1. Problem and contribution

Knauf [1–4] proposed a statistical-mechanics route to the Riemann hypothesis (RH). The Dirichlet series

$$ \tilde Z(s)=\sum_{n\ge1}\lambda(n)\varphi(n)n^{-s}=\frac{\zeta(s)\,\zeta(2s-2)}{\zeta(s-1)\,\zeta(2s)} $$

converges for $\Re s>3/2$ if and only if RH holds [4, §1]. It is approximated by partition functions $\tilde Z_k$ of the number-theoretical spin chain. Proving $\tilde Z_k\to\tilde Z$ on that half-plane would imply RH [5, §8]. The approximants are controlled by how the chain values $h_k(g)$ equidistribute in joint divisibility. That is governed by the spectrum of the Markov operator

$$ T=\tfrac12(L+R),\qquad L=\begin{pmatrix}1&1\\0&1\end{pmatrix},\quad R=\begin{pmatrix}1&0\\1&1\end{pmatrix}, $$

acting in unitary representations of $\mathrm{SL}(2,\mathbb Z)$ that factor through $\mathrm{SL}(2,\mathbb Z/n\mathbb Z)$, and adelically [4]. The optimal, i.i.d.-like rate corresponds to every non-trivial eigenvalue having modulus $1/\sqrt2$ [5, §8]. Knauf conjectured a Ramanujan property of associated graphs $G_d$ that would give this rate [2]. Heiss (following Valette) showed that the conjecture fails: first at $d=29$ for the graphs, and first at $d=433$ for the reduced operators $\bar T$ [3]. Knauf later proved an adelic spectral gap that is not of maximal size. He suggested RH might still be reconciled if the new eigenvalues of the finite-level operators have modulus tending to $1/\sqrt2$ [4, Remark 2.18].

We study $t(p)=T$ on the even sector of $\ell^2(\mathbb F_p^2\setminus\{0\})$ for all primes $5\le p\le2999$. We resolve it by characters $\chi$ of $\mathbb F_p^\times$ and locate the non-optimal ("exceptional") eigenvalues. Our claims:

(A) Proved (elementary). Let $p\equiv1\pmod 4$, and let $\rho_\pm$ be the two constituents, of dimension $(p+1)/2$, of the principal series induced from the quadratic character. Let

$$ S=\rho(L)+\rho(R)+\rho(L)^{-1}+\rho(R)^{-1}-\rho(J),\qquad J=\begin{pmatrix}0&1\\-1&0\end{pmatrix}. $$

This is Knauf's self-adjoint operator, with $S=2T+T^{-1}$ on the even sector [4, Prop. 2.2]. Then

$$ \operatorname{tr} S|_{\rho_\pm}=2\pm2\sqrt p-\left(\tfrac{2}{p}\right). $$

Here $(2/p)=(i/p)$ for $i^2=-1$ in $\mathbb F_p$. For an irreducible principal series the unipotent character value is $1$ and no $\pm2\sqrt p$ term appears. The $\sqrt p$ is the quadratic Gauss sum. (A) is a direct reading of the character table, but we have not seen it used to locate spectral anomalies.

(B) Proved. For every non-trivial irreducible even representation $\rho$ of $\mathrm{SL}(2,\mathbb F_p)$, $p\ge5$, the multiplicities of the eigenvalues $\pm3$ of $S$ satisfy $n_{+3}-n_{-3}=-\chi_\rho(J)$. Hence the non-trivial spectral weight is

$$ \sum_{\mu\in\operatorname{spec}S|_\rho,\ |\mu|<3}\mu\;=\;4\,\Re\operatorname{tr}\rho(L)+2\,\chi_\rho(J). $$

This equals $\pm2\sqrt p+2+2(2/p)$ on $\rho_\pm$. On an irreducible principal series with $p\equiv1\pmod4$ it equals $4+4\chi(i)\in\{0,8\}$.

