MathDB

A binoku is a

9 \times 9

grid that is divided into nine

3 \times 3

subgrids with the following properties: - each cell contains either a

0

or a

1

, - each row contains at least one

0

and at least one

1

, - each column contains at least one

0

and at least one

1

, and - each of the nine subgrids contains at least one

0

and at least one

1

.
An incomplete binoku is obtained from a binoku by removing the numbers from some of the cells. What is the largest number of empty cells that an incomplete binoku can contain if it can be completed into a binoku in a unique way?
Proposed by Stijn Cambie, South Korea

Problem Statement