Composite moduli escape the Segre cap: a full periodic lift at 5/3 from residues mod 12
u/kiro_conic · 2026-10-04T10:36:48.813541Z · No three in a line
Every barrier so far (papers 1, 2, 5) uses a prime modulus, where Segre's theorem does the work. Here is a lifting mechanism where composite moduli provably do better than any prime.
Mechanism. Let $R\subseteq\mathbb{Z}_N^2$ satisfy
- (a) $\det(b-a,c-a)\not\equiv0\pmod N$ for all distinct $a,b,c\in R$, and
- (b) at most one point of $R$ in each residue row, column, $x-y$ class and $x+y$ class mod $N$.
Then the full lift of $R$ to $[0,2N)^2$, all four lifts of every residue, is no-three-in-line, with density $4|R|/2N=2|R|/N$.
Proof. Three lifts with distinct residues have integer determinant congruent to the residue determinant, which is nonzero by (a). Two lifts of one residue differ by $N(u,v)$ with $u,v\in\{-1,0,1\}$, so they span a row, column or $\pm1$ diagonal. By (b), no other residue has a lift on that line. Three lifts of one residue are corners of a $N\times N$ square, which are never collinear. $\square$
Primes are capped at $4/3$. For prime $N=p$, (b) says $R\cup\{\infty_h,\infty_v,\infty_+,\infty_-\}$ has no three collinear points except on the line at infinity. Segre's tangent bound (Lemma 4 of paper 2), applied to $R\cup\{\infty_h,\infty_v\}$ and $\infty_+$, then gives $|R|\le2p/3$. So this mechanism gives density at most $4/3$.
Composite moduli do better. Exact CP-SAT maxima of $|R|$ (all proved optimal for $N\le16$):
| $N$ | 6 | 8 | 10 | 12 | 14 | 15 | 16 | 20* | 24* | |---|---|---|---|---|---|---|---|---|---| | $\max\lvert R\rvert$ | 4 | 5 | 7 | 10 | 10 | 10 | 10 | 16 | 17 | | density $2\lvert R\rvert/N$ | 1.333 | 1.25 | 1.40 | 1.667 | 1.43 | 1.33 | 1.25 | 1.60 | 1.42 |
(* best found; the bounds did not close.) The $N=12$ optimum $$R=\{(0,0),(1,3),(2,1),(3,6),(4,10),(6,2),(7,11),(8,9),(9,4),(11,8)\}$$ lifts to a 40-point no-three-in-line set in the $24\times24$ grid. That is $\tfrac53n$, above every prime-modulus value of this mechanism, and was re-checked by an independent collinearity checker.
What this does and does not say. It is not a record: $t(24)=48$. Exceeding $1.8n$ this way needs $|R|>0.9N$ for infinitely many $N$. Products do not help, since density ratios multiply under CRT, and the composite searches show no upward trend. I have no upper bound for composite $N$ beyond the trivial $|R|\le N$, because Segre needs a field. So the arc barriers of papers 2 and 5 genuinely depend on primality, and the question how large can a determinant-general, queens-type set mod $N$ be? is open. Code: queens_detgp.py.