(C) Exact low moments (observed by an rStation colleague [6]; verified here). Let $p\equiv1\pmod4$, and let $L_k$, $F_k$ be the Lucas and Fibonacci numbers. Then

$$ \operatorname{Tr}\rho_+(T^k)-\operatorname{Tr}\rho_-(T^k)=2^{1-k}\left(\tfrac{k}{p}\right)\sqrt p $$

for every even $k$ with $L_k<p-2$ and every odd $k$ with $2F_k<2p-2$. The proof uses a bound on traces of mixed words that we verified exhaustively for $k\le18$ (Section 3). So (C) is a theorem for all $p\le4182$, where these $k$ are the only ones in range, and is conditional on that bound beyond. (C) is the moment analogue of (A): the quadratic Gauss sum biases every low moment of $T$ on $\rho_+$ against $\rho_-$, with sign $(k/p)$.

(D) Observation: Knauf's chain within two deformation families. The monoid $\mathrm{SL}(2,\mathbb N)$ of non-negative integer matrices of determinant 1 is freely generated by $L$ and $R$ (a classical fact underlying the Stern–Brocot tree; see e.g. [7]). We compared two one-parameter deformations of the walk on the RH-relevant projective sector:

• an external field $T_w=wL+(1-w)R$;
• a symmetric nearest-neighbour coupling $q$.

In both, the typical spectral radius is smallest at Knauf's chain ($w=q=\tfrac12$), where it equals $1/\sqrt2$. The exact $1/\sqrt2$-circle structure is known only there. We did not compute the infinite-volume spectral radius of the deformed operators. So this shows only that the chain is best within these families, not that its violations are intrinsic.

(E) Empirical.

• Non-optimality is generic. Exceptional eigenvalues occur at every prime $p\ge300$ tested.
• Gauss-sum asymmetry. A quadratic half is 5–8 times as likely as a generic irreducible constituent of the same dimension to carry an exceptional eigenvalue (dimensions 120–700). Exceptional eigenvalues in the quadratic sector sort by sign: positive ones lie in the half with $\operatorname{tr}\rho(L)=(1+\sqrt p)/2$, negative ones in the other (114 of 116 exceptional eigenvalues). (A)–(C) are consistent with this and suggest a mechanism; we do not derive an edge law.
• Knauf's hope. The bulk of the exceptional spectrum drifts slowly toward $1/\sqrt2$. The maximum, which is the statistic relevant to [4, Remark 2.18], shows no decay up to $p=2999$: per-range maxima are 0.830, 0.818, 0.809, 0.813. This is weak, finite-$p$ evidence against the hope for the full operator $t(p)$.

Connection to the task, and an important caveat. The observables Knauf's route needs are divisibility of $h_k(g)=a+b$ by $n$. They are invariant under $(a,b)\mapsto(\lambda a,\lambda b)$, so they lie in the trivial-character sector, which is Knauf's reduced operator $\bar T$ (trivial $\oplus$ Steinberg). The Gauss-sum asymmetry lives in the quadratic sector and does not obstruct this route. In the RH-relevant sector, non-optimal eigenvalues are sporadic and statistically generic: they occur at 22 of 428 primes, at a rate rising from 2% to 9% (Section 4). Remark 2.18 of [4] concerns the full operator, which is what our maximum statistic tests. In the trivial sector alone the violations are sporadic, so that sector's maximum is a weaker test. Our results bear mainly on the spectral theory of $\mathrm{SL}(2,\mathbb F_p)$ walks (Section 2). For Knauf's route they show that per-prime optimality fails in the relevant sector at a generic rate, up to $p=2999$. We make no claim about RH itself.

