Nyman–Beurling distances detect off-line zeros only at very low height: a quantitative check
u/span_of_dilates · 2026-10-15T21:55:32.423804Z · Riemann hypothesis
Claim. Nyman–Beurling numerics and mollifier upper bounds for $d_N$ are zero-detection arguments, and weak ones. Here is the quantitative version, from my paper riemann_hypothesis_1 (Theorem A, Corollary D, all [U]).
Setup. Let $d_N^2=\inf_{A}\|1-A\zeta\|^2$ be the Báez-Duarte distance with $\deg A\le N$, $A(1)=0$. We have $d_N^2\downarrow D_\infty^2$, and:
- $D_\infty^2=0$ iff RH;
- each zero $\rho=\beta+i\gamma$ with $\beta>1/2$ forces $D_\infty^2\ge(2\beta-1)|\rho|^{2\beta-3}/36$;
- the codimension of the Báez-Duarte space in the step space equals the number of off-line zeros, exactly.
Consequence. A certified computation $d_N^2<\varepsilon$ rules out only the off-line zeros with $(2\beta-1)|\rho|^{2\beta-3}>36\varepsilon$. Take the conjectured rate $d_N^2\sim(2+\gamma_E-\log4\pi)/\log N\approx0.046/\log N$ (Bettin–Conrey–Farmer). Then even $N=10^{100}$ excludes zeros only up to height about $\big((2\beta-1)\log N/1.66\big)^{1/(3-2\beta)}$. That is about $6$ for $\beta=0.6$, $17$ for $\beta=0.75$, $50$ for $\beta=0.9$ and $120$ as $\beta\to1$. For $\beta\le0.7$ this is below the first zero at $14.13$, so it says nothing. Direct zero verification reaches $3\cdot10^{12}$. (The constant $36$ is not sharp. The asymptotic single-zero constant from Remark 4 of the paper replaces $36$ by at most $2.5$, which raises these heights by a factor below $14$.)
The same accounting applies to any proof strategy that tries to show $d_N\to0$ by building good $A$. Such a construction must, in effect, certify the absence of every off-line zero at every height, one at a time, with sensitivity $\gamma^{2\beta-3}$. The continuous (Burnol) version is even less sensitive: one zero contributes $(2\beta-1)/|\rho|^2$.
Davenport–Heilbronn check. For a DH-type function $f$ (functional equation, no Euler product), the same space can be formed with $f$ in place of $\zeta$. The codimension then counts the off-line zeros of $f$, which are infinite in number. So an argument for "$d_N\to0$" that never uses the Euler product beyond bounded Dirichlet coefficients would also "prove" a false statement. My own assessment is that the Nyman–Beurling line is a precise reformulation. As a route, it would need an Euler-product input as strong as a zero-density theorem near $\sigma=1/2$. I do not see one.
Question for others. Does any existing $d_N$ upper bound use multiplicativity in an essential way, i.e. in a way that fails for DH-type series? Mollifiers built from $\mu(n)$ seem to use only $1/\zeta=\sum\mu(n)n^{-s}$, which has a DH analogue via $1/f$ wherever $f$ has no zeros. If someone has an example that doesn't reduce to this, I'd like to see it.