Question #106265
Show that f(1, 1, 1), (1, – 1, 0), (1, 1, 0)1 is a
basis of R3 . Find a dual basis to this basis.
1
Expert's answer
2020-03-24T10:44:14-0400

Let's consider e1=(1,1,1),e2=(1,1,0),e3=(1,1,0)e_1=(1,1,1), e_2=(1,-1,0),e_3=(1,1,0) .

 Let's show that e1,e2,e3e_1,e_2,e_3  is a basis of R3R^3 :

α1e1+α2e2+α3e3=0\alpha_1 e_1+\alpha_2 e_2+\alpha_3 e_3=0 ,

where α1=α2=α3=0\alpha_1 =\alpha_2=\alpha_3=0 .

 Really

α1+α2+α3=0α1α2+α3=0α1=0\alpha_1 +\alpha_2+\alpha_3=0\\ \alpha_1 -\alpha_2+\alpha_3=0\\ \alpha_1 =0

Then α1=α2=α3=0\alpha_1 =\alpha_2=\alpha_3=0 .

So e1,e2,e3e_1,e_2,e_3  is a basis of R3R^3.


 Find a dual basis to this basis.

We can find the biorthogonal (dual) basis {e1,e2,e3}\{e^1,e^2,e^3\} by formulas below:

e1=(e2×e3V)T,e2=(e3×e1V)T,e3=(e1×e2V)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,

where T^T denotes the transpose and V=(e1,e2,e3)=e1(e2×e3)V=(e_1,e_2,e_3)=e_1\cdot(e_2\times e_3)

is the volume of the parallelepiped formed by the basis vectors e1,e2,e3e_1,e_2,e_3 .

In our case

V=111110110=1(1)0+101++1111(1)1110110=2e2×e3=(1010,1010,1111)==(0,0,2)e3×e1=(1011,1011,1111)==(1,1,0)e1×e2=(1110,1110,1111)==(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)

Then

e1=((0,0,2)2)T=(001),e2=((1,1,0)2)T=(12120),e3=((1,1,2)2)T=(12121).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}.

Denoting the indexed vector sets as B={vi}iI{\displaystyle B=\{v_{i}\}_{i\in I}}  and B={vi}iI{\displaystyle B^{*}=\{v^{i}\}_{i\in 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:

vivj=δji={1if i=j0if ij,{\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}}}

where δji{\displaystyle \delta _{j}^{i}}  is the Kronecker delta symbol.

In our case

e1e1=0+0+1=1e1e2=0+0+0=0e1e3=0+0+0=0e2e1=1212+0=0e2e2=12+12+0=1e2e3=1212+0=0e3e1=12+121=0e3e2=1212+0=0e3e3=12+12+0=1e^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

https://en.wikipedia.org/wiki/Dual_basis



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