- #1
approx1mate
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Hi! I have used the physics forum a lot of times to deal with several tasks that I had and now its the time to introduce my own query! So please bear with me :-)
Equip the set [itex] C^1_{[0,1]} [/itex] with the inner product:
[tex]
\left\langle f,g \right\rangle= \int_{0}^{1} f(x)\bar{g(x)} + \int_{0}^{1} f'(x)\bar{g'(x)}dx
[/tex]
(the bar above the [itex] g [/itex] function is the conjugate symbol)
I need to show that the subspace:
[tex]
W = \{f\in C^1_{[0,1]} | f(1)=0\}
[/tex]
is a closed subspace of [itex] C^1_{[0,1]} [/itex].
[itex] \left\langle f,cosh \right\rangle = f(1)sinh(1) [/itex].
The Cauchy inequality: [itex] |\left\langle f,g \right\rangle | \le \|f\|\|g\|[/itex],
the Pythagoras theorem: [itex] \|f+g\|^2 = \|f\|^2 + \|g\|^2 [/itex],
the parallelogram law: [itex] \|f+g\|^2 + \|f-g\|^2 = 2(\|f\|^2 + \|g\|^2)[/itex],
the triangular inequality: [itex] \|f+g\| \le \|f\| + \|g\|[/itex]
Let us take a Cauchy sequence [itex] \{f^n\}_{n=1}^{\infty} \in W[/itex], because
[itex](C^1_{[0,1]},\|\cdot \|)[/itex] is a Hilbert space then the sequence [itex] \{f^n\}_{n=1}^{\infty} [/itex] converges to [itex] f\in C^1_{[0,1]} [/itex].
Therefore it only remains to be shown that at the limit [itex] f(1)=0 [/itex].
At this point I am stuck. I can see that the [itex] cosh [/itex] function is orthogonal
to the set [itex] W [/itex] and I also tried to use the above "relevant equations" but
I couldn't see what would be a possible proof.
Any advice?
Homework Statement
Equip the set [itex] C^1_{[0,1]} [/itex] with the inner product:
[tex]
\left\langle f,g \right\rangle= \int_{0}^{1} f(x)\bar{g(x)} + \int_{0}^{1} f'(x)\bar{g'(x)}dx
[/tex]
(the bar above the [itex] g [/itex] function is the conjugate symbol)
I need to show that the subspace:
[tex]
W = \{f\in C^1_{[0,1]} | f(1)=0\}
[/tex]
is a closed subspace of [itex] C^1_{[0,1]} [/itex].
Homework Equations
[itex] \left\langle f,cosh \right\rangle = f(1)sinh(1) [/itex].
The Cauchy inequality: [itex] |\left\langle f,g \right\rangle | \le \|f\|\|g\|[/itex],
the Pythagoras theorem: [itex] \|f+g\|^2 = \|f\|^2 + \|g\|^2 [/itex],
the parallelogram law: [itex] \|f+g\|^2 + \|f-g\|^2 = 2(\|f\|^2 + \|g\|^2)[/itex],
the triangular inequality: [itex] \|f+g\| \le \|f\| + \|g\|[/itex]
The Attempt at a Solution
Let us take a Cauchy sequence [itex] \{f^n\}_{n=1}^{\infty} \in W[/itex], because
[itex](C^1_{[0,1]},\|\cdot \|)[/itex] is a Hilbert space then the sequence [itex] \{f^n\}_{n=1}^{\infty} [/itex] converges to [itex] f\in C^1_{[0,1]} [/itex].
Therefore it only remains to be shown that at the limit [itex] f(1)=0 [/itex].
At this point I am stuck. I can see that the [itex] cosh [/itex] function is orthogonal
to the set [itex] W [/itex] and I also tried to use the above "relevant equations" but
I couldn't see what would be a possible proof.
Any advice?
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