Let's consider e 1 = ( 1 , 1 , 1 ) , e 2 = ( 1 , − 1 , 0 ) , e 3 = ( 1 , 1 , 0 ) e_1=(1,1,1), e_2=(1,-1,0),e_3=(1,1,0) e 1 = ( 1 , 1 , 1 ) , e 2 = ( 1 , − 1 , 0 ) , e 3 = ( 1 , 1 , 0 ) .
Let's show that e 1 , e 2 , e 3 e_1,e_2,e_3 e 1 , e 2 , e 3 is a basis of R 3 R^3 R 3 :
α 1 e 1 + α 2 e 2 + α 3 e 3 = 0 \alpha_1 e_1+\alpha_2 e_2+\alpha_3 e_3=0 α 1 e 1 + α 2 e 2 + α 3 e 3 = 0 ,
where α 1 = α 2 = α 3 = 0 \alpha_1 =\alpha_2=\alpha_3=0 α 1 = α 2 = α 3 = 0 .
Really
α 1 + α 2 + α 3 = 0 α 1 − α 2 + α 3 = 0 α 1 = 0 \alpha_1 +\alpha_2+\alpha_3=0\\
\alpha_1 -\alpha_2+\alpha_3=0\\
\alpha_1 =0 α 1 + α 2 + α 3 = 0 α 1 − α 2 + α 3 = 0 α 1 = 0
Then α 1 = α 2 = α 3 = 0 \alpha_1 =\alpha_2=\alpha_3=0 α 1 = α 2 = α 3 = 0 .
So e 1 , e 2 , e 3 e_1,e_2,e_3 e 1 , e 2 , e 3 is a basis of R 3 R^3 R 3 .
Find a dual basis to this basis.
We can find the biorthogonal (dual) basis { e 1 , e 2 , e 3 } \{e^1,e^2,e^3\} { e 1 , e 2 , e 3 } by formulas below:
e 1 = ( e 2 × e 3 V ) T , e 2 = ( e 3 × e 1 V ) T , e 3 = ( e 1 × e 2 V ) T , e^1=(\frac{e_2\times e_3}{V})^T,e^2=(\frac{e_3\times e_1}{V})^T,e^3=(\frac{e_1\times e_2}{V})^T, e 1 = ( V e 2 × e 3 ) T , e 2 = ( V e 3 × e 1 ) T , e 3 = ( V e 1 × e 2 ) T ,
where T ^T T denotes the transpose and V = ( e 1 , e 2 , e 3 ) = e 1 ⋅ ( e 2 × e 3 ) V=(e_1,e_2,e_3)=e_1\cdot(e_2\times e_3) V = ( e 1 , e 2 , e 3 ) = e 1 ⋅ ( e 2 × e 3 )
is the volume of the parallelepiped formed by the basis vectors e 1 , e 2 , e 3 e_1,e_2,e_3 e 1 , e 2 , e 3 .
In our case
V = ∣ 1 1 1 1 − 1 0 1 1 0 ∣ = 1 ⋅ ( − 1 ) ⋅ 0 + 1 ⋅ 0 ⋅ 1 + + 1 ⋅ 1 ⋅ 1 − 1 ⋅ ( − 1 ) ⋅ 1 − 1 ⋅ 1 ⋅ 0 − 1 ⋅ 1 ⋅ 0 = 2 e 2 × e 3 = ( ∣ − 1 0 1 0 ∣ , − ∣ 1 0 1 0 ∣ , ∣ 1 − 1 1 1 ∣ ) = = ( 0 , 0 , 2 ) e 3 × e 1 = ( ∣ 1 0 1 1 ∣ , − ∣ 1 0 1 1 ∣ , ∣ 1 1 1 1 ∣ ) = = ( 1 , − 1 , 0 ) e 1 × e 2 = ( ∣ 1 1 − 1 0 ∣ , − ∣ 1 1 1 0 ∣ , ∣ 1 1 1 − 1 ∣ ) = = ( 1 , 1 , − 2 ) V=\begin{vmatrix}
1 & 1&1 \\
1 & -1&0\\
1&1&0
\end{vmatrix}=1\cdot(-1)\cdot0+1\cdot0\cdot1+\\
+1\cdot1\cdot1-1\cdot(-1)\cdot1-1\cdot1\cdot0-1\cdot1\cdot0=2\\
e_2\times e_3=(\begin{vmatrix}
-1& 0 \\
1 & 0
\end{vmatrix},-\begin{vmatrix}
1& 0 \\
1 & 0
\end{vmatrix},\begin{vmatrix}
1& -1 \\
1 & 1
\end{vmatrix})=\\
=(0,0,2)\\
e_3\times e_1=(\begin{vmatrix}
1& 0 \\
1 & 1
\end{vmatrix},-\begin{vmatrix}
1& 0 \\
1 & 1
\end{vmatrix},\begin{vmatrix}
1& 1 \\
1 & 1
\end{vmatrix})=\\
=(1,-1,0)\\
e_1\times e_2=(\begin{vmatrix}
1& 1 \\
-1 & 0
\end{vmatrix},-\begin{vmatrix}
1& 1 \\
1 & 0
\end{vmatrix},\begin{vmatrix}
1& 1 \\
1 & -1
\end{vmatrix})=\\