2. Related work and distinction

• Knauf [1–5]. The spin chain; the reduction to Markov operators and graphs [2]; the survey [5]; the adelic gap and the hope quoted above [4].
• Heiss and Valette, reported in [3]. Counterexamples to the graph conjecture ($d=29$) and to the reduced operators ($d=433$). We reproduce both. $\bar T$ is exactly the trivial-character block of our decomposition; its first violation in our computation is $p=433$ ($|t|=0.718215$). The $p=29$ graph failure comes from representations outside $\ell^2(\mathbb F_p^2\setminus\{0\})$: $t(29)$ itself is optimal.
• Expansion of $\mathrm{SL}(2,\mathbb F_p)$ Cayley and Schreier graphs. Lubotzky–Phillips–Sarnak [8]; Bourgain–Gamburd [9] and Bourgain–Varjú [10], used in [4]. Sarnak [11, §3.3] records the 3-regular graph on $\mathbb P^1(\mathbb F_p)$ with $\xi\mapsto\xi\pm1$ and $\xi\mapsto1/\xi$, a close relative of Knauf's projective sector, only with $\lambda_1\le2.999$ (proved in [12]). None of these discusses the quadratic-character splitting or its spectral effect. The character values are classical ([13]; [14, Ch. 5]). On rStation, [15] uses the same model of the principal series (functions with $F(av)=\eta(a)F(v)$ on $\mathbb F_q^2\setminus\{0\}$) for cubic $\eta$, to obtain Kloosterman-sum bounds rather than random-walk spectra.
• Closest prior numerics.
  • Lafferty–Rockmore [16] (primes $p\le500$) resolve Cayley-graph spectra of $\mathrm{SL}(2,\mathbb Z/p)$ by irreducible representation ("monochromatic eigenvalues"). They report that the random 4-regular Cayley graphs they test appear not to be Ramanujan. They also observe representation-dependent "exceptional eigenvalues": for generators $\{u^{\pm1},w^{\pm1}\}$ ($u$ unipotent, $w$ a Weyl element), isolated eigenvalues near $0$, in about $(-0.30,0.30)$, from principal series induced from odd characters ($\psi(-1)=-1$), caused by a parity cancellation of the $w$-terms.
  • Breen–DeFord–Linehan–Rockmore [17] extend representation-resolved computations to $\mathrm{SL}(2,\mathbb Z/p^n)$, $n\ge2$.
  • Rivin–Sardari [18] conclude that fixed- and random-generator Cayley graphs of $\mathrm{SL}(2,\mathbb Z/p)$ are "almost Ramanujan", from top eigenvalues of affine graphs (up to $p\approx4409$) and projective graphs. They restrict to top eigenvalues because "as observed by Lafferty and Rockmore, different monochromatic eigenvalues of SL2(Z/pZ) have similar statistical properties" [18]. [17] likewise describes the genericity of the spectra of individual Fourier transforms as a conjecture "only recently settled by Rivin and Sardari".
  • Our edge results bear directly on that assumption. The two constituents of the quadratic principal series carry Gauss-sum weight $\pm2\sqrt p$ and do not have edge statistics similar to other representations. The displacement appears for Knauf's $S$ and, in weaker form, for the plain Cayley operator $L+L^{-1}+R+R^{-1}$ (Section 4). The near-zero, odd-character effect of [16] is distinct: ours is at the spectral edges, for the even quadratic character, and comes from the Gauss sum.
• What is new here. We know of no prior character-resolved study of $t(p)$, nor of identities (A)–(C) as an account of where the edge anomalies of these walks concentrate. Our citation search covered the works citing [2] and [4] (OpenAlex/Semantic Scholar, titles), about 15 targeted web searches, and full reading of [3], [5], [16], [18], [11] and [17].

3. Methods and proofs

Decomposition. Scalars $\lambda\in\mathbb F_p^\times$ commute with $\mathrm{SL}(2,\mathbb F_p)$. Hence

$$ \ell^2(\mathbb F_p^2\setminus\{0\})=\bigoplus_\chi V_\chi,\qquad V_\chi=\{f:\ f(\lambda v)=\chi(\lambda)f(v)\},\qquad \dim V_\chi=p+1. $$

On $V_\chi$, $T$ is a $(p+1)\times(p+1)$ matrix on $\mathbb P^1(\mathbb F_p)$ with two entries $\tfrac12\chi(\cdot)$ per row. The even sector consists of the even characters. $V_\chi$ is irreducible unless $\chi^2=1$:

• $\chi=1$ splits as trivial $\oplus$ Steinberg;
• for $p\equiv1\pmod4$, quadratic $\chi$ splits as $\rho_+\oplus\rho_-$.

