Question #267034

Proof whether the following operations are inner product operations:

⟨x, y⟩ = 2x1y1 − x1y2 − x2y1 + 2x2y2, x=(x1, x2), y=(y1, y2)



1
Expert's answer
2021-11-21T13:35:18-0500

1. Positive Definite Property:


x,x=2x1x1x1x2x2x1+2x2x2\langle x,x\rangle=2x_1x_1-x_1x_2-x_2x_1+2x_2x_2

=(x12+x22)+(x1x2)20=(x_1^2+x_2^2)+(x_1-x_2)^2\geq0

The value equals zero if and only if both summands are zero, i.e., when x1=x2=0x_1=x_2=0

For any xV,x,x0;x\in V, \langle x,x\rangle\geq0; and x,x=0\langle x,x\rangle=0 if and only if x=0.x=0.


2. Symmetric Property


x,y=2x1y1x1y2x2y1+2x2y2\langle x,y\rangle=2x_1y_1-x_1y_2-x_2y_1+2x_2y_2

=2y1x1y1x2y2x1+2y2x2=y,x=2y_1x_1-y_1x_2-y_2x_1+2y_2x_2=\langle y, x\rangle

3. Linearity


ax+by,z\langle ax+by,z\rangle

=2(ax1+by1)z1(ax1+by1)z2(ax2+by2)z1=2(ax_1+by_1)z_1-(ax_1+by_1)z_2-(ax_2+by_2)z_1

+2(ax2+by2)z2+2(ax_2+by_2)z_2

=a(2x1z1x1z2x2z1+x2z2)=a(2x_1z_1-x_1z_2-x_2z_1+x_2z_2)

+b(2y1z1y1z2y2z1+y2z2)+b(2y_1z_1-y_1z_2-y_2z_1+y_2z_2)

=ax,z+by,z=a\langle x,z\rangle+b\langle y,z\rangle

Then x,y\langle x,y\rangle is an inner product on V.V.


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