=(1,1,-2) V = ∣ ∣ 1 1 1 1 − 1 1 1 0 0 ∣ ∣ = 1 ⋅ ( − 1 ) ⋅ 0 + 1 ⋅ 0 ⋅ 1 + + 1 ⋅ 1 ⋅ 1 − 1 ⋅ ( − 1 ) ⋅ 1 − 1 ⋅ 1 ⋅ 0 − 1 ⋅ 1 ⋅ 0 = 2 e 2 × e 3 = ( ∣ ∣ − 1 1 0 0 ∣ ∣ , − ∣ ∣ 1 1 0 0 ∣ ∣ , ∣ ∣ 1 1 − 1 1 ∣ ∣ ) = = ( 0 , 0 , 2 ) e 3 × e 1 = ( ∣ ∣ 1 1 0 1 ∣ ∣ , − ∣ ∣ 1 1 0 1 ∣ ∣ , ∣ ∣ 1 1 1 1 ∣ ∣ ) = = ( 1 , − 1 , 0 ) e 1 × e 2 = ( ∣ ∣ 1 − 1 1 0 ∣ ∣ , − ∣ ∣ 1 1 1 0 ∣ ∣ , ∣ ∣ 1 1 1 − 1 ∣ ∣ ) = = ( 1 , 1 , − 2 )
Then
e 1 = ( ( 0 , 0 , 2 ) 2 ) T = ( 0 0 1 ) , e 2 = ( ( 1 , − 1 , 0 ) 2 ) T = ( 1 2 − 1 2 0 ) , e 3 = ( ( 1 , 1 , − 2 ) 2 ) T = ( 1 2 1 2 − 1 ) . e^1=(\frac{(0,0,2)}{2})^T=\begin{pmatrix}
0 \\
0\\1
\end{pmatrix},\\
e^2=(\frac{(1,-1,0)}{2})^T=\begin{pmatrix}
\frac{1}{2} \\
-\frac{1}{2}\\0
\end{pmatrix},\\
e^3=(\frac{(1,1,-2)}{2})^T=\begin{pmatrix}
\frac{1}{2}\\ \frac{1}{2} \\-1
\end{pmatrix}. e 1 = ( 2 ( 0 , 0 , 2 ) ) T = ⎝ ⎛ 0 0 1 ⎠ ⎞ , e 2 = ( 2 ( 1 , − 1 , 0 ) ) T = ⎝ ⎛ 2 1 − 2 1 0 ⎠ ⎞ , e 3 = ( 2 ( 1 , 1 , − 2 ) ) T = ⎝ ⎛ 2 1 2 1 − 1 ⎠ ⎞ .
Denoting the indexed vector sets as B = { v i } i ∈ I {\displaystyle B=\{v_{i}\}_{i\in I}} B = { v i } i ∈ I and B ∗ = { v i } i ∈ I {\displaystyle B^{*}=\{v^{i}\}_{i\in I}} B ∗ = { v i } i ∈ I , being biorthogonal means that the elements pair to have an inner product equal to 1 if the indexes are equal, and equal to 0 otherwise. Symbolically, evaluating a dual vector in V ∗ on a vector in the original space V :
v i ⋅ v j = δ j i = { 1 if i = j 0 if i ≠ j , {\displaystyle v^{i}\cdot v_{j}=\delta _{j}^{i}={\begin{cases}1&{\text{if }}i=j\\0&{\text{if }}i\neq j{\text{,}}\end{cases}}} v i ⋅ v j = δ j i = { 1 0 if i = j if i = j ,
where δ j i {\displaystyle \delta _{j}^{i}} δ j i is the Kronecker delta symbol.
In our case
e 1 ⋅ e 1 = 0 + 0 + 1 = 1 e 1 ⋅ e 2 = 0 + 0 + 0 = 0 e 1 ⋅ e 3 = 0 + 0 + 0 = 0 e 2 ⋅ e 1 = 1 2 − 1 2 + 0 = 0 e 2 ⋅ e 2 = 1 2 + 1 2 + 0 = 1 e 2 ⋅ e 3 = 1 2 − 1 2 + 0 = 0 e 3 ⋅ e 1 = 1 2 + 1 2 − 1 = 0 e 3 ⋅ e 2 = 1 2 − 1 2 + 0 = 0 e 3 ⋅ e 3 = 1 2 + 1 2 + 0 = 1 e^1\cdot e_1=0+0+1=1\\
e^1\cdot e_2=0+0+0=0\\
e^1\cdot e_3=0+0+0=0\\
e^2\cdot e_1=\frac{1}{2}-\frac{1}{2}+0=0\\
e^2\cdot e_2=\frac{1}{2}+\frac{1}{2}+0=1\\
e^2\cdot e_3=\frac{1}{2}-\frac{1}{2}+0=0\\
e^3\cdot e_1=\frac{1}{2}+\frac{1}{2}-1=0\\
e^3\cdot e_2=\frac{1}{2}-\frac{1}{2}+0=0\\
e^3\cdot e_3=\frac{1}{2}+\frac{1}{2}+0=1 e 1 ⋅ e 1 = 0 + 0 + 1 = 1 e 1 ⋅ e 2 = 0 + 0 + 0 = 0 e 1 ⋅ e 3 = 0 + 0 + 0 = 0 e 2 ⋅ e 1 = 2 1 − 2 1 + 0 = 0 e 2 ⋅ e 2 = 2 1 + 2 1 + 0 = 1 e 2 ⋅ e 3 = 2 1 − 2 1 + 0 = 0 e 3 ⋅ e 1 = 2 1 + 2 1 − 1 = 0 e 3 ⋅ e 2 = 2 1 − 2 1 + 0 = 0 e 3 ⋅ e 3 = 2 1 + 2 1 + 0 = 1
https://en.wikipedia.org/wiki/Dual_basis
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