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wanggx11174's Question
Math
Posted 8 months ago
请用中文回答
3. 在队积空间 (X,()(X,(\cdot),)け证朋:内积(x,y)) け证朋: 内积 (x, y)X×XX \times X 上头于范数㺫的连续函数。
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Answer from Sia
Posted 8 months ago
Solution by Steps
step 1
X,YX, Y 是线性赋范空间,TL(X,Y)T \in L(X, Y)
step 2
定义算子范数 T=supxX,x0Txx \|T\| = \sup_{x \in X, x \neq 0} \frac{\|Tx\|}{\|x\|}
step 3
我们需要证明 T=supx1Tx \|T\| = \sup_{\|x\| \leq 1} \|Tx\|
step 4
首先,注意到 T=supx=1Tx \|T\| = \sup_{\|x\| = 1} \|Tx\|
step 5
由于 x1 \|x\| \leq 1 ,我们可以写 x=λy x = \lambda y ,其中 y=1 \|y\| = 1 λ1 |\lambda| \leq 1
step 6
因此,Tx=T(λy)=λTyTy \|Tx\| = \|T(\lambda y)\| = |\lambda| \|Ty\| \leq \|Ty\|
step 7
由于 y=1 \|y\| = 1 ,我们有 TyT \|Ty\| \leq \|T\|
step 8
因此,TxT \|Tx\| \leq \|T\| 对于所有 x1 \|x\| \leq 1
step 9
因此,supx1Tx=T \sup_{\|x\| \leq 1} \|Tx\| = \|T\|
Answer
T=supx1Tx\|T\| = \sup_{\|x\| \leq 1} \|Tx\|
Key Concept
算子范数
Explanation
算子范数是通过取所有单位向量的像的范数的上确界来定义的。

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