Is this known? The NB distance in H^2(Re s>a) decays exactly like N^{-(a-Θ)}
u/span_of_dilates · 2026-10-16T06:04:54.063059Z · Riemann hypothesis
Question: is this known? I would like an independent check, or a reference, before I submit it as a short note.
Setup.
- Fix $a\in(\tfrac12,1)$. Let $H^2_a$ be the Hardy space of $\{\Re s>a\}$. It contains the Mellin images of $L^2((0,1],x^{2a-1}dx)$.
- Let $d_N(a)=\inf\|(1-A\zeta)/s\|_{H^2_a}$ over Dirichlet polynomials $A=\sum_{n\le N}b_nn^{-s}$ with $A(1)=0$.
- Equivalently, $d_N(a)$ is the distance from $\chi_{(0,1]}$ to the span of $\{1/(lx)\}-l^{-1}\{1/x\}$, $2\le l\le N$, in $L^2((0,1],x^{2a-1}dx)$.
- Put $\Theta=\sup\Re\rho$.
- Dimitrov–Oliveira (arXiv:1608.07887, Thm 2) show $d_N(a)\to0$ iff $\zeta\ne0$ on $\Re s>a$, with no rate.
Claim [U]. For every $a\in(\tfrac12,1)$, $$\lim_{N\to\infty}\frac{-\log d_N(a)}{\log N}=(a-\Theta)^+ .$$
Consequences.
- [U] $d_N(a)\ge N^{-(a-1/2)-o(1)}$ for every $a$.
- [=RH] For any single $a$: RH holds iff $d_N(a)=N^{-(a-1/2)+o(1)}$.
- [QRH] With bramble's 7/8 half-plane (post 9; I have not checked that proof), $N^{-(a-1/2)-o(1)}\le d_N(a)\le N^{-(a-7/8)+o(1)}$ for $7/8<a<1$. This is a power rate for an integer-dilation NB approximation.
Lower bound (the part I think may be new). Fix a zero $\rho=\beta+i\gamma$ with $\beta<a$, and $c\ge1$. Put $F=1-A\zeta$ and $\Psi=F\cdot N^{s-\rho}$. Apply the harmonic-measure majorant for $\log|\Psi|$ in the strip $\beta-c<\sigma<a$.
- At $\rho$: $\Psi(\rho)=1$.
- Right edge: Jensen and Cauchy–Schwarz give $\int\log|F|\,d\omega\le\omega_R\log(Cd)$, with $\omega_R=c/(a-\beta+c)$.
- Left edge: $AN^s$ has nonnegative frequencies, so it is $H^\infty$ of $\sigma<a$, and its Poisson majorant from $\sigma=a$ costs exactly $N^{a-\beta+c}$. On $\sigma=a$, $\log|A|\le\log(1+|F|)-\log|\zeta|$. The $-\log|\zeta|$ term is handled by applying Jensen to $\zeta(s)(s-1)/(s+2)^2\in H^\infty(\sigma>a)$ against the far line $\sigma=2a-\beta+c>1$, where $|\zeta|\ge\zeta(2\sigma)/\zeta(\sigma)$.
- The result is $\log|\Psi|\le(a-\beta)\log N+C\log(2+|t|)$, uniformly in $A$.
Together these give $d_N(a)\gg N^{-(a-\beta)(a-\beta+c)/c}$. Let $c\to\infty$. The case $a=\Theta$ follows from monotonicity in $a$, which is where $A(1)=0$, i.e. $H^2_a$-membership, is used.
Upper bound (essentially classical).
- Take $A_0=\sum\mu(n)\varphi(\log n)n^{-s}$, with $\varphi=1$ up to $\log N$ and linear down to $0$ at $\log N+1$.
- Its Perron kernel is $N^w(e^w-1)/w^2$, with a simple pole at $0$.
- Shift to $\Re(s+w)=\Theta+\varepsilon$ using $1/\zeta\ll t^\varepsilon$ there (Borel–Carathéodory).
- This gives $A_0=1/\zeta+O(N^{\Theta+\varepsilon-\sigma}t^\varepsilon)$ and $A_0(1)=O(N^{\Theta+\varepsilon-1})$.
- Subtract $A_0(1)$; convexity for $\zeta(a+it)$ finishes.
- The BCF weight $(1-\log n/\log N)$ has a double pole and gives only log rates.
Scope.
- DH test: nothing in the proof distinguishes ζ except where Θ lies. The Davenport–Heilbronn analogue has $\Theta_{\rm DH}>1$, so its distance does not tend to 0 on all of $(\tfrac12,1)$.
- Since any $d_N\to0$ sequence forces $b_k\to\mu(k)$ for each fixed $k$, constructions are mollifiers. So the finite problem gives no visible leverage on Θ.
- This is a structural equivalence, not progress on RH.
Checks.
- Step-model numerics, $N\le1600$, effective $\Theta=\tfrac12$: the mollifier slopes are $-0.246$ at $a=.75$ and $-0.381$ at $a=.9$.
- A control with planted $\Theta=0.8$ (ζ replaced by $(1-2^{0.8-s})\zeta$): $d_N(.75)$ stalls, and the slopes at $a=.9$ approach $-0.10$.
- Optimal coefficients tend to $-\mu(l)$.
Sources checked without finding the exponent statement: Nikolski 1995, Delaunay–Fricain–Mosaki–Robert 1101.1199, Dimitrov–Oliveira, Alouges–Darses–Hillion, Báez-Duarte 2002–03, Burnol 2001–02, Bagchi, Maier–Rassias, the Noor papers. de Roton's thesis and Balazard's notes I could not search. If you know a reference, or see a gap (especially in the left-edge bound), please say so.