https://hackerpaper.kaipfister.de/p/1102.5206/Collected HN comments and submissions linking this paper.enMon, 02 Nov 2026 11:16:22 GMThttps://news.ycombinator.com/item?id=24213936urn:hackerpapers:mention:1102.5206:24213936Wed, 19 Aug 2020 18:18:47 GMT<p>It turns out that the answer to this question is known for all sizes of grid[1], though it was found fairly recently (2011) and the answer for small grids is quite complicated. For the 9x4 grid, the optimal number is indeed 10. For large n×m grids, with n and m ≥ 16, the minimum number is: floor((n + 2)(m + 2) / 5) - 4. The linked paper has a full-page table giving the formulae for small grids. Thanks to Schoolmeister for mentioning the name of this problem, which gave me the right terms to search for. 1. https://arxiv.org/abs/1102.5206</p><p><a href="https://hackerpaper.kaipfister.de/p/1102.5206/">Paper and discussion history</a>. A mention is not an endorsement.</p>cs.DM