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## Hot answers tagged spectral-theory

4

A good example of an operator with one point in the spectrum is the integral operator $$Lf=\int_{0}^{t}f(u)\,du$$ defined on $X=L^{2}[0,1]$, or defined on $X=C[0,1]$. In both cases $\sigma(L)=\{0\}$. This operator is also compact but it has no eigenvalues, which makes it a good counterexample to remember when studying compact operators.

2

I think you are mixing up notation from two different sources. Your work shows that there are indeed eigenvalues (the open unit disc) for this definition of the unilateral shift. However there is another definition for the unilateral shift: $S(\alpha_1,\alpha_2,\ldots) = (0,\alpha_1,\alpha_2,\ldots)$. This incarnation has no eigenvalues. This incarnation is ...

2

Your definition is not the unilateral shift. If your book gave you $Se_{n}=e_{n+1}$, then that is correct. The coefficient of $e_{1}$ is mapped to become the coefficient of $e_{2}$. So, $$S(\alpha_{1},\alpha_{2},\alpha_{3},\cdots)=(0,\alpha_1,\alpha_2,\alpha_3,\cdots).$$ The operator $S$ never sends anything to $0$ except $0$ because $\|Sx\|=\|x\|$. ...

2

It turns out that for any bounded linear operator on a Hilbert space, the spectrum of that operator is a compact subset of $\mathbb{C}$. In turn, any compact subset, $K$, of $\mathbb{C}$ is the spectrum of some linear operator. In particular, if $\chi(z)$ is the indicator function of that set, then $M_{\chi}$ has spectrum $K$. Here $M_{\chi}$ is the ...

2

The equation must be considered in the context of the Hilbert space $L^{2}(-1,1)$ because the filter of requiring eigenfunctions to be in $L^{2}$ is what eliminates the non-regular solutions, and it is what determines the eigenvalues. For $m = 1,2,3,\cdots$, the operators $$L_{m}f = -\frac{d}{dx}(1-x^{2})\frac{d}{dx}f + \frac{m^{2}}{1-x^{2}}f ... 2 If you are talking about T^{-1}, it means you are assuming that T is invertible. So 0\not\in\sigma(T)\cup\sigma(T^{-1}) You have$$ T^{-1}-\lambda I=-\lambda T^{-1}(T-\lambda^{-1} I). $$So \lambda\in\sigma(T^{-1}) if and only if 1/\lambda\in\sigma(T). That is$$ \sigma(T^{-1})=\{1/\lambda:\ \lambda\in\sigma(T)\}. $$2 It appears you saying that you want to assume that 1=1'. Also, presumably A and A' sit together within a larger algebra (perhaps you want to think of them as subalgebras of the same B(H)?). Let B be the C^*-algebra generated by A and A'. Then the identity 1 for A and A' is also the identity for B. Thus you know that the spectra ... 1 Hint: T-\lambda I is invertible with inverse A \in \mathcal{B}(H) iff$$ I = (T-\lambda I)A = A(T-\lambda I) $$The above holds iff$$ I = A^{\star}(T^{\star}-\overline{\lambda}I) = (T^{\star}-\overline{\lambda}I)A^{\star}. 1 In the indicated line, we can just expand the right hand side \begin{align} (x-\lambda)^{-1}[(x-\lambda_0) - (x-\lambda)](x-\lambda_0)^{-1} &= [(x-\lambda)^{-1}(x-\lambda_0) - (x-\lambda)^{-1}(x-\lambda)](x-\lambda_0)^{-1}\\ &=[(x-\lambda)^{-1}(x-\lambda_0) - I](x-\lambda_0)^{-1}\\ &= (x-\lambda)^{-1}(x-\lambda_0)(x-\lambda_0)^{-1} ... 1 Is there any relationship between V_{\lambda_i} and  E_{\lambda_i}? Yes. V_{\lambda_i} \subseteq E_{\lambda_i}, with equality iff the algebraic and geometric multiplicities of \lambda_i are equal Is there any relationship among V, V_{\lambda_i},E_{\lambda_i} (maybe something with direct sum) ? If \mathcal F is algebraically closed, ... 1 There is a general method for finding sequences which satisfy linear recurrent relations with constant coefficients. In this case, we have the relation x_{n+1}+x_{n-1} = \mu x_n \text{ for } n\geq 1. \text{ (*)} $$Here is how we solve it: consider all the sequences (not nesessarily from l^2) which satisfy (*). They form a linear space (it's easy to ... 1 I'm going to build on another problem of yours, Tobias: Why is this operator self-adoint . In the above problem, it is shown that, if A : \mathcal{D}(A)\subseteq H\rightarrow H is symmetric with (A\pm iI) surjective, then A is densely-defined and selfadjoint. As you noted, your operator O is symmetric on its domain. To see that (O\pm iI) are ... 1 Definitely not. A self-adjoint operator just need A^T=A. Note the matrix of T^* is the transposed of the matrix of T. 1 For \lvert\lambda\rvert > \lVert T\rVert, we have a non-negative lower bound (strictly positive for x\neq 0) for \lVert (T-\lambda I)x\rVert, namely$$\lVert (T-\lambda I)x\rVert \geqslant (\lvert\lambda\rvert - \lVert T\rVert)\lVert x\rVert.$$Now set x = (T-\lambda I)^{-1} y to obtain the inequality$$\lVert y\rVert = \lVert(T-\lambda ...