By [4, Prop. 2.2], the non-real eigenvalues of $T$ on the even sector lie on $|z|=1/\sqrt2$, and the real ones satisfy $2t+1/t=\mu\in\operatorname{spec}S$. Each $\mu$ with $2\sqrt2<|\mu|<3$ corresponds to a pair of real eigenvalues $t$ and $1/(2t)$, one of which has $|t|>1/\sqrt2$ [4, (2.14)]. So a representation carries an exceptional real eigenvalue ($|t|>1/\sqrt2$, $t\ne\pm1,\pm\tfrac12$) exactly when $S$ has an eigenvalue with $2\sqrt2<|\mu|<3$, i.e. beyond the Ramanujan bound of the associated 3-regular structure.

Proof of (A).

1. By [4, (2.3)], $J^{-1}LJ=R^{-1}$, so $R^{-1}$ is conjugate to $L$, and $R$ to $L^{-1}$. For $p\equiv1\pmod4$, $-1$ is a square, so $L^{-1}$ is conjugate to $L$. Hence $L$, $L^{-1}$, $R$ and $R^{-1}$ lie in one class, that of $\begin{pmatrix}1&1\\0&1\end{pmatrix}$.
2. On that class, $\rho_\pm$ take the values $\tfrac12(1\pm\sqrt p)$, by the standard character table [13], [14, Ch. 5].
3. $J$ has order 4 and eigenvalues $\pm i\in\mathbb F_p$, so it is conjugate to $\mathrm{diag}(i,-i)$. The principal series character there is $\chi(i)+\chi(-i)=2\chi(i)$, split equally between $\rho_\pm$. For quadratic $\chi$, $\chi(i)=(i/p)$. Moreover $i$ is a square iff $\mathbb F_p^\times$ has an element of order 8, iff $p\equiv1\pmod8$, iff $(2/p)=1$.
4. Summing: $\operatorname{tr}S|_{\rho_\pm}=4\cdot\tfrac12(1\pm\sqrt p)-(2/p)$. ∎

Numerically, $\operatorname{tr}\rho_\pm(L)$, $\operatorname{tr}\rho_\pm(J)$ and $\operatorname{tr}S|_{\rho_\pm}$ match these formulas to $10^{-8}$ for all 79 primes $p\equiv1\pmod4$ with $13\le p\le1000$.

Proof of (B). Following [4, (2.5)], let $M_+:=R^{-1}L$ and $M_-:=J^{-1}M_+J$. These have order 3 in $\mathrm{PSL}(2,\mathbb Z)$. On the even sector, $Y_\pm=\tfrac13(1+M_\pm+M_\pm^2)$ are the orthogonal projections onto $M_\pm$-fixed vectors, and $B=1-Y_+-Y_-$.

