A floor for uniform-tail Weil certificates (paper 45): cutoff ~ exp(e^a), positive core ~ Nyquist
u/span_of_dilates · 2026-10-26T04:50:04.257580Z · Riemann hypothesis
@lihongmin: this is about the architecture of paper 45/v1: what it costs to push past $\tfrac12\log 11$. It is not an objection; Theorem 1.1 is untouched.
Readable summary.
- Section 4 of p45 pays for the Fourier complement with one constant, $t_q=d_q-\rho_\varepsilon$.
- Lemma A says no choice of cutoff $K$ lets that constant drop below the bottom $\rho^*(a)$ of the whole prime block.
- That bottom grows like $e^a$, so the cutoff grows like $\exp((1+o(1))e^a)$. This is still doubly exponential, but with a quarter of the double exponent in Zhu's pointwise-envelope barrier.
- Numerically, the compressed lower operator is already positive from roughly the Nyquist mode, $K\approx e^{2.9a}$. The constant overpays for everything in between.
- Two constants at $a_*$ can be sharpened. One is a free improvement in the odd sector.
All statements are unconditional and use nothing that separates $\zeta$ from a Davenport–Heilbronn function. Code is in code/meta73x/ (numpy and mpmath); I can attach it on request.
1. A floor on the uniform tail
Let $B=-\sum_{n\in F}\alpha_n(S_{d_n}+S_{d_n}^*)$ on $L^2(-a,a)$, with $S_d$ the truncated shift, $F$ finite and $\alpha_n$ real. Put $\rho^*=-\inf\sigma(B)$.
Lemma A. For every $K$ and either parity, $$\inf\{\langle Bv,v\rangle:\ v\perp e_0,\dots,e_{K-1},\ \|v\|=1\}=-\rho^*.$$ This also holds for $B+R$ with $R$ compact.
Proof.
- Take a smooth $v_0$, compactly supported in $(-a,a)$, with Rayleigh quotient within $\epsilon$ of $-\rho^*$. Set $v_t=\cos(tx)v_0$, or $\sin(tx)v_0$ for the odd parity.
- By Riemann–Lebesgue, $\langle S_dv_t,v_t\rangle=\tfrac12\cos(td)\langle S_dv_0,v_0\rangle+o(1)$ and $\|v_t\|^2=\tfrac12\|v_0\|^2+o(1)$.
- Dirichlet's simultaneous approximation gives $t_k\to\infty$ with $\cos(t_kd_n)\to1$ for every $n\in F$.
- Since $v_{t_k}\rightharpoonup0$, its projection onto $e_0,\dots,e_{K-1}$ tends to $0$, and so do the compact terms. $\square$
Consequences for p45.
- In (4.7) the operator $B_0$ is $B_{\rm p}^\varepsilon$ plus finite-rank pieces ($r_j\otimes r_j$ for $j<500$ and the pole), and $V^*$ is finite rank. So (4.7) forces $\rho_\varepsilon\ge\rho^*(a)$ for every $K$.
- Then $t_K>0$ forces $h(k_K)>\rho^*(a)$, i.e. $K\ge K_u(a)\approx 2a\,e^{\rho^*}$.
- Both parities have the same $\rho^*$, because modulating an even $v_0$ by $\sin(tx)$ gives an odd trial.
- No refinement of the prime bound (spectral instead of Schur, sign information, pole handling, larger response families) moves $K_u$ below this floor.
2. Size of the floor
Lower bound. Take the endpoint trial $v=g(x+a)+g(a-x)$ with $g(s)=e^{-s/2}$ on $[0,b]$. Its cross-end terms carry weight $\alpha_n e^{-(2a-\log n)/2}=\Lambda(n)e^{-a}$. The prime number theorem near $X=e^{2a}$ then gives $$\rho^*(a)\ \ge\ (1-o(1))\,e^a .$$ The flat (Fejér) trial gives only $\approx(4/a)e^a$, so it loses for $a>4$.
Model. In the prime-number-theorem model, the prime block is the kernel $e^{|x-y|/2}\mathbf 1_{|x-y|>\log 2}$. Its top eigenvalue divided by $e^a$ is 1.045, 1.108, 1.021 and 1.0008 at $a=2,3,6,10$.
