><math></ins>A = \begin{bmatrix}</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & 2 & 3 & 1 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & 2 & 3 & 1 \\</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & 1 & 2 & 1 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & 1 & 2 & 1 \\</div></td></tr>
<tr><td colspan="2" class="diff-lineno" id="L75" >Line 75:</td>
<td colspan="2" class="diff-lineno">Line 75:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 1 & 1 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 1 & 1 & 0 \\</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 0 & 0 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 0 & 0 & 0 \\</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix} = R<del class="diffchange diffchange-inline">$</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix} = R<ins class="diffchange diffchange-inline"></math></ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* we see that one of the rows are $\mathbf 0$ - so the nullspace of $A^T$ should have something apart from $\mathbf 0$</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* we see that one of the rows are $\mathbf 0$ - so the nullspace of $A^T$ should have something apart from $\mathbf 0$</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td colspan="2" class="diff-lineno" id="L88" >Line 88:</td>
<td colspan="2" class="diff-lineno">Line 88:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Example cont'd  </div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Example cont'd  </div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* <del class="diffchange diffchange-inline">$</del>\left[ \begin{array}{cccc|ccc}</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* <ins class="diffchange diffchange-inline"><math></ins>\left[ \begin{array}{cccc|ccc}</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & 2 & 3 & 1 & 1 & 0 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & 2 & 3 & 1 & 1 & 0 & 0 \\</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & 1 & 2 & 1 & 0 & 1 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & 1 & 2 & 1 & 0 & 1 & 0 \\</div></td></tr>
<tr><td colspan="2" class="diff-lineno" id="L97" >Line 97:</td>
<td colspan="2" class="diff-lineno">Line 97:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 1 & 1 & 0 & 1 & -1 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 1 & 1 & 0 & 1 & -1 & 0 \\</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 0 & 0 & 0 & -1 & 0 & 1 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 0 & 0 & 0 & -1 & 0 & 1 \\</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>\end{array}\right]<del class="diffchange diffchange-inline">$</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>\end{array}\right]<ins class="diffchange diffchange-inline"></math></ins></div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* now if we take <del class="diffchange diffchange-inline">$</del>E = \begin{bmatrix}</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* now if we take <ins class="diffchange diffchange-inline"><math></ins>E = \begin{bmatrix}</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>-1 & 2 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>-1 & 2 & 0 \\</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & -1 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & -1 & 0 \\</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>-1 & 0 & 1 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>-1 & 0 & 1 \\</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix}<del class="diffchange diffchange-inline">$ </del>and multiply it by $A$, we get  </div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix}<ins class="diffchange diffchange-inline"></math> </ins>and multiply it by $A$, we get  </div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>** <del class="diffchange diffchange-inline">$</del>\begin{bmatrix}</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>** <ins class="diffchange diffchange-inline"><math></ins>\begin{bmatrix}</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>-1 & 2 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>-1 & 2 & 0 \\</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & -1 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>1 & -1 & 0 \\</div></td></tr>
<tr><td colspan="2" class="diff-lineno" id="L117" >Line 117:</td>