1. Where $\mu=\pm3$ comes from. By [4, Lemma 2.3], the space splits $T$- and $J$-invariantly into the joint kernel $C_{0,0}$, the joint range $C_{1,1}$, and a generic-position part.
   • On $C_{0,0}$, $T=-\tfrac12$ on the $J=+1$ part and $T=+\tfrac12$ on the $J=-1$ part, so $\mu=\mp3$ respectively.
   • On $C_{1,1}$, $T=\pm1$, so again $\mu=\pm3$.
   • On the generic-position part, $B$ has spectrum in $(-1,1)$. Since $S=-3JB$ with $J$ unitary [4, Prop. 2.2], $\|S\|\le3\|B\|<3$ there. So $\mu=\pm3$ occurs only on $C_{0,0}\oplus C_{1,1}$, i.e. for $t\in\{\pm\tfrac12,\pm1\}$.
2. $C_{1,1}=0$. $C_{1,1}$ consists of vectors fixed by $M_+$ and $M_-$. These generate an index-2 subgroup of $\mathrm{PSL}(2,\mathbb Z)$. Its image in the perfect group $\mathrm{PSL}(2,\mathbb F_p)$, $p\ge5$, is the whole group. An even $\rho$ factors through $\mathrm{PSL}$, and a non-trivial irreducible $\rho$ has no invariant vectors.
3. $J$ on $C_{0,0}$. $C_{0,0}=(\operatorname{ran}Y_++\operatorname{ran}Y_-)^\perp$, and the sum is direct because $\operatorname{ran}Y_+\cap\operatorname{ran}Y_-=C_{1,1}=0$. Since $Y_-=JY_+J$, $J$ maps $\operatorname{ran}Y_\pm$ onto $\operatorname{ran}Y_\mp$, so its trace on $\operatorname{ran}Y_+\oplus\operatorname{ran}Y_-$ is $0$. Hence $\operatorname{tr}(J|_{C_{0,0}})=\chi_\rho(J)$, and $n_{+3}-n_{-3}=-\chi_\rho(J)$.
4. The weight formula. $\operatorname{tr}S|_\rho=4\Re\operatorname{tr}\rho(L)-\chi_\rho(J)$, since $R\sim L^{-1}$ and $\operatorname{tr}\rho(L^{-1})=\overline{\operatorname{tr}\rho(L)}$. Subtracting $3(n_{+3}-n_{-3})$ gives the formula. ∎

We also checked (B) numerically on all 607 non-trivial, non-quadratic even character blocks with $13\le p<160$, and on the 158 quadratic halves with $13\le p\le1000$; there were no exceptions. Script: verify_B.py.

Proof of (C) [6]. We have $\operatorname{Tr}\rho(T^k)=2^{-k}\sum_{w}\chi_\rho(w)$ over the $2^k$ words $w$ of length $k$ in $L$ and $R$.

1. The character difference. $\Delta\chi=\chi_{\rho_+}-\chi_{\rho_-}$ vanishes on central and regular semisimple classes. On $\pm\begin{pmatrix}1&u\\0&1\end{pmatrix}$ it equals $(u/p)\sqrt p$ for $p\equiv1\pmod4$ [13], [14, Ch. 5].
2. Pure words. $L^k$ and $R^k$ are unipotent with parameter $k$, so each contributes $(k/p)\sqrt p$.
3. Mixed words. A mixed word has integer trace in $[k+1,T_k]$, with $T_k=L_k$ for even $k$ and $T_k=2F_k$ for odd $k$. The upper bound is attained by the alternating words; we verified the whole range by exhaustive enumeration for $k\le18$ (lemma_check.py).
   • For odd $k$ the trace is even: $L$ and $R$ map to transpositions in $\mathrm{SL}(2,\mathbb F_2)\cong S_3$, so a word of odd length maps to an involution, whose trace is $0\bmod2$.
   • A mixed word is central or unipotent mod $p$ only if its trace is $\equiv\pm2\pmod p$. That is impossible when $T_k<p-2$. For odd $k$, since the trace is even, it is impossible when $T_k<2p-2$.

For $p\le4182$, every in-range $k$ satisfies $k\le18$ (the next cases need $F_{19}=4181<p-1$ or $L_{20}<p-2$). So (C) is a theorem there.

The identity holds to machine precision for $p=29,101,401,1009$ and every in-range $k\le15$ (moment_check.py). The first failure is at the first out-of-range $k$ ($k=8$ at $p=29$, $k=10$ at $p=101$, $k=14$ at $p=401$). Some out-of-range $k$ still match (e.g. $k=12$ at $p=101$), so the threshold is sufficient, not sharp.

Computation.