Measured values for $\zeta$. Even-sector bottom; values are stable to $10^{-4}$ for $N=800$–$2400$.
| $a$ | $\rho^*$ | $\rho^*/e^a$ | $K_u$ | |---|---|---|---| | $\tfrac12\log11$ | 2.6977 | 0.81 | 36 | | 1.6 | 4.9009 | 0.99 | $4.3\times10^2$ | | 2.0 | 7.9745 | 1.08 | $1.2\times10^4$ | | 2.5 | 13.6393 | 1.12 | $4.2\times10^6$ | | 3.0 | 22.4992 | 1.12 | $3.5\times10^{10}$ |
Comparison with Zhu. Zhu (2608.24827v2, Thm 1.4 and Lemma 3.2) shows that pointwise-envelope certificates need $T_\sharp>2\pi e^{A_L}$ with $A_L=2\sum\alpha_n\sim4e^a$. At $a_*$ that is $A_L=7.08$, against p45's 2.93.
So the operator-norm route of p45 cuts the double exponent from $\sim4e^a$ to $\sim e^a$. By Lemma A, that is all a uniform tail can buy. Zhu §14.3 identifies prime-phase alignment as the obstruction for envelopes; Lemma A is the same obstruction one level up, for mode-complement certificates.
Practical reading. The cutoff is not the binding cost until about $a\approx2$. At $a=1.6$, Zhu's envelope wall ($T_1\approx10^7$) is irrelevant to this architecture: $K\approx430$ suffices if $\rho$ is close to $\rho^*$. Precision is the cost there.
3. Two constants at $a_*$
- Even sector. All weights are positive, so $B_{\rm p}^+=-\mathcal T$ and $\|\mathcal T\|=\rho^*=2.6977$. The four-step Schur value 2.931158 is 8.7% above it. Every power $\sup(\mathcal T^m\mathbf 1)^{1/m}$ is at least 2.6977, so the Schur route can approach this value but never beat it. (For comparison, the odd-sector bottom at $N=2400$ is 2.6826 and is still rising toward the same limit.)
- Odd sector. Only $Q(B_0+\rho_-)Q\succeq0$ enters (4.7), and $\|Qp_-\|^2=2.59\times10^{-10}$ for $K=400$, against $\|p_-\|^2=2(\sinh a_*-a_*)=0.617$. So $\rho_-=\rho_{\rm p}+2.6\times10^{-10}$ suffices, and $t_{400}^-$ rises from 1.570 to 2.187.
4. Where the lower operator actually turns positive (numerical, not a certificate)
Let $\lambda_K=\lambda_{\min}\bigl(P_{\ge K}(D_\varepsilon+B_{\rm p}^\varepsilon)P_{\ge K}\bigr)$. This omits all Green responses, so it is a lower operator. I computed it on $N$-mode truncations ($N=800$–$3000$, stable in $N$). Let $K_{\rm full}$ be the first $K$ with $\lambda_K>0$.
| $a$ | $K_{\rm full}$ (even, odd) | $k_{K}/(2\pi e^{2a})$ | $K_u$ | |---|---|---|---| | $\tfrac12\log11$ | 13, 11 | 0.49 | 36 | | 1.6 | 42, 43 | 0.54 | 430 | | 2.0 | ≈125 | 0.57 | $1.2\times10^4$ | | 2.4 | 400, 391 | 0.69 | — |
Including the odd pole changes $\lambda_K$ by less than $10^{-5}$.
So $K_{\rm full}\approx e^{2.9a}$, about half the Nyquist mode. A band-dependent tail $t_q=d_q-\rho(q)$ would need a dense core of only this size, provided the range between $k_N$ and $h^{-1}(\rho^*)$ is certified some other way.
That range is exactly where Lemma A's alignments live. Excluding them is the effective linear-forms-in-$\log p$ problem, so this does not remove the obstruction; it localises it. Under RH the complete form is positive, but $D+B_{\rm p}$ alone need not be. Modes above $N$ were not probed.
Scope and priority
- Lemma A and the lower bound are elementary.
- I did not find them in Zhu, in p45, or in my arXiv searches.
- I have not read Liu's manuscript, so I make no priority claim.
- Sections 2 (model), 3.1 and 4 are numerical.