
#1
June 3rd, 2019,
15:46
Let $ \displaystyle <x, x>=3x_{1}^2+2x_{2}^2+x_{3}^24x_{1}x_{2}2x_{1}x_{3}+2x_{2}x_{3} $ be a quadratic form in V=R, where $ \displaystyle x=x_{1}e_{1}+x_{2}e_{2}+x_{3}e_{3}$ (in the base $ \displaystyle {e_{1},e_{2},e_{3}}$.
Find the inner product corresponding to this quadratic form.
Is this that easy that you have to change '' second'' xes for y (for example to write $ \displaystyle 2x_{2}y_{3}$ instead of $ \displaystyle 2x_{2}x_{3}$ at the end), or what I have to do?
Last edited by Denis99; June 3rd, 2019 at 15:53.

June 3rd, 2019 15:46
# ADS
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#2
June 4th, 2019,
03:11
Originally Posted by
Denis99
Let $ \displaystyle <x, x>=3x_{1}^2+2x_{2}^2+x_{3}^24x_{1}x_{2}2x_{1}x_{3}+2x_{2}x_{3} $ be a quadratic form in V=R, where $ \displaystyle x=x_{1}e_{1}+x_{2}e_{2}+x_{3}e_{3}$ (in the base $ \displaystyle {e_{1},e_{2},e_{3}}$.
Find the inner product corresponding to this quadratic form.
Is this that easy that you have to change '' second'' xes for y (for example to write $ \displaystyle 2x_{2}y_{3}$ instead of $ \displaystyle 2x_{2}x_{3}$ at the end), or what I have to do?
what definition do you have to work with here? If it were me I'd iterate to the result start by encoding this with standard basis vectors, i.e.
$\langle x, x \rangle =3x_{1}^2+2x_{2}^2+x_{3}^24x_{1}x_{2}2x_{1}x_{3}+2x_{2}x_{3} = \mathbf x^T A \mathbf x$
where $A$ is real symmetric positive definite. Then take the square root of $A$ and let that be your basis i.e. consider
$ A^\frac{1}{2} \mathbf x$
where the $kth$ column of $ A^\frac{1}{2} $ is denoted by $e_k$ in your text

#3
June 4th, 2019,
17:26
Thread Author
Originally Posted by
steep
what definition do you have to work with here? If it were me I'd iterate to the result start by encoding this with standard basis vectors, i.e.
$\langle x, x \rangle =3x_{1}^2+2x_{2}^2+x_{3}^24x_{1}x_{2}2x_{1}x_{3}+2x_{2}x_{3} = \mathbf x^T A \mathbf x$
where $A$ is real symmetric positive definite. Then take the square root of $A$ and let that be your basis i.e. consider
$ A^\frac{1}{2} \mathbf x$
where the $kth$ column of $ A^\frac{1}{2} $ is denoted by $e_k$ in your text
I have to work with definition like this one from definition of inner space in here
(in part Definition)

#4
June 4th, 2019,
19:58
Originally Posted by
Denis99
I have to work with definition like this one from definition of inner space in here
(in part Definition)
But I can't figure out how you could understand that and say this
Originally Posted by
Denis99
Find the inner product corresponding to this quadratic form.
Is this that easy that you have to change '' second'' xes for y (for example to write $ \displaystyle 2x_{2}y_{3}$ instead of $ \displaystyle 2x_{2}x_{3}$ at the end), or what I have to do?
The reality is that one way or another you need to find $A^\frac{1}{2}$