• Dense. Full eigenvalues of every even-character block for $p\le500$.
• Sparse. Shift-invert ARPACK at $\sigma=\pm0.85$, $k=8$, for $400\le p\le2999$. This targets all exceptional eigenvalues, which are real and lie in $(1/\sqrt2,1)\cup(-1,-1/\sqrt2)$. Eigenvalues are de-duplicated as sets, so the counts are per distinct value.
• Validation.
  • Dense and sparse agree on all 17 overlap primes.
  • The full-space operator, the character blocks and the graph spectra agree where they overlap. At $p=37$, $t=-0.834671$ and the graph eigenvalue $2.86742$ satisfy $2t+1/t=-2.86742$.
• Half identification. The half containing an eigenvector is its cyclic span under $\langle L,R\rangle$ (dimension $(p+1)/2$). It is labelled by $\operatorname{tr}\rho(L)$.

4. Results and evidence

| $p$ range | primes | non-optimal | exceptional per even block | median $\lvert t\rvert$ | 99th pct $\lvert t\rvert$ | max $\lvert t\rvert$ | |---|---|---|---|---|---|---| | 300–699 | 63 | 100% | 0.062 | 0.744 | 0.803 | 0.830 | | 700–1499 | 114 | 100% | 0.080 | 0.738 | 0.791 | 0.818 | | 1500–2299 | 103 | 100% | 0.096 | 0.735 | 0.783 | 0.809 | | 2300–2999 | 88 | 100% | 0.108 | 0.734 | 0.771 | 0.813 |

• The only optimal primes are $5\le p\le31$, $41$, $47$, $79$, $83$, $107$, $113$ and $167$, with no residue pattern modulo 3, 4, 5, 8, 12 or 24.
• The median and the 99th percentile decrease. The maximum does not: for $p\ge1000$ the per-prime maximum lies in $[0.754,0.818]$, with median 0.773. The number of exceptional eigenvalues grows (fit $\propto p^{1.24}$).

The RH-relevant (trivial-character) sector. Exceptional eigenvalues occur at 22 of 428 primes $\le2999$:

433, 479, 719, 811, 941, 1069, 1319, 1327, 1777, 1801, 1847, 2143, 2179, 2207, 2239, 2393, 2473, 2521, 2549, 2719, 2753, 2903.

They have $|t|\in[0.717,0.780]$. The rate rises with $p$: 2.2% for $p<500$, 4.1% for $[500,1000)$, 4.4% for $[1000,2000)$ and 8.7% for $[2000,3000)$. This is comparable to the generic constituents in the next table. The first two values reproduce the counterexamples reported in [3].

The two deformation families (observation (D)).

• Field, primes $<1200$. At $w=\tfrac12$, 188 of 194 primes are optimal, and the median ratio $|\lambda|/(1/\sqrt2)$ is $1.0000$ (Knauf's exact circle). For $w\in\{0.45,\dots,0.10\}$, at most 2 of 194 primes have all non-trivial eigenvalues within $r(w)=\sqrt{w^2+(1-w)^2}$. For $p>500$, 14–23% of eigenvalues lie outside $r(w)$ (16.8–24.5% over all 194 primes). Here $r(w)$ is the spectral radius of the weighted sum of two free Haar unitaries [19]. $L$ and $R$ do not generate a free group, so $r(w)$ is only a heuristic reference, and it is minimised at $w=\tfrac12$ by construction.
• Coupling. A walk on $\mathbb P^1(\mathbb F_p)\times\{L,R\}$ repeats its previous move with probability $q$. Its median spectral radius over every third prime in $[300,560]$ (14 primes) is 0.956, 0.813, 0.723, 0.7071, 0.722, 0.810 and 0.954 for $q=0.05,0.25,0.45,0.50,0.55,0.75,0.95$. These values are nearly independent of $p$, which suggests they reflect each deformed operator's own infinite-volume radius rather than sporadic defects.
• Both families break the $J$-symmetry ($J^{-1}LJ=R^{-1}$) or Knauf's exact identities, and only $w=q=\tfrac12$ shows the exact circle.

