# Cyclic vectors for the shift operator

Let $S:\ell^2\to\ell^2$ be the shift operator $$S(x_1,x_2,\dots)=(0,x_1,x_2,\dots).$$ Let $x\in \ell^2$ with $x_1=1$. Is $x$ cyclic for $S$? In other words, is the span of the vectors $x,Sx,S^2x,\dots$ dense in $\ell^2$? If this does not hold for every $x$, is there a handy criterion to decide whether a given $x$ is cyclic or not?

• You mean the span is dense? And you mean $x_2$, $x_3$, etc. are zero? Is Halmos: Hilbert space problem book, problem 160 related to your question? – András Bátkai Feb 19 '16 at 7:22
• If the $x_2,x_3$ etc, are zero, the answer is clear. So I don't assume this to holds. – user1688 Feb 19 '16 at 7:33
• Intuitively yes, because you can recursively approximate the basis vectors: $x-x_2\cdot Sx + (x_2^2-x_3)S^2x+ \ldots$. But I am travelling now, cannot think about it seriously. Sorry. I hope this works out. – András Bátkai Feb 19 '16 at 8:40

This is part of what the theory of Hardy spaces is for. There is a complete characterization of the cyclic vectors for the shift, though it applies to the Fourier transform of $x$ rather than the sequence $x=(x_0, x_1, \dots)$ itself. (For convenience I am starting the indexing at 0 rather than 1.) As in Fedor's answer, we can view the $x$ sequence as the sequence of Fourier coefficients of an $L^2$ function $h$ on the unit circle, which can be extended analytically to the disk $|z|<1$ by $$h(z) = \sum_{n=0}^\infty x_n z^n.$$ The function $h$ is said to belong to the Hardy space $H^2$, which sits as a closed subspace of $L^2$ on the circle. In this model, the right shift becomes the map $h(z)\to zh(z)$. It is then a fact that $\log|h|$ belongs to $L^1$ of the unit circle, and the inequality $$\log|h(0)|\leq \frac{1}{2\pi} \int_0^{2\pi} \log|h(e^{i\theta})| \,d\theta$$ always holds. Such an $h$ is called an outer function if equality holds (so in particular $x_0=h(0)\neq 0$ is necessary but not sufficient). Finally, it is a theorem that $h$ is cyclic for the shift if and only if $h$ is outer. All of these facts may be found e.g. in the book Banach Spaces of Analytic Functions by Kenneth Hoffman.

• Nice. I had some vague memory that Hardy spaces were involved but I didn't remember just how. – Nik Weaver Feb 19 '16 at 17:22
• Hm, I do not understand something here. At first, if $h(z)=1-z$, then $h$ is cyclic, since $1-(z+z^2+\dots+z^n)/n=:h(z)p_n(z)$ tends to 1 in $H^2$. But $1/h\notin H^2$. – Fedor Petrov Feb 20 '16 at 17:18
• At second, if $h$ does not has zeroes inside the unit disc, $\log |h|$ is locally harmonic function (it is a real part of analytic function $\log h$ which is locally well defined), hence globally harmonic in the unit disc, hence it coincides with its average over a circle, i.e. your inequality is always an equality. – Fedor Petrov Feb 20 '16 at 17:21
• Of course $1/h$ need not belong to $H^2$ when $h$ is outer, my edit was too hasty and I have removed it. However the inequality is not always an equality for nonvanishing functions; in particular it is a strict inequality whenever $h$ has a singular inner factor...the simplest example is $h(z)=exp((z+1)/(z-1))$. – Mike Jury Feb 20 '16 at 17:38
• OK, I see the source of confusion, I had a superfluous $r$ inside the integral...the integral is meant to be taken over the unit circle. – Mike Jury Feb 20 '16 at 17:49

Not always. Let $(e_n)$ be the standard basis of $l^2$ and take $x = e_1 - 2e_2$. For any $k$ the vector $S^kx$ is orthogonal to the sequence $(2^{-n})$, so $x$ is not cyclic.

You ask for a handy criterion that $\overline{\rm span}(x, Sx, S^2x, \ldots)$ equals $l^2$. I don't know, but note that this subspace is invariant for $S$, so it is necessary and sufficient that it contain the vector $e_1$. So a sufficient condition for cyclicity is that the series mentioned by András in the comments should converge. I guess that is probably pretty close to a necessary condition, too, because you need to use a series close to this one in order to get close to $e_1$.

I suppose my conjecture is that $x$ is cyclic if and only if András's series converges weakly. You can see that weak convergence might be necessary by taking $x = e_1 - e_2$. This vector is cyclic because anything that is orthogonal to $S^kx$ for all $k$ must be a constant sequence and hence, if it is in $l^2$, the zero sequence. But the series you get by trying to solve $\sum a_k S^kx = e_1$ is $x + Sx + S^2 x + \cdots$, which only converges weakly.

Consider $x=(x_0,x_1,\dots)\in \ell^2$ as a function $F_x:=\sum_{k=0}^{\infty} x_k z^k\in L^2({\mathbb S}^1)$ (this is norm-preserving). Then shift operator multiples functions by $z$. If $x$ is so that $F_x(a)=0$ for some complex $a$, $|a|<1$, then also $a^k F_x(a)=F_{S^k x}(a)=0$, and since $$f(z)\rightarrow f(a)=\frac1{2\pi i}\int \frac{f(z)dz}{z-a}$$ is a continuous functional on $L^2$ restricted to our space (well, not just our, but Hardy space), we see that orbit of $x$ under $S$ lies in a proper closed subspace. In Nik Weaver's example function is $1-2z$ and $a=1/2$.

• I am starting to suspect that you are more than one person. You know too many things ... – Nik Weaver Feb 20 '16 at 17:56
• @NikWeaver have you ever been in St. Petersburg? Every busker on Nevsky knows Hardy spaces better than me. – Fedor Petrov Feb 23 '16 at 11:23