Found by randomized valid-set generation, ejection-chain walks, and remove-and-regrow search. Direct enumeration gives exactly 20 ordered solutions to a+3b=2c+2d modulo 999, all diagonal. Exhaustive growth searches from this set found no 21-element witness obtainable by removing and replacing up to four elements.
David Renshaw · 2026-08-06 22:28:53 UTC