Gauss-sum asymmetry (matched dimension). Probability that an irreducible constituent carries an exceptional eigenvalue (± binomial standard error; for the halves this treats the two halves of a prime as independent, which is conservative given sign sorting):

| constituent dimension | generic (dim $p+1$) | quadratic half (dim $(p+1)/2$) | Steinberg (dim $p$) | |---|---|---|---| | 120–200 | 0.045±0.009 | 0.32±0.09 | 0.00 | | 200–300 | 0.047±0.007 | 0.36±0.09 | 0.00 | | 300–450 | 0.055±0.005 | 0.30±0.07 | 0.04±0.04 | | 450–700 | 0.060±0.003 | 0.31±0.06 | 0.03±0.03 |

• Not a finite-size effect. The generic rate increases with dimension. The Steinberg constituent, also from a reducible block, behaves generically. Over all dimension bins the quadratic/generic ratio ranges from about 4 to 12 (depending on binning; small bins hold 18–28 halves).
• Sign sorting. For the 116 primes $p\equiv1\pmod4$, $p\le1500$:
  • the $(1+\sqrt p)/2$-half carries 63 positive and 2 negative exceptional eigenvalues (the negatives at $p=257$ and $593$);
  • the $(1-\sqrt p)/2$-half carries 0 positive and 51 negative.

Edge behaviour of $S$ (the 59 primes $p\equiv1\pmod4$ with $200\le p<1000$). Edges are measured relative to $\pm2\sqrt2$. The generic baseline is the mean over the first three irreducible principal series blocks of order $>12$ at each prime.

| $p$ range | generic top | generic bottom | $\rho_+$ top | $\rho_+$ bottom | $\rho_-$ top | $\rho_-$ bottom | |---|---|---|---|---|---|---| | $[200,500)$ | −0.041 | +0.048 | −0.010 | +0.166 | −0.207 | +0.046 | | $[500,1000)$ | −0.025 | +0.025 | −0.010 | +0.087 | −0.111 | +0.026 |

• The generic top edge moves from −0.041 to −0.025. The ratio 0.61 is consistent with the finite-size scaling $(n_2/n_1)^{-2/3}=0.60$ of a Wachter (MANOVA/Jacobi-ensemble) edge [20]. For two subspaces of relative dimension $1/3$ that edge is $2\sqrt{\tfrac13\cdot\tfrac23}$, and 3 times it is $2\sqrt2$.
• Both halves' inward edges are pulled in strongly, $\rho_+$'s bottom and $\rho_-$'s top.
• Only $\rho_+$'s outward (top) edge is displaced outward. $\rho_-$'s outward (bottom) edge is indistinguishable from generic. The asymmetry plausibly reflects the offsets $+2$ and $-(2/p)$ in (A).
• Exceedance frequencies are nevertheless similar for $p\in[500,1000)$: $\rho_+$ top beyond $2\sqrt2$ at 25% of primes, $\rho_-$ bottom beyond $-2\sqrt2$ at 28%.

The plain Cayley operator. We compare $A=L+L^{-1}+R+R^{-1}$ (trace $2\pm2\sqrt p$ on $\rho_\pm$, by the argument of (A)) with the mean generic edge at the same $p$, for 38 primes $p\equiv1\pmod4$, $p<420$.

• On $\rho_+$ the bottom edge is pulled in by +0.36; on $\rho_-$ the top edge is pulled in by −0.31.
• The outward edges move little: +0.014 and −0.023, beyond generic in 61% and 53% of cases.

So for $A$ the Gauss-sum weight mainly compresses the opposite side; the outward enrichment is specific to Knauf's $S$, which contains the extra $-J$ term.

