math.NT — Number Theory
Convolution Smoothing in the Turán Hierarchy: The Blind Zone Extends from Order 43 to Order 61, and Empirically to Order 68
Let Xi(t)=xi(1/2+it) be the Riemann xi-function on the critical line, Xi(t)=sum_{k>=0}(-1)^k c_k t^{2k} its even Taylor expansion, and D(n,r)=det[c_{n+i-j}]_{i,j=0}^{r-1} the associated Toeplitz minors. Writing G(z)=sum_{k>=0}c_k z^k and beta_j=-1/z_j for the zeros of G, the dual Jacobi-Trudi identity gives D(n,r)=c_0^r s_{(r^n)}(beta), the Schur function of the rectangular partition (r^n) in the alphabet beta. Positivity of D(n,r) for all n>=0 and all r>=1 is equivalent to the Riemann hypothesis (RH) by the Schoenberg-Edrei characterisation of Polya frequency sequences; for each fixed r, the inequalities D(n,r)>=0 are a necessary condition, and for r=2 they hold unconditionally. A previous paper of the author proved that every level r<=43 is blind: the minors D(n,r) stay positive even in the presence of non-real zeros. The mechanism there was the two-variable Chebyshev criterion U_k(cos theta)>0 iff (k+1)theta<pi, together with the uniform bound theta<=theta_max=2 arctan(1/(2 gamma_1)) =0.07071826... The present paper shows that the bound 43 is not the truth but an artefact of ignoring two structural features of the true alphabet. First, the evenness Xi(-t)=Xi(t) forces every zero of G to occur with multiplicity two in beta, so the relevant constituent is not U_r but the self-convolution W_r = sum_{i=0}^{r} U_i(cos theta) U_{r-i}(cos theta), with generating function sum_{r>=0} W_r z^r = 1/(1-2z cos theta + z^2)^2. We prove the identity W_r = (1/2) U'_{r+1}(cos theta) and the pole-free sign criterion of Lemma (sign), and we show that W_r>0 for all r<=61 while W_62<0: the convolution of the Chebyshev sequence against itself smooths away the sign change that occurs at U_44, pushing the threshold from 43 to 61. Secondly, the zeros on the critical line contribute a positive real sub-alphabet whose complete symmetric functions h_i(X)>0 convolve with the violating part, h_r(X union Y)=sum_{i+j=r} h_i(X) h_j(Y), pushing the threshold further. Main theorem (proved). For every r<=61 and every n=1, and regardless of whether RH is true, D(1,r)>0. Consequently the first 61 levels of the Turan hierarchy are blind on the shift-1 row. High-precision computation of the full rectangular Schur functions s_{(r^n)}(beta) on the true alphabet (truncated at N critical-line zeros) shows that the first discriminating order is r*=69 at shift n=1 and n=3, so that all orders r<=68 are blind; the value is stable under truncation (N=45,60,80 all give 69). A scaling law r* ~ 1.5 pi/theta is observed across two and a half decades of the violating offset. At shift n=2 no negative minor was found for any r<=400, suggesting a qualitatively stronger phenomenon. The number 61 is proved; the number 68 is numerical, and the Status section separates the two without ambiguity. No claim of a proof of RH is made.