<td colspan="2" class="diff-lineno">Line 117:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>- & - & - & - \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>- & - & - & - \\</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 0 & 0 & 0 \\</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>0 & 0 & 0 & 0 \\</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix}<del class="diffchange diffchange-inline">$</del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>\end{bmatrix}<ins class="diffchange diffchange-inline"></math></ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>** so indeed we manage to get the last row with zeros  </div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>** so indeed we manage to get the last row with zeros  </div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* so we need the last row of $E$ to get $\mathbf 0^T$</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* so we need the last row of $E$ to get $\mathbf 0^T$</div></td></tr>
<tr><td colspan="2" class="diff-lineno" id="L128" >Line 128:</td>
<td colspan="2" class="diff-lineno">Line 128:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* vectors of $V$ that correspond to $\sigma_i = 0$ are from the nullspace  </div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* vectors of $V$ that correspond to $\sigma_i = 0$ are from the nullspace  </div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div> </div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline"> </ins>def null(A, eps=1e-15):</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline"><pre></del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">    </ins>u, s, vh = np.linalg.svd(A)</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>def null(A, eps=1e-15):</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">    </ins>null_space = np.compress(s <= eps, vh, axis=0)</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">    </del>u, s, vh = np.linalg.svd(A)</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">    </ins>return null_space.T</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">    </del>null_space = np.compress(s <= eps, vh, axis=0)</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div></div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline">    </del>return null_space.T</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div></div></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div><del class="diffchange diffchange-inline"></pre></del></div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>From [http://stackoverflow.com/questions/1835246/how-to-solve-homogeneous-linear-equations-with-numpy]</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>From [http://stackoverflow.com/questions/1835246/how-to-solve-homogeneous-linear-equations-with-numpy]</div></td></tr>
</table>
Alexey
http://mlwiki.org/index.php?title=Nullspace&diff=519&oldid=prev
Alexey at 17:10, 26 April 2015
2015-04-26T17:10:51Z
<p></p>
<p><b>New page</b></p><div>== Nullspace ==<br />
Nullspace $N(A)$ of a matrix $A$ is one of the [[Four Fundamental Subspaces]] of the matrix $A$<br />
<br />
The nullspace of $A$ contains all $\mathbf x$ that solve the [[System of Linear Equations|system]] $A \mathbf x = \mathbf 0$ (this system is called [[Homogeneous Systems of Linear Equations|''homogeneous'']])<br />
<br />
<br />
=== Example ===<br />
$A = \begin{bmatrix}<br />
1 & 1 & 2 \\<br />
2 & 1 & 3 \\<br />
3 & 1 & 4 \\<br />
4 & 1 & 5 \\<br />
\end{bmatrix}$, $\mathbf x = \begin{bmatrix}<br />
x_1 \\ x_2 \\ x_3<br />
\end{bmatrix}$, $\mathbf b = \mathbf 0_4 = \begin{bmatrix}<br />
0 \\ 0 \\ 0 \\ 0<br />
\end{bmatrix}$<br />
* There are 3 columns and they are 4-dim vectors <br />
* the [[Column Space]] $C(A)$ is a subspace of $\mathbb R^4$, but $\text{dim } C(A) = 2$ (because the [[Rank (Matrix)|rank]] of this matrix is 2)<br />
* since there are only 3 columns, the number of unknowns is 3 - so $N(A)$ is a subspace of $\mathbb R^3$<br />
<br />
<br />
Let's find what's inside $N(A)$<br />
* i.e. all such $\mathbf x$ that solve $A \mathbf x = \mathbf 0$ <br />
* $\mathbf 0 \in N(A)$ always<br />
* $\begin{bmatrix}<br />
1 \\ 1 \\ -1<br />
\end{bmatrix}$ or any multiple of this vector $c \cdot \begin{bmatrix}<br />
1 \\ 1 \\ -1<br />
\end{bmatrix}$<br />