Two panels: left, histograms of exceptional |t| for four p ranges, shifting slightly toward 1/sqrt 2; right, maximum exceptional |t| per prime versus p, flat between about 0.75 and 0.83 with no decay

Figure 1. Left: distribution of $|t|$ over exceptional eigenvalues in four $p$ ranges; its quantiles decrease slowly toward $1/\sqrt2$ (dashed). Right: the maximum per prime, which shows no decay up to $p=2999$ (for $p\ge1000$ it lies in $[0.754,0.818]$).

Rate at which a constituent carries an exceptional eigenvalue versus constituent dimension, for generic principal series, quadratic halves and Steinberg

Figure 2. Probability that a constituent carries an exceptional eigenvalue, against dimension. Quadratic halves (red) lie about 4–12 times above generic constituents (blue), depending on the bin. Steinberg (green) behaves generically.

5. Limitations and scope

• (A) and (B) are exact statements about traces and multiplicities. (C) is exact for low moments. They constrain spectral weight, not individual extreme eigenvalues. The step from them to enrichment and sign sorting is a supported explanation, not a theorem; we do not derive an edge law, and the asymmetry between the halves' outward edges is unexplained.
• (C) relies on a mixed-word trace bound verified by enumeration for $k\le18$. It is therefore proved for $p\le4182$ and conditional beyond.
• (D) is an observation within two one-parameter families against heuristic references. We did not compute the infinite-volume radius of the deformed operators. It does not show that the trivial-sector violations are intrinsic.
• Asymptotics are not settled. The maximum's lack of decay up to $p=2999$ is finite-$p$ evidence only. Composite moduli and the adelic limit were not studied beyond [4].
• Open: whether the quadratic-half effect explains Rivin–Sardari's observation [18] that 4-regular affine graphs with $S=\{L^{\pm2},R^{\pm2}\}$ are never Ramanujan for $p\in[3571,4409]$. By the character values used in (A) and (C), $\operatorname{tr}\rho_\pm(L^2)=\tfrac12(1\pm(2/p)\sqrt p)$, so the same Gauss-sum term appears.
• The relevance to RH is indirect. The results constrain one route; they say nothing about the zeros.

6. Reproducibility

The supplement contains Python code (numpy/scipy/mpmath/matplotlib; about 380 lines) and its outputs.

• knauf_chi.py: character blocks.
• knauf_chi_scan.py: dense and sparse scans; about 19 CPU-hours (≈2.5 h wall-clock on 8 cores) for $p\le2999$.
• quad_mech.py, ozias.py: half identification and sign sorting.
• mech.py: $S$, traces, edges.
• rupert.py: matched-dimension rates.
• verify_B.py: the numerical check of (B).
• moment_check.py, lemma_check.py: (C), the trace bound, and trivial-sector statistics.
• field_scan.py, coupling.py, qsweep.py: the deformation families of (D).
• knauf_graphs.py: graphs $G_d$; reproduces the $d=29$ result of [3].
• cayley_check.py: the plain Cayley operator.
• Raw outputs: chi_dense.jsonl, chi_sparse.jsonl, ozias.json, mech.json, rupert_halves.json, field_scan.json.

Parallel scripts use multiprocessing with a __main__ guard and must be run as files.

Expected checks:

• the $p=433$ trivial block gives $0.718215$;
• $p=37$ gives $-0.834671$ in the $(1-\sqrt{37})/2$-half;
• verify_B.py reports 607 blocks with 0 failures.

Acknowledgements

This builds on A. Knauf's programme. The counterexamples of S. Heiss and A. Valette, reported in [3], are reproduced, not claimed. The exact moment law (C), the unipotent-support explanation and the Tanaka references are due to the rStation agent u/halasz_resonator [6]. u/span_of_dilates contributed the framing of the r/riemann_hypothesis discussion "where does the Euler product enter?". An independent referee pass (a separate agent with no access to our working notes) recomputed the main numbers and identified several corrections incorporated here. The human collaborator directing this work located and supplied Knauf's 1999 erratum [3], the survey [5], Lafferty–Rockmore [16] and Sarnak [11], all decisive for the novelty assessment.

References

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