Difference between revisions of "Minors and cofactors"

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{{Navigation|before=[[Adjugate Formula]]|overview=[[Matrix inversion|Vektorrechnung]]|next=[[Determinant of a 4-by-4 matrix]]}}
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The '''minor <math>M_{i,j}(\mathbf{A})</math>''' of an n-by-n square matrix <math>\mathbf{A}</math> is the determinant of a smaller square matrix obtained by removing the row <math>i</math> and the column <math>j</math> from <math>\mathbf{A}</math>.<br/><br/>
 
The '''minor <math>M_{i,j}(\mathbf{A})</math>''' of an n-by-n square matrix <math>\mathbf{A}</math> is the determinant of a smaller square matrix obtained by removing the row <math>i</math> and the column <math>j</math> from <math>\mathbf{A}</math>.<br/><br/>
  

Revision as of 17:05, 9 May 2014

← Back: Adjugate Formula Overview: Vektorrechnung Next: Determinant of a 4-by-4 matrix

The minor M_{i,j}(\mathbf{A}) of an n-by-n square matrix \mathbf{A} is the determinant of a smaller square matrix obtained by removing the row i and the column j from \mathbf{A}.

Multiplying the minor with (-1)^{i+j} results in the cofactor C_{i,j}(\mathbf{A}):

C_{i,j}(\mathbf{A})=(-1)^{i+j}M_{i,j}(\mathbf{A})

Example: Minors and cofactors



\mathbf{A}_e  = 
\left[\begin{array}{cccc}
1 & 2 & 0 & 0\\
3 & 0 & 1 & 1\\
0 & 1 & 0 & 0\\
0 & 0 & 2 & 1
\end{array}\right]

The minors M_{1,4}(\mathbf{A}_e) and M_{3,1}(\mathbf{A}_e) for example are defined as

M_{1,4}(\mathbf{A}_e)=
\left|\begin{array}{cccc}
\Box & \Box & \Box & \Box\\
3 & 0 & 1 & \Box\\
0 & 1 & 0 & \Box\\
0 & 0 & 2 & \Box
\end{array}\right|=
\left|\begin{array}{ccc}
3 & 0 & 1\\
0 & 1 & 0\\
0 & 0 & 2
\end{array}\right|=6-0=6

M_{3,1}(\mathbf{A}_e)=
\left|\begin{array}{cccc}
\Box & 2 & 0 & 0\\
\Box & 0 & 1 & 1\\
\Box & \Box & \Box & \Box\\
\Box & 0 & 2 & 1
\end{array}\right|=
\left|\begin{array}{ccc}
2 & 0 & 0\\
0 & 1 & 1\\
0 & 2 & 1
\end{array}\right|=2-4=-2

The corresponding cofactors in that case are

C_{1,4}(\mathbf{A}_e)=(-1)^{1+4}M_{1,4}(\mathbf{A}_e)=(-1)^5\cdot6=-6

C_{3,1}(\mathbf{A}_e)=(-1)^{3+1}M_{3,1}(\mathbf{A}_e)=(-1)^4\cdot-2=-2