* so it's a subspace - a line in $\mathbb R^3$ through the origin<br />
<br />
<br />
=== Is $N(A)$ a Subspace? ===<br />
Does it form a [[Vector Space]] on its own?<br />
* so we need to check that all possible $\mathbf x$ for that solve $A \mathbf x = \mathbf 0$ form a subspace<br />
* let $\mathbf v$ and $\mathbf w$ be two solutions<br />
** $A \cdot (\mathbf v + \mathbf w) = A \mathbf v + A \mathbf w = \mathbf 0 + \mathbf 0 = \mathbf 0$. so $\mathbf v + \mathbf w$ is also a solution<br />
* if $A \mathbf v = 0$, then $A \cdot (c \cdot \mathbf v) = (c \cdot A) \cdot \mathbf v = 0$<br />
** this would just multiply all columns of $A$ on the same number <br />
* so yes, it is a subspace<br />
<br />
<br />
=== [[Basis (Linear Algebra)|Basis]] of $N(A)$ ===<br />
Basis for $N(A)$ is formed by the "special" solutions<br />
<br />
<br />
<br />
== Left Nullspace ==<br />
We can also consider another nullspace of $A$ - the nullspace of $A^T$ (this is the 4th [[Four Fundamental Subspaces|fundamental subspace]] of a matrix)<br />
<br />
Let's have a look at a system $A^T \mathbf y = \mathbf 0$<br />
* $A$ is an $n \times m$ matrix, so $A^T$ is $m \times n$<br />
* $y$ is $n$-len column vector<br />
<br />
Let's take the transpose of $A^T \mathbf y = \mathbf 0$:<br />
* $(A^T \mathbf y)^T = \mathbf 0^T$<br />
* $\mathbf y^T A = \mathbf 0^T$<br />
* so now we have a row vector $\mathbf y^T$ that is on the left side of $A$ <br />
<br />
$\big[ - \, \mathbf y^T - \big] \Bigg[ ~ ~ ~ ~ ~ {A} ~ ~ ~ ~ ~ \Bigg] = \big[ - \, \mathbf 0^T - \big]$<br />
<br />
<br />
=== [[Basis (Linear Algebra)|Basis]] of $N(A^T)$ ===<br />
Let's consider this example <br />
<br />
Let $A$ be some rectangular matrix and we find it's rref $R$<br />
* $A = \begin{bmatrix}<br />
1 & 2 & 3 & 1 \\<br />
1 & 1 & 2 & 1 \\<br />
2 & 3 & 5 & 2 \\<br />
\end{bmatrix} \leadsto <br />
\begin{bmatrix}<br />
1 & 0 & 1 & 1 \\<br />
0 & 1 & 1 & 0 \\<br />
0 & 0 & 0 & 0 \\<br />
\end{bmatrix} = R$<br />
* we see that one of the rows are $\mathbf 0$ - so the nullspace of $A^T$ should have something apart from $\mathbf 0$<br />
<br />
<br />
How to best find this left nullspace?<br />
* Let's do Gauss-Jordan Elimination: create the augmented matrix by appending $I$ and reduce it to the echelon form:<br />
* $\big[ A_{m \times n} \ I_{n \times n} \big] \to \big[ R_{m \times n} \ E_{n \times n} \big]$<br />
* So $E$ is the elimination matrix - the matrix that brings $A$ to rref $R$<br />
* $E A = R$<br />
** If $A$ is square and invertible, then $E \equiv A^{-1}$<br />
** but since $A$ is rectangular, it has no inverse <br />
<br />
Example cont'd <br />
* $\left[ \begin{array}{cccc|ccc}<br />
1 & 2 & 3 & 1 & 1 & 0 & 0 \\<br />
1 & 1 & 2 & 1 & 0 & 1 & 0 \\<br />
2 & 3 & 5 & 2 & 0 & 0 & 1 \\<br />
\end{array}\right] \leadsto <br />
\left[ \begin{array}{cccc|ccc}<br />
1 & 0 & 1 & 1 & -1 & 2 & 0 \\<br />
0 & 1 & 1 & 0 & 1 & -1 & 0 \\<br />
0 & 0 & 0 & 0 & -1 & 0 & 1 \\<br />
\end{array}\right]$<br />
* now if we take $E = \begin{bmatrix}<br />
-1 & 2 & 0 \\<br />
1 & -1 & 0 \\<br />
-1 & 0 & 1 \\<br />
\end{bmatrix}$ and multiply it by $A$, we get <br />
** $\begin{bmatrix}<br />
-1 & 2 & 0 \\<br />
1 & -1 & 0 \\<br />
-1 & 0 & 1 \\<br />
\end{bmatrix} \cdot <br />
\begin{bmatrix}<br />
1 & 2 & 3 & 1 \\<br />
1 & 1 & 2 & 1 \\<br />
2 & 3 & 5 & 2 \\<br />
\end{bmatrix} = <br />
\begin{bmatrix}<br />
- & - & - & - \\<br />
- & - & - & - \\<br />
0 & 0 & 0 & 0 \\<br />
\end{bmatrix}$<br />
** so indeed we manage to get the last row with zeros <br />
* so we need the last row of $E$ to get $\mathbf 0^T$<br />
** recall the row picture from [[Matrix Multiplication]]<br />
<br />
<br />
== Numerical Computation ==<br />
Use [[SVD]] to compute the nullspace<br />
* $A = U \Sigma V^T$ <br />
* vectors of $V$ that correspond to $\sigma_i = 0$ are from the nullspace <br />
<br />
<br />
<pre><br />
def null(A, eps=1e-15):<br />
u, s, vh = np.linalg.svd(A)<br />
null_space = np.compress(s <= eps, vh, axis=0)<br />
return null_space.T<br />
</pre><br />
<br />
From [http://stackoverflow.com/questions/1835246/how-to-solve-homogeneous-linear-equations-with-numpy]<br />
<br />
<br />
<br />
== Sources ==<br />
* [[Linear Algebra MIT 18.06 (OCW)]]<br />
* http://en.wikipedia.org/wiki/Kernel_%28linear_algebra%29#Numerical_computation<br />
<br />
[[Category:Linear Algebra]]</div>
Alexey