Inspired by Board with all 2020s :
Zeroes are written in all cells of a n×n board. We can take an arbitrary cell and increase by 1 the number in this cell and all cells having a common side with it.
- Is there a highest n for which an equal positive number can be reached in all cells simultaneously?
- Is there a highest n for which an equal positive number can not be reached in all cells simultaneously?
Note: It is possible for n = 1,2,4 and 5. It is not possible for n=3 and n=6
My LP solver tells me under 100 it is solvable for
n = 1,2,4,5,8,9,10,14,15,19,20,22,24,25,29,32,34,39,44,59,64,71,76,77,82,84,94,97 (I do not see a pattern)
Obviously at least 1 of the answers is no. But are there an infinite number of solvable and an infinite number on unsolvable sizes, or has one of the types a finite number of sizes? (I don't know myself)*
Hint: impossibility for specific cases can be proven mathematically:
- If a balanced matrix with only positive increment values exists, then a fully symmetric balanced matrix with only positive increment values can be constructed from it by adding mirror images. Hence: If no fully symmetric balanced matrix with only positive increment values exists, the case is infeasible
- Looking at increments for fully symmetric n = 3: - Corner total: T = 2*side+corner - Side total: T = 2*corner+centre+side - Centre total: T = centre+4*side Eliminating side and corner from these equations yields centre = -Total/7 -> infeasible
- I applied the same technique to prove n=6 is infeasible
It seems likely that if, with size, the number of equations increases, the chance of a negative value increases. However, there might a a pattern or redundant equation might appear, making (some, or all) high n cases feasible.