1

I suppose that $0\in\mathbb{N}$. If you follow the other convention, replace $e_0$ with $e_1$ in the following. If $T+ \epsilon S$ is surjective, then there is an $x\in \ell_2$ with $\lVert x\rVert = 1$ and $$(T+\epsilon S)(x) = c\cdot e_0$$ for some $c\neq 0$. Then $\lVert\epsilon Sx\rVert^2 = \lVert c\cdot e_0 - Tx\rVert^2 = \lvert c\rvert^2 + 1$ and ...

1

Ok, so I think I've got it (I use $\Bbb N = \{1,2,...\}$): Let $S' \in \mathcal{B}$ be given by $S'y = Sy - \langle e_1,Sy \rangle e_1$. $T + \epsilon S'$ is an isomorphism for small enough $\epsilon \ (>0)$ from $H_0 = \ell_2$ to $H_1 = \{ x \in \ell_2 | \langle e_1,x \rangle = 0 \}$. Now suppose that $e_1 \in \text{range}(T + \epsilon S)$: there exists ...

1

Since $\lVert L\rVert = 1$, it follows that $L-\lambda I$ is invertible for all $\lambda$ with $\lvert\lambda\rvert > 1$, we can see that using the Neumann series. For $\lvert \lambda\rvert > \lVert L\rVert$, the series $$\sum_{n=0}^\infty \lambda^{-n}L^n$$ is absolutely convergent, and since $B(H)$ is a Banach space, it is convergent. One computes ...

1

For a regular Sturm-Liouville eigenvalue problem on a finite interval $[a,b]$, say $$Lf = \left[-\frac{d}{dx}p\frac{d}{dx}+q\right]f = \lambda f,$$ there are two types of standard endpoint conditions: Separated conditions $$\cos\alpha f(a)+\sin\alpha f'(a) = 0 \\ \cos\beta f(b) + \sin\beta f'(b) = 0.$$ The linear ...

1

First, in general case the spectrum consists of much more than eigenvalues: the resolvent set is $$R(L)=\{z\in\Bbb C: L-zI \text{ is continuously invertible}\}$$ and $$\sigma(L)=\overline{\Bbb C\setminus R(L)}.$$ Second, in your case, you can explicitly write $$(L-zI)x = (x_2-zx_1,x_3-zx_2,\ldots).$$ Fix $z\in \Bbb C$. If for an arbitrary $\epsilon>0$ ...

1

Consider the unilateral shift on $\ell^2(\mathbb N)$, i.e. $$S(a_1,a_2,\ldots)=(0,a_1,a_2,\ldots).$$ It is easy to verify that $S$ is injective, so $0$ is not an eigenvalue of $S$. But $S$ is not invertible (because it is not surjective). So we have $0$ as an element of the spectrum of $S$, but not an eigenvalue. In finite dimension, two things happen: ...

1

As you acquire examples, please be sure to include this multiplication operator: $$X = L^{2}_{\mu}(-1,1),\\ (Af)(x) = xf(x).$$ Here, $\mu$ is a finite Borel or Lesbesgue measure on $[-1,1]$. The reason to include this one in your collection of examples is that bounded selfadjoint operators with ...

1

The map $(T-\lambda I)^{-1}$ is defined as the solution mapping of the equation $$(T-\lambda I) y = z,$$ i.e. $y= (T-\lambda I)^{-1}z$. In case of the multiplication with $v$, this is equivalent to $$(v(t)-\lambda) y(t) = z(t).$$ First consider the case that $\lambda\ne v(t)$ for all $t\in [0,1]$. Then the above equation has a unique solution, ...

1

I'll be verbose. For a bounded operator $T$, the resolvent set $\rho(T)$ is the set of $\lambda$ for which $(T-\lambda I)$ is injective and surjective. That is, $\lambda\in\rho(T)$ iff $(T-\lambda I)x=y$ has a unique solution $x\in C[0,1]$, regardless of the choice of $y\in C[0,1]$. Claim: $\sigma(T) = \mathcal{R}(v)$ where $\mathcal{R}(v)=\{ v(t) : 0 ... 1$\text{dom}(A) = \text{dom}(T)$is wrong. By the Spectral Theorem, you can essentially assume$T$is multiplication by the variable$x$on$L^2(\mu)$for some positive measure$\mu$on$[0,\infty)$, with$\text{dom}(T) = \{f \in L^2(\mu): x f \in L^2(\mu)\}$. Then you want$A$to be multiplication by$\sqrt{x}$, with$\text{dom}(A) = \{f \in L^2(\mu): ...

1

The spectral theorem for an unbounded selfadjoint operator allows you to represent $T$ as $$Tx = \int_{0}^{\infty}\lambda dE(\lambda)x,\\ \mathcal{D}(T) = \left\{ x \in H : \int_{0}^{\infty}\lambda^{2}d\|E(\lambda)x\|^{2} < \infty \right\}.$$ The unique positive square root of $T$ is  \sqrt{T}y = ...

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